THE
A
AUK'
QUARTERLY
JOURNAL
OF
ORNITHOLOGY.
VOL.
XX¾.
THE
No.
APRIL, 1908.
ECOLOGICAL
BY
SUCCESSION
CHARLES
C.
OF
2.
BIRDS.
ADAMS.
CONTENTS.
I.
II.
IlI.
Introduction.
Representative Literature on Habitats and Succession.
1.
Habitat
2.
Succession.
Preference.
The Major Arian Environments.
IV.
V.
Minor Arian
Arian
Environments
and their Associations.
Succession.
1.
General Remarks.
2.
a.
Successionon Isle Royale.
The Aquatic Association and Habitat.
b.
The Shore and Marsh Association
and Habitat.
c. Bog-forestAssociationand Habitat.
d. Aspen-birchAssociationand Habitat.
e.
3.
4.
The Climax Association
Internal Factors.
]Snvironmental Factors.
and Habitat.
5. F,nvironmental and AssociationalConvergence.
6.
VI.
Successional and Environmental
Evolution.
7. The Relation of Successionto Organic Evolution.
Some Advantagesof a Knowledge of the Laws of Succession.
"Of all truths relating to phenomena,the most valuable to us are those
which relate to their order of succession. On a knowledge of these is
founded every reasonable anticipation of future facts, and whatever
power we possessof influencingthose facts to our advantage."--John
STUART MILL,
"Indeed, some geologistsseemto take pride in lack of knowledgeof
principlesand of their failureto explainthe factsobservedin the trainsof
110
Aox•s, Ecological
Succession
o/ Birds.
I-auk
[April
the elementarysciences. I have heard a man say: 'I observethe facts
as I find them, unprejudiced by any theory.' I regard this statement
as not only condemningthe work of the man, but the positionas an impos-
sibleone.... The geologistmust selectthe factswhich he regardsof sufiqcient note to record and desc•Sbe. But such selectionimplies theoriesof
their importanceand significance. In a given casethe problem is therefore reducedto selectingthe facts for record,with a broad and deep comprehensionof the principlesinvolved, a defi•,iteunderstandingof the rules
of the game,an appreciationof what is probableand what is not probable;
or elsemaking mere randomobservations. All agreethat the latter alter•mtive is worsethan useless,and therefore the only training which can
make a geologistsafe, even in his observations,is to equip him with such
a knowledgeof the principlesconcernedas will make his observationsof
value."--
PRESIDENT C. R. VAN H1SE.
I.
INT•ODUCT)ON.
Almosteveryobserverof animalshasnotedthat certainkinds of
birds are usuallyfound associatedin certain conditions,as, for
exampleparticularspecies
of sandpipers
andploversuponthesandy
beach,or the Meadowlarkand Dickcissel
upon certainprairies;
but this is rarely considered
a subjectworthyof seriousscientific
study. To discussthe significanceand value of suchecological
study and suggestphasesfor investigation
is the objectof this
paper. By the ecological
distributionof birdsis meantthat correlation betxveen environmental
conditions and the occurrence and
association
of certainspecies
of birds. In suchstudyspecialattention must be devotedto the placesof breeding;nevertheless
the
associations
of birdsat all seasons
of the year are of importance.
It is not the isolatedoccurrence
of thesespecies,but their relative
abundance,
the association
of certainspecies,
andtheirpersistent
occurrence
in suchconditions
xvhichis significant. In theliter•tture
of ornithology
thereis a vastamountof isolated
databearingon
this subject,but very little of it has beenorganizedand systematically studied.
When oncethe factsand generalecologicalrelationshave been
detennlned,sothat the representative
bird associations
or societies
of givenlocalities
havebeencorrelated
withtheir properenvironments,it will thenbepossible
to determine
howonesociety
becomes
transformed
intoanother,
whether
thisisdueprimarilytoother
birds
Vol.
XXV'l
1908
J
Au.•.•s,Ecological
Succession
o/ Birds.
11 I
or to otherenvironmental
influences.A knowledge
of thesucccssiou
of bird societies
and of the lawsof change
will notonlyleadto new
ideas as to the influenceof the enviromnent,but will also have a
markedinfluenceuponthe practicalfield studiesof the bird student. It shouldlead to a moreintelligentunderstanding
of the
relation of birds to the world about them, or even better, to the
worldof whichtheyform a part.
Attentionshouldfurtherbe directedto thefact that simplythe
occunxmce
of the bird in a definitehabitatis notby anymeansthe
sole aim of such work.
The influence of the enviromnent should be
studiedin itsbearinguponall phases
of birdlife. Not onlyshould
the mostfavorablehabitator optimumbe recognized,
but alsothe
influenceof the lessfavorableconditions;thusthe nestingsite,
composition
of thenestmaterial,food,abundance,
feeding
grounds,
•nigrating
habitsandall like relations
areneededfor an adequate
and exhaustive
studyof the ecological
distributionand succession
of birds.
It is thereforenot surprisingthat suchrequirements
will be
difficult to meet because the facts themselves are difficult to secure.
Then there are further difficulties which are due to the limitations
of the studenthimself,and are psychological
in theirnature. As
examples
of thisclassof difficnlties
twomaybe cited,because
they
are of frequentoccurrence
in all kindsof scientificwork and not
by any•neansconfined
to thestudyof birds. For, contraryto our
youthfnlideas,naturalistshavethe sa•nelimitationsas hu•nanity
in general! We maydividenaturalists
intotwoclasses,
depending
upon their primarytype of •nind. First, thosewho tend to see
only.the infinite detail of isolatedfacts and observations.This
type of mind is particularlyimpressed
with the multiplicityand
varietyin natnre,and is oneto whicha generalstatement
is almost
a causeof irritationbecausethereare nsuallyexceptions
to any
generalstatement.The constructive
imaginationseemsfeebly
developedin this type. To this classbelongmany extremely
valuableand usefulstudents,
because
of the datawhichthey,often
with extremeconscientiousness,
collect. They arc collectors
of
factsratherthanstudents
of relations. To thesecond
classbelongs
that type of mind whoseprimary interestis in generalizations,
principles,relations,and whichtendsto neglectisolatedfactsand
112
AD^•lS,œcological
Succession
o] Birds.
I-Auk
kApril
observations.The constructiveimaginationis liable to be developedin this type. This includesmanyextremelyvaluableand
usefulstudentson accountof their tendencyto condense,
sift and
formulategreat massesof isolatedfacts. They are studentsof
ideas and relations rather than collectors of "facts."
Eachclass,especially
thewell-markedtypesand extremes,
often
findsit difficultor impossible
to understand
the pointof viewof the
otherclass. This frequently
leadsto misunderstandings
and often
to mutual contempt. Cope and Marsh clearly illustratethese
two typesof mindsamongour Americannaturalists.
By this time somemaywonderwhy thissubjecthasbeenintroduced. It has beenwith a definitepurpose,becausefrequently
theseopposedpointsof view causcdelaysin the development
of
many subjects. Thus a forewarningto studentsof bird ecology
mayproducegoodresultsif theindividualstudentmakesa conscious
cffortto counterbalance
suchdeficiencies
as go with his particular
typeof mind. In thepast,detailshavetendedto produce
confusion
throughthe neglectof generalideas. It is rarely that a word of
warning on this subjectis out of place, becausethe balanced
"goldenmean"investigator
isnevertooabundant. Tile quotations
at the head of this article have an immediatebearingupon the
subjectat this point.
Not onlyis habitatpreference,
the association
of arian species,
their succession,
and the laws expressing
theserelationsof much
interest,but theyare of muchimportance
scientifically
as well as
in a practicalway. It is thereforedesirable
that naturalistsrealize
the necessity
of understanding
the "rulesof the game"if the true
relationsof birdsare to be studiedto the bestadvantage. No adequatesubstitute
hasyet beendevisedto replacea graspof general
principles.
Throughoutthis paperemphasis
is placedon the defir•iteaess
of
the dominant major environmentalinfluencesand complexes
becausethe irregular featureshave apparentlyreceivedundue
emphasisand have retardedthe recognitionof certainimportant
definite relations.
Vol.
XXV]J
1908
I[.
ADAMS,
Ecological
Succession
o[ Birds.
113
P•EPRESENTATIVE LITERATUllE ON ttABITATS AND SUCCESSION.
1. Habitat Pre]erence.The Americanliteratureon habitat
preferenceand succession,
as a subjectof specialinvestigation,
is
very limited. By succession
is meantthe changeor replacement
at a givenplaceof oneor severalspecies(an association)
by others;
aswhena swampisinvadedby a duneandtherepresentative
swamp
birds are replacedby thoseof the dune; or evenagainwhenthe
dunebecomes
fixedby vegetationand is inhabitedby still another
association
of bird life. This is a muchneglected
subject;however,
isolatedobservations
on habitatsare abundantin the biographies
of the variousspecies.The fragmentary
character
of thesebiographiestendsto makethem composite
and they losewhat peculiaritiesthey may havewhichare due to a response
on the part of
the bird to its particularconditions
of life. Theseunfortunate
limitationsclearlyshowthat here is an extensivefield worthy of
careful investigation.The work already done will be a useful
guidein many cases,but the studentwho wishesto developthis
subjectmustturn to the fieldsand forestsratherthan to the literature, bothfor his inspirationand his data.
Perhapsa furtherword shouldbe addedconcerning
the limitationsof the composite
life-history
method,asthiswill aidin making
clearthe kind of work neededin the future. This colnposite
or
generalized
methodof describing
habitatsandlife historiesand the
response
of birdsto thcm, tendsto lay undueemphasisuponthe
averageconditionsof life and habits,and tendsto neglectthose
detailed
responses
totheenvironment
which
reflect
thclawsofl•cal
influence.Theseresultsaresimilarto thoseproducedby systematic students
who are "lumpers"andwho do not recognize
local
racesor varieties. Thus a nestmay be built uponthe groundat
the baseof a shrubor bunchof grass,or in the brush,bnt what
conditions
determine
suchsites? In a drymeadowa SongSparrow
maybuilddirectlyuponthe ground,but in a swamp,in orderto
havea dry nestingsite,it buildsin a willowshrub. In manycases
the causesof thesedifferences
will bc difficultto determine,but in
othersit is a relativelysimplequestion
for anyonefamiliarwith the
speciesto solve. There are alsogeographic
variationsof habits
114
ADAMS,
Ecological
Succession
of Birds.
fAuk
[April
as well as those of habitats just cited, and for this reason it is
necessary
not to confusesuchvariationswith thoseconfinedto
some restrictedarea. These local and geographicrelationsare
very intimatelyrelated,but they are subjcctswhich can only be
worked out in detail when local studiesgive properattentionto
local enviromnental
responses.
In the followingaccountof the literatureno attemptis madeat
completeness,
but thepaperscitedarebelievcdto be representative.
Thesepaperswill helpto givesomeideaof thekind of observations
and recordsalreadymade,andwill be suggestive
as to futurework.
Mentionwill firstbe madeof the literatureon habitatpreference,
and then of that on succession.
By far thebestdiscussion
we havefoundonhabitatprefercnce
of
the birds of a givenregionis that by Townsend('05) on Essex
County, Massachusetts.The pri•nary arian environmentsare
described,
the representative
birdslisted,and their preferredhabitatsare brieflydiscussed.Thus, the oceanand its birds,the sand
beach and its birds, the sand dunesand their birds, the salt marshes
andtheirbirds,andthefreshmarshesandtheirbirds,givea general
idea of the subjectstreated. Regardingthe birds of the sand
beaches,
he remarks:"Amongthe Plover,the Black-bellied,
Semipalmated,and Piping Ploversare aboveall birds of the beach,
althoughthe first two are occasionally
foundin the marshes,while
thelast-namedrarelystr.aysfromthe beachand the adjoiningsand
dunes. The GoldenPlover,althoughat timesfoundon the wet
sands,is muchmorelikely to hunt for foodon the dry sandsabove
thel•ighesttides,or stillfartherinland,whiletheKilldeergenerally
avoi;ds
thebeach
altogether,
preferring
thefields"
(p.21). And
regardingthe birds of the sand duneshc remarks: "Savanna
Sparrowsnestin numbersat thefoot of clumpsof tall beachgrass
throughoutthe dunes,and on the edgesof the tidal inletsfromthe
marsh. The nestsof theRed-wingedBlackbirdsand the Bronzed
Gracklesare abundantin the bogsand grovesof the birches. The
Crow, in the absenceof tall trees,buildsperforcein the stunted
pinesandbirches,at ti•nesonlytenor twelvefeetfromtheground"
(p. 34). In the caseof the Crow,notethat he recordsthe rcsponse
to the dune environment.
WhileTownsendrecognizes
changes
in theenvironment,
asin the
Vok
XXV]
1908
J
ADa•is,Ecological
Succession
o[ Birds.
115
dunesand beach(pp. 21, 30), yet he doesnot seetheir relationto
the bird life in the definiteway in whichhe seestheirhabitatpref.erences,
nor doeshe appearto dearly recognize
the fundamental
relation of association
within the breedinghabitat. To him the
enviromnent
is static.
However
an excellent feature of his work
is the record of seasonalchangesin the bird lifc of the various
h•bitats.
In this connection attention should be called to certain
paperswhich will greatlyaid in the study of the d•Tnamics
or
changingenvironmental
factorswhichinfluence
seaor lake shores
bordcredby dunesand swamps;conditionsrepresented
on the
Massachusetts
coast. Gilbert ('85) has discusscd
the general
principlesand topographic
featuresof lake shores;and Gulliver
('99) the shoreline of the sea. But in additionto thesephysiographic forces,the vegetationalso has a dominatinginfluence
upon bird life. For generalprinciplesrelatingto this subject
Cowles('01) shouldbe consulted
for his discussion
of the vegetation of inlandshorcsand dunes,and Ganong('03 and '06) for his
treatment
of the Atlantic
coastal conditions.
These
authors dis-
cuss the succession
of the vegetation,a factor of the utmost
i•nportance
in the studyof arian succession.
While considering
Townsend'sresults,it may be well to outline
brieflya generalsuccession
of ]birdlife alongthe shoreasindicated
by his records. It is evidentfrom the map accompanying
his
volumethat the currcntsand wavesare constantlymodifyingthe
coastline andformingspits,barsandislands;andthat thebarrier
beachareais increasing,
and thustendingto becomecontinuous
at
the expense
of someof theoceanhabitat. As the continuity
of the
beachdevelops,
the areaof swamplandbehindit tendsto increase
and thusto furtherrestrictthe openwaterand increase
the swamp
habitat. The beachsands,oncefree from the waycsor ground
water,arecaughtup by thewindsto formdunes,andmaymigrate
into the swampsand thustransformthem. Thus with the extensionof the beachthe seabirdsare replacedin dominance
by the
shorebirds,and a succession
is produced.
In a similarmannerthe dunesencroachuponthe swamp,and
swampbirdsaresucceeded
by thoseof thedunes. As thewandering
dunesbecomeanchored
by vegetation
andforestsgrowuponthem,
still otherbirdswill invadethem. Thus all stages
maybeexpected,
116
A•)A•S,Ecological
Succession
o/ Birds.
[Auk
[April
from oceanto beachbirds, onwardto thosecharacteristicof wander-
ing and fixedforesteddunes. Theserelationsare outlinedsimply
to-indicate
theproblemandits causes,
whichneeddetailedinvestigation.
In Michigana fewhabitatstudies
havebeenmade. One in the
PorcupineMountains,on the southshoreof Lake Superior,and
anotheron Isle Royale. Bothareby MeCreary; thepaperon the
latter area is unpublished.The summerbirds of the Porcupine
Mountainsare listed(MeCreary'06) by selected
localitiesand the
habitat preferences
are discussed
as follmvs:water birds, birds
frequentingthe shorcsand banksof streams,birds frequenting
grassymeadows
andalders,birdsfrequenting
tamarackandcedar
swamps,
birdsfrequenting
hemlocks
andmaples,
andbirdsfrequenting thecliff andlnountaintop. In itsemphasis
uponhabitatpreferencethispaperis the onlyonesofar seenwhichat all approaches
Townsend's
discussion
of thissubject. MeCreary'sworkwasdone
without a knowledgeof Townsend's.
In }outheastern
3•lichigan,Brown ('06) madea localitystudy
and outlines the habitats as follows: birds found in orchards,
birdsof theopenwoods,
birdsof theopenfields,birdsof thethicket,
andbirdsof the•narshes
andriver. Brown'spaperis intermediate
in characterbetweenthe precedingpapersand thoseof an econonfienature, to be mentionedlatdr, becausethe area studiedhas
beenso muchinfluencedby man.
There are a few paperswhich, althoughprimarilyfaunisticor
geographic,
containhabitatdata. Such,forexa•nple,
is Ridgway's.
('74) discussion
of the birdsof the WabashValley and ('89) the
birds of the Illinois prairie (pp. 13-16). An exceptionally
good
paperof thischaracteron the Louisianabirdsis by Beyer,Allison
andKopman('06),although
itsaimandmethodoftr6atment
differs.
from that of Townsend. The bird life is, however,doselycorrelatedwith the vegetation
andthe physicalconditions
of the State.
The paperspreviously
mentioned
hayebeenwrittenfrom a
regionalstandpoint.The studyhoweverof all the variousconditionsfrequented
by a givenspecies
or somenaturalgroupis alst>
an importantandneglected
methodof ecological
studywhichpossesses
certaini•nportantadvantages.As an illustrationof this
methodmaybe mentioned
Palmefts('00) studyof the Maryland
Vol.
XXVq
1908
/
Yellow-throat.
ADx•s, Ecological
Succession
o[ Birds.
117
He has shown that different varieties have differ-
ent habitatpreferences.Jacobs('04) hasgivenus an interesting
habitatstudyof a singlespecies
in Pennsylvania,
theGolden-winged
Warbler.
Let us now turn to another classof habitat studies,thosewhich
throughnmn'sinfluence
throwonlya subordinate
lightupon"natural" habitatsand succession,
and are primarilyof economic
importance but contain valuable habitat data.
An interesting
and ratheruniquepaperbelongingto this class,
basedon observations
in Southeastern.
Michigan,is by Watkins
('00). It is entitled'Michigan Birdsthat Nestin OpenMeadows.'
A few of his statements
explainhis pointof view: "To makemore
plainthelimit andscopeof thistreatise,which,of necessity
mustbe
longerthanI hoped,I will includein my list onlysuchspeeics
asI
havefoundnestinguponthe groundin the openfieldsand meadows,excludingthosefoundnestingupon the boundaryfencesor
ground; alsothosenestingin the openmarshlandswhich are
undrainedandboggyto theextentof beingunfitfor hayor pasture"
(p. 67). The papercontainsnumerous
noteson the habitatpreference and variations in these traits.
By far themostcomprehensive
andthorough
studyof anylimited
farm area is that by Judd ('02) of a farm in Maryland. In this
paper habitat preferences
are clearlyrecognized,and discussed
ratherfully (pp. 12-20). The birds are associated
thus:-- birds
that nestin opcnfields,birdsthat dcpendon covers,birdsof less
limited distribution(consistslargely of remarkson haunts),and
birds
of varied
distribution.
His
last two
sections are rather
miscellaneous
in characterand showthat the principlesof classificationfor habitatswerenotclearlydefinedin hisownmind.
The onlyother paperdiscussing
habitatpreference
in detailis
alsothe latestuponthe suhjeet,and is by Forbes('07). This is a
preli•ninaryreporton a bird census
acrossthe cornbelt of Central
Illinois in the early autumn; a studyof the feedinggroundsand
preferences
as influencedby the dominantcropsof the area traversed,corn,pasture,and stubble. By meanso• this census,
the
habitatpreferences
for differentcropsandtheassociation
of species
in them is statisticallydetermined. The paper is particularly
suggestive
for its bearingon the subjectof dominance;
however,
118
ADA•S,Ecological
Succession
of Birds.
FAuk
LApril
the suggested
methodof studyhas evengreatersignificance
when
,appliedduring the breedingseason. Doubtlessopinionswill vary
,as to the validity of the methodas appliedby Forbes,even'by
thosewho would approveof it for the detailedstudyof a limited
area, or a breedinghabitat. For large areassomeco6perative
methodmaybe necessary.
2. Succession.Turning now to the literature on succession,
it is foundto be extre•nelylimited in amount. So far as knownto
the writer, only two Americanauthorsseemto have realizedthe
.existenceof succession. In his discussionof tl•e biotic succession
in'the PorcupineMountainsof Michigan,Ruthyen ('06) clearly
includedthebirds,althoughtheydidnotreceiveseparate
treatment,
and might for this reasonbe overlooked. His positionis clearly
stated(2. 43) as follows: "Owing to the dependence
of formsof
life on their environment,biotic changesare necessarilyclosely
'relatedto environmental
changes. Thesebioticchanges
•nayoccur
in two ways; the formsmusteitherbe ableto respondto the new
conditions,
or be supplemented
by otherforms. That they tend
to becomeadjustedcannotbe questioned,but in many casesat
least,thisadjustment
lagsbehindthe changingconditions,
andthe
forms are replacedby othersfrom adjacenthabitatswhich are
adjustedto the conditions
towardwhichthe particularhabitatis
changing,
thusbringingabouta succession
of societies."In speaking of the biota of the hard-woodforesthe furthersays: "This
regionhas beenreservedfor the last, for the conditionsare evidently thosetoward which the other habitatstend to be changed
under the presentconditions
.... This societythus represents
the
climax societyof the region. It consistsof the forms that are
adaptedto or associated
with the conditionswhich prevailin this
regionin thelaststagesof themutualadjustment
of all theenvironmental processes.As the processesbecome adjusted to one
another,
thehabitatof thcclimaxsociety
isincreased
at theexpense
.
of the other habitats,and the associated
biota tendsto becomeof
generalgeographic
extentin the region."
The only other paper discussing
avian succession
is that by
•Frothingham
('06), and this is not a "natural" succession
but one
influencedprimarilyby man. He clearlyexpresses
a bird succes.sion correlated with the reforestation
of burned lands.
The
area
¾ol.
XXVq
1908
I
ADAMS,
Ecological
Succession,
o] Birds.
,
119
studied is the Michigan forest reserveon Higgins Lake. The
regionwas originallycoveredwith White and Norway pine, but
repeatedfiresfirst kill off the pines,later the oak and •naple; and
finally the dominantvegetationis sedge,sweetfern, huckleberry
andprairiewillow. •¾iththefireprotection
affordedby thereserve,
Frothinghamanticipatesa reversalof the above succession
of
(testruction,
and furtherrmnarks: "With the typesof vegetation
whichmarkthedifferentstages
of theplantsuccession
justdescribed
there seem to be correlated certain definite bird forms.
These
for•nsareforthemostpartsuchasfrequentobservations
in northern
Michiganhaveidentified
asgenerally
characteristic
of therespective
enviromnents." This is followedby listsof birds characteristic
of
different kinds of vegetation. While theselists do not correlate
perfectlywith the i•npliedsuccession,
yet the generalstatementof
the problmnis clearlyexpressed.
The burningof forestshaslongbeenknownto changethe characterof the vegetationand faunaof areas,but this is oftenreferred
to as the changeof a "life zone." Thus Merriam ('99, p. 47)
statesthat a firein the CanadianzoneonMr. Shastais followedby
the Transition
zone and remarks:
"But
in the meanti•ne anew
growthof Shastafirshasstarted,andin tenor twentyyearsislikely
to overtopand drownout the Transitionzonespecies,
enablingthe
Canadianzoneto reclaimthe burn.... But on the steeperslopes,
,especially
rock slopes,if the vegetablelayer is burnedoff, the
(lower)zonewhichcreepsup to replacethe (higher)onedestroyed
beeo•nes
permanentor nearlyso.... Deforestation
of an areathereforetendsto lowerits zoneposition." Birdsare not mentioned
in
this discussion
nor the relationof "zones"to the generalproblmn
of succession'.
Such"zones"
arethusonlyparticular
phases
of
succession.
It is thus seen from the above outline of literature that habitat
preferences
havebeenoutlinedfor a fewwidelyseparated
localitiesand for someagriculturalconditions,
but there has beenno cmnprehensive
discussion
of the problemsof habitatsand succession,
evenin a preliminarymanner,eitherfrom a scientificor econo•nie
standpoint. This fact seemsrather rmnarkablein view of the
greatutility of a knowledge
of the generalprinciplesunderlying
•econo•nic
practice. There are, however,certainphasesof biotic
120
ADx•s,Ecological
Succession
o[ Birds.
[^uk
1April
succession
which have been discussed
by a fewauthors. These
subjectshaveeitherbeendiscussed
in a verygeneralmanneror are
detaileddiscussions
of specialregionsor groupsof plants and
animals. For thisreason,
perhaps,
theirbearinguponothergroups
thanthosespecifically
mentioned
are verylikelyto be overlooked
by thosewhotakelittle interestin anysubjector discussion
which
doesnotspecifically
mention
theirspecialty
orlocality. This phase
is mentioned in order to show that while arian successions have
beenconsiderably
neglected,
advances
havebeenmadeelsewhere,
by memos
of whichsomegeneralprinciplesappearto havebeen
fairlywellestablished.This is particularlytrue of plantsuccession,asshownby thewritingsof Cowles('01), andin eonslderable
detailby Clements('05). The discussion
by Clementswill be
particularlyvaluableto the studentof aviansuccession.
III.
T•m
MxJoI•
Awx•
EN'wao•'am•TS.
As has beenseenin the precedingreviewof the literatureon
haunts,no cmnprehensive
discussion
hasbeengivenof theenvironmentalinfluences
or ecological
distribution
of (extra-tropical)
North
Americanbirds. Variousauthorshavediscussed
their geographic
distribntion,and certaingeographic
variationshavebeenreferred
to certainenvironmentalinfluences,but a generalccologictreatment, as contrasted
with a primarilyfaunisticone,has not been
made.
This is remarkable when we recall the fact that the collec-
tionsof North Americanbirds are, consideringthe large area concerned,the bestin the world both as to qualityand as to quantity
(Stejn•ger,'03). This meansthat therehaw beenmanytrained
collectors; but what has become of the notes and observationson
the environments and conditions of life of these birds, which must
necessarily
have been known to successful
collectors?Part of
theseobservations
havebeenpublished,
andperhapsno oneis to
blamebecause
morehavenot; but the pointof significance
is that
we have,in fact, hardlymadea beginningin the carefuldetailed
study of the bird enviromnentand its developmentas a distinct
fieldof study. In common
withtheremainder
of theNorthAmerican biota, severalgeneralprinciplesare known,but they donot
appearto be currentamongornithologists.
Vol.
XXV-I
1908
J
ADA•S,Ecological
S•ccession
o[ Birds.
]2]
The followingdiscussion
and suggestions
on the largerenvironmentalunitsattenlptonlyan outlineof certainphases
of the problem, in order to call attentionto certainprincipleswhich seem
usefulas a background
for theintelligentstudyof birdhabitatsand
succession.Fromsucha standpoint
asthis,thedominant
influences
of givenareasand environments
are of particularinterestand of
fundamental
value. By focussing
attentionuponthe importance
of recognizing
thesedominantenvironmental
influences,
we may
hopeto escapesomeof the confusion
whichappalsthosewho are
keenlyimpressed
with the chaosand complexity
of the problem.
These dominantfactorsare usuallynot singleisolatedforces,but
resultantsof severalor manyinfluences.Thus, as in the caseof
the vegetation,
it is not onefactor,but a complex,
whichinfluences
differentbirdsin differentways. Nevertheless
thereiswhat may
be called a mass or dominant effect.
A major habitat unit lnay be considered
as a combinationof
conditions
whichare dominantin a certainarea. The •:er• dominancemeansthata relati•:ely
limitednumbero[ [orcesor complexes
are operati•:e.With departurefrom sucha centerof influencethe
donfinancechanges,as other influencesare encounteredand other
dominants
are established.
When we considerthat certain ecologicalgroupsof birds are
world-widein their environmentalrelations,it becomesevidentthat
suchcharacters
areof fundamental
importance.Thuswaterbirds
lnay occurin any part of the world wherewater is a dominant
enviromnental
factor. This isnota simpleecological
groupofbirds,
but oneof thegreaterunitsof association
whichmayhe subdivided
intomanynilnorclasses;asthosewhichfrequentthesea,andothers
the inland bodies of water.
The shore birds form another natural
ecological
group,and alsothe inlandbirdsa third. There may
thusbe considered
to be threeprimaryecological
groupsof birds
whichare closelycorrelated
with definiteand dominantenvironmental influences: thus :-1.
Water
birds.
Thosefrequentingthe seaand the a.djaeentrockson which they
nest, and inland waters.
2.
Shore and Marsh birds.
Thosefrequentingshoresof all kinds, seas,lakes, swampsand
rivers.
122
3.
ADA•S,
Ecological
Succession
o] Birds.
Inland
rAuk
LApp'i!
bir•ls.
Those frequentiug deserts, gr•ss 1,'•nds•d
forests.
Of coursetheseeco]oglcal
classes
arenot sharplydefined,andyet
theyare sodistinctthat theycanbe easilyrecognized. It shouldbe
notedthat the abovegronpsarc closelycorrelatedwith certain
dominantphysicalfeaturesof the earth- the sea,the shoreand
the inland
environments.
The relative abnndance and dominance of these classes of birds
will be determinedlargelyby the dominanceof snchphysicalconditionsasmostdistinctlyfavora particularecological
group. Thus
at sea the water birds are dominant; on shore, the shore birds;
and inland, still other kinds. The linear character of the shore
habitatand the adjacentbreedinggroundsgivesit a ratheruniqa•e
character,as the two otherhabitatsoccupylargeexpanses.However,the swampy,somewhat
shorc-llkeconditions
of the far north
mostnearlyapproach,for the shorebirds,the expansive
character
so usual for water bodies and inland areas.
In the presentdiscussion
the emphasis
placeduponthe inland
vegetation
doesnot meanthat the dominance
of otherinfluences
is
not recognized,
but simplythat it anakes
a convenient
andfairlyreliableindexto •manyothere,•viron,nenlal
i•/tuer•ces,
as,for example,
the climateand topo•aphy. A further importantadvantageof
the plantindexis that the science
of plantecology
andmanyof its
generalprinciples
andmethods
are applicable
to birds. A general
knowledge
of plant ecologyis thereforebecomingoneof the most
valuabletoolsin the ha•ds of the field ornithologist.Every field
natm'alisthasobserve(]
the generalcorrelationof certainbirdswith
certainkindsof vegetation.This relationis clearlyexpressed
by
Rid•,•vay('89, p. $) asfollows:"There is probablyno betterindex
or key to the distribution
of birdsin any countrythanthat afforded
by the characterof the vegetation;shouldthis vary.essentially
withina givenarea,a corresponding
difference
in the bird-lifeis a
certainty." This phaseof the subjectclearlyillustratesthe oftrepeatedexperience
of naturaliststhat in orderto thoroughly
understandone subject--perhapsthe favoriteone--it becomes
necessary
to studyanother,or even several. Thais in order to
knowthe bird life of a regionit hasbecomenecessary
to studythe
ecological
relationsof its vegetation.
Vol.
xxv]/
1908
ADx•Is,Ecological
Succession
o[ Birds.
The studyof ecological
plant geographyis an extensiveone,but
manyof the details,soimportantto the botanist,are of muchless
concernto the ornithologist,
whoneedsprimarilyto knowthe rimjot
plant associations
or formationsand their successional
relations.
This impliesability to recognizedominanceamongplant species
and the generalmethodof transformnation
from the dominanceof
one to that of another.
By a plantformationis•neantthat association
of species
(orplant
society)which is correlatedwith thoseconditionswhich tend to
prew•ilover a large geographic
area in the laststagesof mutual
adjustmentof all environmental
and biotic processes.Such an
association
or formationtendsto occupysuchan areato the exclusionof all others,and is thus a elimaxsociety.
But absolute dominance of a formation does not occur,because
local conditionsbreak the lnonotonywherestrea•ns,water basins,
bare rock, and sbnilarinfluences
may interruptthe desert,grasslandor forest,andproducenilnorhabitatsand associations
of both
plantsand animals.
It is not my purposeto discuss
in detailthe variousplantformationsof (extra-tropical)North America,but to outlinethosewhich
are of evidentornithological
utility. The followingmay be recognized provisionall.y:-1.
2.
3.
4.
The
The
The
The
Arid Deserts of Southwestern U.S. and the Mexican
Grasslands of the Great Plains.
Deciduous Hardwood Forest of Southeastern U.S.
Coniferous Forests of Eastern Canada.
Plateau.
5. The Giant Conifer Forest of the Pacific Coast and the Rocky Mountains.
6.
The Barren
Grounds
or Cold Desert.
7. The Alpine Deserts.
A mereinspection
of thislist of avianandvegetational
formations
shows
thattherecognition
of theselargeenvironments
is relatively
simple. It is alsoseenthat they representfairly definitephysical
or environmental
complexes
of suchfundamentalimportancethat
therecanbe no doubtasto theirgeneralvalidity. As to therelative
value, influence,boundaries,and the dynamic relationsof these
formations,muchis alreadyknown,but not as an organizedbody
of factsandprlneiples.It will alsobe notedthat theseregionsdo
'[24
ADAMS,
Ecological
Succession
o[ Birds.
['Auk
[April
not closelycorrespond
with currentfaunal'areas,althoughthereis a
veryclose
correlation
in some
cases.An arianformation
may,in
generalterms,be considered
the analogucof a vegetational
formation, althoughthis doesnot imply that they necessarily
have the
same'boundaries.
As-the
literature
treating
ofthevegetation
ofthese
areasisextensiveandscattered,
a fewpaperswill be citedasan indexto others:-1. Arid Deserts; Bray, '06; Coville and MacDougal, '03.
2. Grasslandsor Plains; Clements, •05; Pound and Clements, '00.
3. SoutheasternH•rdwoods; Cowles,'01; Harper, '06; Transeau, '05.
4. Eastern Can•di:m Conifers; Whitford, '01; Transeau, '03, '05-'06;
Ganong, '03, '06; Harvey, '03.
5. Rocky Mountain and Pacific Conifers; Whirford, '05; Gray and
Hooker, '81; Piper, '06; Young, '07.
6. Alpine; Merriam, '90, '99; Coville, '93; Fern•ld, '07.
Theseenvironmental
unit areasas foundto-day,aretheresultof
many successions
which, in somecasesat least,reachrather far
back into the past. This is becausesomeoccupyancientland
areas,suchas much of the SoutheasternHardwood area. On the
otherhand, someoccupyrelativelynew regions,that is, at least
with regard to the dominantfactorsnow in control,as in the
glaciatedpart of North Americaand on the CoastalPlain. So far
as the presentis concernedsuchrelationsclearlyshowthat these
areasare onlythe end resultsof extensive
pastchanges
or successionswhich representthe terminalbranchesand crosssectionsof
development.It is to the studyof suchregionsand associations
that we must turn for the fundamentalorganizationor associariohal relationsof the variousclementswhich coinpose
not only
the cnviromnents but also the associations of animals.
In order to make as definiteas possiblethe structuraland ecologicalcharacteristics
of theseformations,
certaingeneralrelations
are hereformulated. Throughoutthispaperit shouldbe re•nemberedthat the individualbirdsandassociations
of givenareasfor•n
the unitsof comparison.Sucha distinctionis necessary
because
many speciesshow considerable
geographicvariationin habits
and in the habitatsfrequented. The writer clearlyrecognizes
the
risksand difficulties
of suchan attempt. They are deliberately
put in their presentform to in•'itecriticisra
andqualification
from
¾ol.
XXV]
1908
3
A•)A•s,Ecological
Succession
o[ B2rds.
125
fieldworkers. It is desirableto knowthe validityof theseformatlons,their internalecologicalrelationsand dynamictendencies,
their relation to dominant environmentalinfluences, etc. A com-
pletelist is not attempted,and someof the statements
may be only
fragmentsof largergeneralizations;
but it is just suchrelationsas
thesewhichwill developif the entiresubjectisconsidered
critically
and synthetically. Some of the leading characteristics
of these
larger cnviromncntalunits and their arian [ormat•onsmay be
briefly outlinedas follows:-1. The dominance
of a limitednumberof physicalconditions
or complexes,
as climate,topography,vegetation,animals,etc.,
in a given area producesthe primary environmental
unitsand
formations.
2. Secondary
environmental
dominance
is shownby a secondary
avian association.
Thus in the Northeastern
biotic center there is
a secondarydominancedue to water basinsin the forestarea.
3. A formationor climaxsocietyis composed
of a relatively
(andusuallyabsolutely)
limitednumberof species
whicharedominant in a givenenvironmentof geographic
extent. Such dominance,in general,impliesextensive
range,relativeabundance,
and
ability to indefinitelysucceedor perpetuateitself under given
conditions.
4. Where dominanceobtains,avian variety is limited so that
the greatestdiversityoccurswherelocalinfluences
prevail,and at
the marginsof the formation.
5. Correlatedenvironmentaland biotic dominanceproduces
what may be considered
a bioticbase,stratum,or optimum,from
which departuresmay be considered
lessfavorable. This is a
relativeequilibrium,resultingfrom completeenvironmental
and
bioticadjustment,
undergivenconditions.
6. In eachformationthereis a normalinter-adjustment
of the
arian speciesand individuals,in additionto the adjustmentwith
the dominantphysicalenvironment. The former is dominated
by their structure,habits, and instinctsor behavior; hencethe
colonialbreedingor spacing,migration,etc.
' 7. Eachlargeenvironmental
areaor formation
tendsto havea
full complement
or set of species,
of diversebut supplementary
ecological
character,suchaswater,shoreor inlandbirds. One set
is likely to be dominant.
126
A•x.us,Ecological
Succession
o] Birds.
I.April
8. Relative stability in an association
is correlatedwith the
climaxdominance,andgenerallywith extremeand slowlychanging
local influences.
Fluctuation
is correlated with intermediate con-
ditions.
9. Diversifiedassociations
and isolationare greatestwith imperfectdonfinance,but donfinanceitself producesisolationof the
climax association.This diversificationproduces associations
surrounded
by othersandhencetheirisolation.
10. The taxonomic
elements
in differentformations
varymuch,
but thereare closeanalogies
in the kindsof taxonomicandecological groupsin differentforenations,-asthe Mniotiltid•eof theNew
are represented
by the Sylviid•eof the Old World. C)•. Osborn,
'02. I,c Conte,'50, p. 239*. C]e.No. 7.
11. The roughly zonal arrangementof societiesabout the
climax society(= formation) or the environmentaloptimum,is
primarily due either to local reversals,the lagginginfluenceof
local or neutral conditions,or to the influenceof adjacentformations. This is a resultof tlm retardationof the completecycleof
successions.
12. The primary environmentalconditionstend to encroach
upon all others. The local conditionsthus tend to becometransformed
in the direction of the dominant
environment
and to be
appropriated
by it. The corresponding
avianassociations
are thus
givena definitedynamictrend.
13. The mobilityof birds during the breedingseasonis very
generallyoverestimated.The presenceof the nest and young
rendersthem for a time relativelysedentary.There are many
causesinfluencingthis, such as other individuals,proximity of
foodfor young,homing,instinctetc.
IV.
M•oR
Aw^•
E•WRO•ME•TS
.a•
ThEm
ASSO-
CIATIONS.
We haveseenthat the largergeographic
environments
or formationsare characterized
by definiteconditions
and associations,
and
at the sametime that eventhroughoutthesefavorableregionsthe
elinmx associationis not distributedwith absoluteuniformity
Vol.
XXV]
1908
3
A•)A•s,Ecological
Succession
o] Birds.
127
because
of loc•dvariationsin the physicalfeatures,suchas vegetation, water basins, streams,mountains,etc. For the student of
local bird life the real work beginswhen one attemptsto examine
into the canscsand influences
exertedby theseconditions
which
breakthe monotony
of the formationandmakepossible
a diversified avifauna. But birdsdonot alwaysrespondascloselyto sl;gbt
localinfluences
as doesthe vegetation,and for thisreasononemust
learnby experience
just what sizeof unitsmustbe used. Thus in
the foresta fewwind-falls
will attractbut little attention,
but•a
burn of a few acreswill have a noticeableinfluencein harboring
thosespecies
of birdswhichfrequentopenings;while swiftsand
swallows
ignoremanylocalinfluences
whichdominateotherspecies.
It shouldalsobe notedthat wheneverpossibleit is of distinct
advantageto examineall habitatsin their originalstate, unlnfiuencedby man.
Insteadof discussing
the leadingfeaturesof local conditions•nd
their societiesor associations
in detail, only an outline of them
will be given,andthat in a formto facilitateuseandrevision.
1. Minor environments
are primarily dependentnpon local
conditions,
and are thusin a sensecorrespondingly
independent
of
the dominantforcesof the region. This is, of course,a relative
condition.
2. Minor enviromncnts
are,asa rule,relativelylimitedin area.
In generaltheir limited area favorstheir short duration,but age
is pri•narilya resultof therateof change.
3.
Marked isolation, even when of extensivelinear extent,-
as a shoreline, along a stream,or an elongaterocky ridge,--is
al.socharacteristic
of •ninorenviromnents.
4. Minor enviromncnts
tend to beco•neencroached
uponby
the do•ninantregionalinfluences
and ulti•natclyto beco•neextinct.
The succession
of societiesin local habitats'is a decliningone,
while that of the geographic
or climaxhabitat is an increasing
andascending
one.
,
5. Localhabitatsproduce•nostof the varietywithinthe dmninant area, and •nakepossiblea diversifiedavifauna. The structuraldifferentiation
withina for•nation(zones,etc.)is thuslargely,
in additionto variationsin the formationitself,of localorigin.
6. Local associations
or societies,
in general,furnishthe essen-
128
AD^•s,Ecological
Succession
o] Bird,s.
tial clues as to the earlier successions which have attended
[•Xuk
lApril
the
evolutionor development
of regionaldominance.The variations
in theseare duebothto tile kind of life andto tile influenceof adjacent associations and centers of dolninance.
7. kVlargina]
societiesarc particularlyliable to variationin
coinposition,
dueto thecombined
influences
of adjacentforinations
or centers of dominance as well as to local conditions.
8. Colnparativestudiesof local habitatswill forin the most
generalandpracticalguidein the determination
of the successions
in the forination.
9. Local habitats and societies,in commonwith the larger
environinental
complexes,
are characterized
by the dominanceof
fewphysical
andbioticfactors,andby a limitednmnberof spccics.
¾.
Aye,iN
SUCCESSION.
1. GeneraIRemarks.Sincethe breedinggroundsare of fundainental iinportancein the ecologyof birds, the studyof them in
suchsituationsfurnishesthe greatestsourceof insightinto their
life relations. By an avian association,
formationor societyis
meant differentcombinations
of specieswhich •'egularlyoccur
togetherin the samebreedinghabitator area. These breeding
grounds
inustbe considered
broadly,andincludenotonlythenesting sitebut alsothe feedinggrounds,evcnwhentheyare physically
very different,becauseecologicallytheseconditionsform a unit
during the breedingseason.
It is well knownthat whena givensetof physicalconditionsare
doininant,as in a denseconiferforest,a s•vaInpor an extensive
orchard,relativelyfew individualsand kindsof breedingbirdsare
characteristic
of suchconditions,
exceptin the caseof thosenesting
in colonies.
The
field relations of these colonial and isolated
breeders
arcquitedifferent.It isalsoof importance
to recallthat
abundanceis a relativeterin, with a very differentmeaningin the
caseof secd-eating
and predaceous
species.
Bearingin mindtheseconditions,
bird succession
meansa change
from the dominanceof certainspeciesor associations
to that of
others. Thus in the beginninga slightchmlgein abundanceof a
Vol.
XXV]
i908
J
A•)x•s,Ecological
Successio*•
of Birds.
.
129
speciesmay be noted,•vith a corresponding
decrease
in another;
and this proportionmay continueto changeuntil the intruderbecomesdominantand the rival form •nay disappear entirely.
Thisprocess
of change,
asa rule,is notlimitedto a singlespecies,
but usuallyinvolvesseveralor all of the membersof the association,
aswhena duneinvadesa swampand the swampbirdsare completelyreplacedby thosefrequentingthe sanddunes.
2. S•ccession
onIsleRoyale. •¾iththcsepreliminaryconsiderationsin mind,we will turn to the ecological
succession
of bird life
uponIsle ttoyale,Lake Superior. The fieldworkuponthe island
was carried on by a party from the UniversityMuseum of the
Universityof Michigan,underthe directionof the writer. Aside
from snccession,
the generalecologicalrelationsof the birdswere
studiedby Otto McCrearyandMax M. Peet,andelsewhere
tictailed
descriptions
of the regionanddctailednoteswill be published. The
writer hasbasedhismainrecords
of habitatpreference
upontheir
work. k'or this outlineof succession
onlythe primaryfeaturesof
thelocationneedbe given.
In the presenttreatmentan attemptwill be •nadeto followthe
geneticsuccesslon•
at leastin its broaderoutlines. Variousqualificationsand reservations
have been •nadc,and othcrswill follow,
soit is hopedthat no confusion
will be produced
by thismethodof
treatment.
Geographically,
Isle 11oyalc,:Michigan,is an island in Lake
Superior,nearthe North Shore,not far fromPortArthur,Ontario.
The topography
formsa part of an ancicntpeneplainof moderate
relief, glaciatedand with an abundanceof elongatedlow ridges
andvallcyswithnumerous
waterbasins. The ,soil,whichislocally
absent,is generally
humicor mixedin character,
borderingandin
the depressions;
but is mineral, stony and residualelsewhere.
The combinedshoreand beachesare extensive,
largelystonyand
gravelly,and contain but little sand; much of the shoreline is
rocky and precipitous;many outlyingislands. Vegetation,herbaceousin shallowinland watersand as a groundcoverexcept
wherethe shadeis toodense,and uponrocks;shrubson protected
beaches,
in moreopenplacesin the forestand in burns; the forest
consists
of Tamarack,Black Spruceand Arbor Vitroin bogs; and
elsewherein mesophyticconditionsof BalsamFir, Arbor Vitro,
130
AD•s, Ecological
Succession
o[ Birds.
['Auk
LApril
White and Yellow Birch, and rarely Sugar5•laple. Upon the dry
ridges,JackPine; andin burnedareas,Aspenand Paper Birch.
Climate,seasonal
changes
verypronounced;wintersverylongand
cold, and summersshort and cool; a relativehumidityof about
80% in Decemberand of about70% in July (c]. Johnson,'07);
a meantemperature
for Januaryof 7.97¸ F.; andfor July,62.24¸
F. (Port Arthur data). Early, deepsnows. Predaceous
animals,
as the Lynx, 5Iarten,weasels,
Red Squirreland batsare directly
in competition
withthebirdsfor food,or preyuponthebirds.
The
above environmental
factors
are dominant
features
and
giveus a generalpictureof the conditions,
largelyin termsof common experience.In the life of the birds, however,a eomplete
reassortment
and changeof intensityin thesefactorsoccurswhen
theyarecombined
ashabitats. The surrounding
lake,thenumerousbays,smalllakesand pondscompose
the aquatichabitatand
makeit a characteristic
feature. The veryirregularand extensive
shore line and limited beach area characterize the coastal border,
while inland, exceptingthe main bodiesof the few larger lakes,
theencroachment
of thebogvegetation
u•)onthesilores
is suchas
to preventan extensive
development
of'sandyopenbeaches. The
abovementioned
habitatsareopentanforested
areas;theremainder
of the island,with the exceptions
of the bare rocky ridges,the
clearingsand burned over areas, are forested. Very extensive
swaanp
forestsaboundin the elongatevalleysand the bordersof the
water bodies,and are composed
of Tamarack, Black Spruceand
ArborVitro. The mesophytic
forestoccurson drainedareasandis
characterized
by BalsamFir, White Spruceand PaperBirch; the
burnedareasby second
growthsof aspensandPaperBirch. Then
there are also influenceswhich are exertedupon the bird life in
general,as for example,migration. In this case,undoubtedly
both external conditions and tile habits and the behavior must be
correlated. Another general and dominantinfluenceshouldbe
reiteratedhere,and that is that all openareastend to becomeinvadedwith vegetation
and finallyforested,whethertheyare lakes,
ponds,bogs,rockopenings
on the ridges,burnsor clearings.The
mesophytic
Balsam-spruce
foresttendsto monopolize
all habitats,
and givesa definiteness
to all succession
uponthe island.
Froana geneticstandpointthe pastandpresentdominance
of the
Vol.
XX'V1908
II
A•)A•IS,Ecological
Succession
of Birds.
131
surroundingLake must be recognized. This formerlystoodat a
level muchabovethat of the highestridgesuponthe island,as is
clearlyevidencedby the abandonedbeacheson the north shoreof
Lake Superior. Suchrelationsprovethat Isle Royalewas oncea
rockyreef in the lake, which,as the Lake levelwaslowered(it is
quiteunlikelythat the islandhasbeenmateriallyelevated)became
exposed
as a wave-washed
beach. Theseconditions
are approximated to-dayby the low outlyingislands. The beachor shoreis
thustheo•'iginalhabitatuponIsleRoyale,andin general,all others
havebeenderivedor developed
fromit. To discuss
theseasa truly
geneticserieswould requirethat thesebe described
s;raultaneously,
as the differentiation
took place. Thesehabitatsdid not develop
asisolatedphenomena,
but severaldeveloped
at the sametime,or
abreast. Thus as soonasenoughof the land surfacehad become
exposed
sothatitsinequalities
beganto havean influence,
theridges
wouldbe the partsbestdrained,andcertaindepressions
wouldtend
to accumulatethe drainage. This processwouldlead to a simultaneousdevelopmentor differentiationof the well, moderately,
and poorly drained habitats. Ahnost all of the residualsoil
formedasthe regionwasbaseleveled
wasprobablyclearedawayby
the glaciers;or later, as the wavesfell froln the island,by the
poundingof the waves. Thus the relativeabsenceof a soil must
characterizeall habitats. At what period life first reachedthe
islandin post-Glacialtime is not definitelyknown; but it is likely
that the pioneervegetation
of lichens,mosses
and low herbaceous
vegetationreachedit soonafter its exposure. If the biotareached
the islandaboutthe time of the formationof the Algonquinbeach,
which, roughlyspeaking,may have been at about the present
elevationof 475feetabovetheLake surface,it hassincespreadupward and downwardfrom that level. The colnpositionof the
initial societies is not liable to as much variation as the later ones.
Thusif theHerringGullsreturnedto theregionat thisearlyperiod
of the exposure,
theywereprobablythe pioneerbirds; but if only
at a muchlater date,still otherspecies
mighthaveaccompanied
theln. While such variationsas this may be expected,anddue
allowancemustbe madefor them,yet therecanbe little reasonable
doubtbut that waterbirdsandthosefrequentingopenplacestended
to become
thepioneers,
andthat late•,with thedevelopment
of a
soil and'forests, other associationsof birds became established.
132
Au.x•s,Ecological
Succession
o[ Birds.
[Auk
LApril
There are at least five importantfactorswhich enter into the
composition
of the pastandpresentconditions
whichhavemoulded
and are evennow mouldingthe formationof the habitatsupon
Isle Royale. Thesefiveare:-- first,pastclimaticchanges;second,
the localtopography;third,the fallinglake surface;[ourth,dynamic tendencyof the vegetation;andfifth, the habitsand structure
of the birds. With thcseguidingprinciples,let us now turn to
certain details of the resultant succession.
a. The AquaticAssociation
and Habitat.
The expanse
of Lake Superior,the irregularshorelineproducing
coves,the inland water bodiesand streams,togetherfumlsh an
extensive
andexpansive
areaof habitat. The cuttingof the Lake
wavesencroaches
uponthelandhanoitat,
andthedeposition
by them
elsewherecausesminor extensionsof the land habitat (as at Rock
Harborwherea sandspithn'nlshes
a nestingsitefor a Kingfisher).
Inland the encroachment
of the vegetationtends to restrictthe
water areas,as the falling Lake level has, in the past, tendedto
increasethe land habitat. Theseprocesses
mustbe recognized
in
orderto graspthedynamictendencies
of thehabitat.
The characteri,'tic
aquaticsocietyis composed
of the Herring
Gull, Loon, Americanand HoodedNlergansers,
and the Piedbilled Grebe; mainlyfish eatersand scavengers.Other species,
of greaterinlandtendencies,
are attractedby the fishfood,as the
Eagle,Ospreyand the Kingfisher.The Gulls showa decided
preference
for the greatLake, and the Loon for the inlandwaters.
The presence
of the Kingfisherwasinfluenced
by the harborwith
its attendant sand banks and bars.
As all these water bodies near
Isle Royale freezeover in winter, the strictlyaquaticbirds must
normallymigrateto securefood. Of coursenoneof thesebirds
nestin the openwaters,but ontheislandbeaches
(Gulls),nearthe
mouthsof streams,
andinlandin marshyplaces;but all, asa rule,
nestnear the water. The very youngsoonattendtheir parents,
and are thusin the water at an agewhcnmanyland birdsare yet
helpless
in thenest,thusconfirming
theiraquatichabitsandhabitat. During migrationsmany other speciesfrequentthis habitat.
WhereIsle Royalenow is, oncerolledthe openLake; and it is
Vol.
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ADA.•S,
Ecological
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o[ Birds.
,
133
not improbablethat as the islandappearedthe Herring Gull was.
oneof thefirstspecies
to discover
it. Sucha birdmightevenreach
the islandunderclimaticconditions
of the Ice Age,for the species
nowrangesfar north alongthe shoreof theArctic Sea. A species
of suchextensive
chronological
and geographical
rangewill tendto
givenmehstabilityto succession.The presentbreedingrangeof
theMergansers
andtheLoonisnot sofar north,andfor thisreason
they may have arrivedundermilder climaticconditions.But if
the islandbecameexposed
undermild post-Glacialconditions,
all
of thesespecies
mayhavearrivedat muchthesame'time. But even
with the chances
for suchvariationsthe generalsuccession
seems
to havebeeninitiatedwith the aquaticassociation
as the pioneer
society.
In followingthegenesis
of thehabitatsandassociations
fromthis.
point onward,divergence
and differentiation
becomes
so nmrked
that it is impossible
to developall linesabreast. A lineartreatment becomesnecessary,
and thereforecertaingeneralrelations
are liable to becomeobscuredunlessspecifically
mentionedin
advance.
The aquaticand beachhabitatspossess
a markedtendencytowarda zonalarrangement.•FromtheSuperiorbeachthetransition
isthroughopenor shrubzonesintothe climaxforest. The topographyof the islandwith its longitudinal
rktgesandvalleysforma
dominantfactorin impressing
thiszonalstructureupon the biotic
associations.This series,--from the water, throughthe beach,
openand shrubmarginalzone,into the climaxforest,--may be
consideredas the genetic vegetativesuccession.They change
simultaneously
and are dueto the samegeneralcause,--the falling
Lake surface, which transforms the water area into beach, the
beachinto forestmargin, and forestmargininto the climaxassociation. But as mentioned,it is manifestlyimpossible
to discuss.
all thesetransitions
at once,andeachecological
unit musttherefore
receiveseparategenetictreatment.
This tensionline or marginalzone betweenthe Lake and the
forestshowssucha wonderfuldiversityand complexityin its conditions,thatseveralplantandanimalassociations
areformedwithin
134
A•.•s, Ecological
Succession
o[ Birds.
['Auk
[April
thiszone. With its onwardmarchtherearesimultaneous
changes
in severalassociations
which,whiletheywill vary in their changes,
yet all tendto converge
in harmonywith the dominantfactors.
Theseconditions
migrateor radiatefromthe highestland. On the
other hand, the inland marginalzones,which borderthe smaller
water bodies,migrateinwardly; and being closedareas,tend to
becomeextinct. This marginal zone, particularlybeyond the
upperbeach,formsoneof the mostinterestingand complexconditionsfounduponthe island. It is not an ecological
unit, but is
composed
of severalof them. This iswheremostof the confusion
arises in actual field work of habitat studies.
b.
The Shore and Marsh
Association and Habitat.
As the area of the islandsexpandedand the shoreline was
lengthened,
the habitatfor shorebirdsincreased;but the steep
and rockyshoreswereunfavorable
for the development
of beaches
becauselooserock, as tools for the waves, was limited in amount.
The local characterof the shingleand gravelto-dayfoundin the
variouscovesclearlyindicatestheir local origin; and much the
sameconditions
have obtainedin the past. On accountof these
conditions,
thesandybeaches
are veryconspicuously
absent. The
dynamic
tendencies
ofthebeacharethose
whichcause
theextension
or restrictionof the aquaticand beachhabitats,supplemented
by
the drift which is tosseduponthe shore. Where there is shallow
water, and mud accumulates,favorableconditionsare furnished
for invertebrate
foodfor birds. Inland, thenumerous
lakes,ponds
and marshes furnish shore conditions which tend to become extinct
throughdrainageor overga'owth
of the vegetation,
exceptin those
partsof the largerlakeswherewaveactiontendsto scattersuch
accumulations
as rapidly as formed,or to preventits formation
altogether.
Althoughobservations
on this subjectare quite limited,yet it
seemsfairly safeto considerthe Spottedand SolitarySa•dplpers
ascharacteristic
birdsof thisassociation.Uponsucha rockycoast,
sandyandgravellybeaches
are quiteexceptional
andare confined
to protectedcoves. Additionaldiversityis producedwheresmall
streamsenter thesecovesand producedeltas.
Vol.
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ADAMS,
Ecological
Succession
o[ Birds.
135
Little isgainedby sharplysegregating
the marshandshorebirds,
althoughthe lnarshbirdsshowa preference
for comtitions
better
represented
or correlatedwith topographically
older coasts,protectedand inland conditions. Attention shouldbe directed, however, to the significantfact that successions
initiated with such
diversitywill producea variationin thecomposition
of theassociations. Alsothat sofar as possible
thesevariationsshouldbe con-
sideredcomparatively
and syntheticallyin reconstructing
and
anticipatingsuccessions.
The AmericanBittern, LesserYellow-legs,SwampSpan•ow
and Marsh Itawk belongto thissocietyof marshbirds. As in the
caseof the aquaticassociation,
thesebirds generallynestin close
proximityorentirelywithintheseshoreor marshconditions.Still
otherspecies
frequentthisbelt to feed,as it is an openarea; but
their presenceis mainly conditionedby the adjacentshrubsor
forest. The very limited numberof speciesin the aquaticand
shoreassociations
is worthyof particularmention.
The Yellow-legs,SpottedSandpiper,Bittern and Marsh Hawk
rangefar to thenorth,evento theBarrenGrounds,andthussuggest
chances,
asin thecaseof theaquaticassociation,
of an earlyanSval
and succession
uponthe island.
Withthegrowthof theisland,
therehasbeena c•rresponding
extensionof the outer and inner shore habitats, althoughthe
encroaching
vegetation
hashad a markedtendencyto restrictthe
area of the inland habitat.
The dominant environmental influences
in thishabitatappearto be, 1, the physicalcharacterof the shore
andbeaches;2, thedynamicforcesof thewaterbodiesandstreams;
3, the cncroachment
of the vegetation;4, the downwardmigration
of the shore; and 5, the habits and structureof the birds.
As a generalrule, we may saythat the beachof the outerlake
tendsto be succeeded
by eitherthe bogor tfptandassociations,
and
thoseinlandby the bogassociation.
c. Bog~]orest
Associationa,d Habitat.
As just statedthe outercoastor an inlandonemaydevelopinto
a marshor boghabitator association.In thebog,theTamarack,
BlackSpruceandArborVittearethepioneertreesin transforming
136
A.A•s, Ecological
Succession
o] Birds.
['Auk
LApri!
the openmarshinto a forested
one; whileupontheoutershorethe
aldersand aspenstend to precedethe conifersas a generalrule.
From the bog forestthe transitionto the Balsam-WhiteSpruce
forestmay be perfectlycontinuous,
and thustherewill be a series
characterized
by the dominantconifers. In placesArbor Vitro
may form the dominantswampforest,but this is only a variation
in the coniferdominance. ]¾ith improveddrainageor the accumulationof vegetabledebris,thesehabitatsbecomeconvertedinto
the Balsam-spruce
climax forest and hencethe environmental
dynamictendency.
As the forest encroaches
upon the open bogsthe Tamarack,
Black Spruce,Arbor Vit•e, Cassandra,Labrador Tea and alders
are accompanied
by birds characteristic
of this early stage; such
as the Red-breasted
Nuthatch,Yellow-belliedFlycatcher,Goldencrowned Kinglet, Cedar Waxwing, Chickadee,Canada Jay,
White-wingedCrossbill. Where aldersaboundthe conditionsare
favorablefor the liedstart and the White-throatedSparrow. But
later, as the bogconiferforestbecomescontinuousand dominant,
the Waxwing,Redstartand White-throatedSparrowsdiminishin
numbersandfinallydisappear.Still later,asthe swampbecomes
eliminatedwith the development
of the clbnaxforest,the YellowbellledFlycatcherwill alsobecomeexcluded.
This is perhapsthe simplestsuccession
from the water to the
elimax forest,via the bog forest. This seriesis very perfectly
preserved
in all stagesand hasan extensive
range. The number
of speciesin the association
is rather large when comparedwith
the precedipg
associations.
d. Aspen-birch
ztssociation
and Habitat.
This seriesdevelopsfroln the beachas the wavesfall from the
ridgesor low rock stirfacesand leavethe bare expanses.As the
rock disintegrates,
decomposes,
and humusaccumulates,
a soil is
formed,•nainlyin depressions
or at thebasesof theridges,andfrom
theseit tendsto encroach
liponthe openplaceswith a zoneof Jack
Pine, aspens,or White Birches. These areasare largelystrips
along the crestsof ridgesor sinall park-like openingson rather
level rock. In no caseare thesesingleareas large, so that the
¾ol.
XXV]J
1908
AD•s, Ecological
Succession
o[ Birds.
137
habitatis onlyextensive
in theaggregate.With thepresence
of the
openaspenand birch woods,the followingsocietyis likely to be
characteristic:-Junto, Oven Bird, lied-eyed Vireo, Chipping
Sparrow,•¾hite-throated
Sparrow,Flicker, CedarWaxwing,Wilson's Thrush and the Chickadee.
As the deciduous trees are re-
placedby the openencroaching
coniferforest,the SongSparrow,
the Nashville, Myrtle and Black-throatedGreen Warblers and
Wilson'sand Olive-backedThrushes,which frequentthe forest
lnargins,increase
in abundance.The Own Bird hasan extensive
northernrangefromLabradorintotheYukonValleyandmaywell
havebeena veryearlypioneeruponthe islandas the aspensand
bircheswere probablythe first broad-leaved
tree arrivals. From
the aboveit is seenthat thismeansan extensive
variety,but asthe
dominanceof the climax forest encroaches
this number again
becomes reduced.
The con/position
of thesocietyvariessomewhat,
depending
upon
the surroundings,
as proximityof the presentshoreor distance
from it. Many of theseopenings
are continuous
with the present
beach. It is not improbablethat this was a prominentsociety
wheneverthe watersfell rapidly from the islandbetweenrather
.stationary
levels. This hasbeena societydecidedlyon the decline
with the encroachment
of the forest.
Probablythis association
variesconsiderably
in its composition,
andhasdonesoin thepast; but itsmainfeaturesarefairlyconstant.
Thesevariations
seemlikely,throughthe influence
of openings
produeed by fires which, when extensive,may have causeda new
equilibriumamongthosespecies
frequentingopenings.
The B•rned
.4tea Association.
This phaseshouldperhapsbeconsidered
assupplementary
to the
•spen-birchassociation
just considered.A fire brings about a
reversalof conditionsthroughthe destruction
of the forest,and in
somecases,a part of the soil aswell. As thereare all degrees
of
extentand completeness
in this process,
thereis a corresponding
variation in the detailsof the resultingsuccession,
at leastin its
earlystages. It is onlywhenthereis a verycomplete
destruction
of the vegetationthat the continuitywith former occupancy
is
wholly broken.
138
ADx•s,Ecological
Succession
o/ Birds.
[Auk
LAp ri!
The easilyinflammable
characterof theseconifers,evenwhenin
a greencondition,
makesit likelythatnaturalcauses,
suchas lightning or marshgas(c[. Penhallow,'07), may havebeeninfluential.
The proximityof the gassupplyand the conifersis of interestas
this may influencetheir liability to fire and thus to this sort of
reversalof conditions.Thus liabilityto firesis rathercharacteristicof the region,and rnan'sinfluencehastendedrnerelyto reinforceratherthanto introducethisfeature. Thus it seems
probable
thatfireshavebeena factorin supplementing
thenaturalpark-like
openings. In additionto theburnedareasfounduponIsle Royale,
otherlimitedopenareasaredueto cultivation
andarekeptopen.
The birdscharacteristic
of themoreopensituations
aretheSharptailedGrouse,SongandChippingSparrows,
Flicker,andthePurple
Finch. The Grouse is a Plains forrn, is near its eastern limit,
and is perhapsa late arrivaluponthe island. The otherspecies
are •viderangingin the Canadianconiferous
forestsbut arenot of
suchnorthernrangeas the aquaticand shoreassociations.There
is nothingin their rangeto suggesttheir arrival earlierthan the
forestassociation.Taking all the birdsof the openings
together,
it is not irnprobable
that they arrivedat aboutthe sametime as
those of the forests,but frequenteddifferentsituations,--the
forestkindsoccupying
the slopesand driervalleys,m•dthe others.
the openings.
e.
The Climax
Association
or Formation
and Habitat.
The climaxassociation
shouldnot be considered
in sucha way
as to lead one to think that it is di,'ti•t
frorn the other associations.
It belongsto all of them as the end of their seriesunderexisting'
biotic and environmental
conditions. Thus the aquaticassociation, throughthe bog conifers,is transforlned
into tile Balsarnspruceassociation;
and from the beachthroughthe aspen-birch
association
againto the balsamsand spruces.The climaxassociation is the conditionof adjustmenttowardwhich all societies.
move under the presentconditions.For this reasonthe earlier
stages,conditions
andassociations
of the climaxhavebeenoutlined
in the precedingdiscussion.
In the dominantforestthe denseshadepreventsan extensive
Vol.
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]
A•)AX•S,
Ecological
Succession
o[ Birds.
139
groundcoverof herbaceous
plants;andalthoughGroundHemlock
is abundantlocally,yet in placesthe forestflooris quiteopenand
free from lower shrubgrowth. The remarkablepreservation
of
trailsor roadsthroughsuchtractsshowsclearlyhowslowlychanges
take place. Such a habitat must be relativelyequablein its
temperature
andmoisturerelations.
Geographically
speaking,
theprimarycharacteristic
of theclimax
is its relativestability,due to a dominance
or relativeequilibrium
producedby the severeenvironmental
and biotic selectionand
adjustment
throughout
the process
of succession.
At thispointattentionshouldbe calledto thefactthatdominance
is a resultantof an equilibriumproducedby neutralizingor overcomingotherforcesand influences.We may thinkof the process
of succession
asa streamof forceswhosedevelopment
maybe comparedwith the transformation
of a drainageline,--such as, for
example,that of a rivuletinto a creek,andtheninto a river. The
streamand the characterof the groundmutuallyinfluenceeach
other and the coursefollowedis a resultantof the mutual adjustments. The streamis deflected
by oneconditionandthenanother,
just as succession
varieswith localconditions;yet the water continuesto run downgradeand seeksan equilibrium,and similarly,
bioticsuccession
continues
on its coursedeflectedhereandthereby
local influences,yct forevertendingtowar.
d a stateof biotic equilibrium. The donfinance
of the climaxsocietyor formation,consideredas a process
ratherthan a product,hasmuchin it that is
analogous
to thedominance
produced
by theprocess
of baseleveling.
The characteristicbirds of the climax forestare:-- the Chickadee,
Golden-crowned
Kinglet, Red-breastedNuthatch, Canada Jay,
Downy,Hairy, ArcticThree-toedand PileatedWoodpeckers,
and
the White-winged
Crossbill.Here againthe association
becomes
smallin varietyof species
and comparable
with the smallsociety
whichmusthavebeenassociated
with the complete
dominance
of
the Lake waters. Thus therehasbeena development
of diversity
fromsimplicity,with later a returnto simplicity. To thesebirds
of the forestshouldalsobe addedthosespecies
of generaldistribution, as the Eagle, Swift, Swallows,etc., a classof birds whose
predaceous,
insect-feeding
and wide ranginghabitsmake them
particularlydifficultto properlyassociate.A carefulstudy of
140
A•)•Ms,Ecological
Succession
o/ Birds.
[Auk
[April
this classof birdswill be necessary
beforetheycanbe satisfactorily
•correlated
with theirproperavianassociations.
But lct usnot overlookthe fact that eventhis dominanceis only
relative,for sincethe Ice Ageeventhisentire[ormationhasmigrated
northward,and a true succession
hasbeenproducedwith its attendant changes
in the conditions
and in the composition
of the associations. Just as uponIsle Royalea definitedynamictrendwas
givento the completeenvironment
by the fallingLake surface,so
in the post-Glacialnorthwardmigrationthere was a northward
migratingclimate. Theseconditions
determined
that on the north
side of this immensesuccession
or migrationhabitatsand asso•ciationswere developedwhich are comparableto thoseattending
the downwardmarchof theIsle ]Royalebeach; and evento-day,by
passingfrom Isle Royaleto the tree limit with its zoneof aspens
.andbirches,one may find representatives
of the variouskindsof
associations
whichin all probabilitymovednorth,just as to-dayin
passing
fromthe forestto the rockybeachbalsamsand spruceare
encountered
beforethe aspensand birch. If however,this is only
another case of convergenceand not at bottom the sa•ne or a
comparableprocess,
we are then certainlyfar from an understandingof eventhegeneralnatureof theproblem.
3. InternalFactors. With the ideaof succession,
asexemplified
by Isle Royale, let us turn to other factorswhich influencethe
internal relationsof the birds within an association
or society,
becausesuchrelationsare alsonecessary
to an intelligentunderstandingof succession.Someof thesegeneralrelationshave been
outlined, but certain others are needed which have been well
expressed
by Brewster('0•, p. •2-•3): "Many if not mostbirds
showa markcdpreference
for breedingin certainregions,
throughout which they are •noreor lessevenlyand generallydistributed,
but within which their numbersdo not seemto increasebeyond
fixed maximumlimits no matterhow (.arefullythe birds may be
protected
or howsuccessful
theymaybe in rearingtheiryoung....
I have observed--as, indeed,who has not!--that few birds-exceptingthosewhich, like Swallows,Terns, Herons,and Gulls,
are accustomed
to nestin colonies
-- tolerateverynear neighbors
of their own species
duringthe seasonof reproduction.At its
beginning
eachpair takespossession
of a definitetractof wood-
Vol.
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AD•ax•s,
Ecological
Succession
o/ Birds.
141
land, orchard,swampor meadow,whichthe •naleis everon the
alertto defendagainsttrespassers
of hisownkind andsex,although
he oftenseemsquitewillingto sharehis domainwith birdsof other
and perhapscloselyrelatedspecies. The extentof the area thus
monopolizedvaries exceedinglywith birds of different species.
An appleorchardwhichaffordssufficient
roomfor -- let us say-two pairs of Yellow Warblers,two pairs of Orioles,three or four
pairsof Chippiesand four or fivepairsof Robins,seldomor never
harbors
morethana single
pairof•ingbirds
orCrested
Flycatchers
.... As a rule, the species
whichroamoverthe •nostgroundin the
courseof their daily wanderingsclai•nand maintain the broadest
preserves,
whilethoseof sedentary
habitsoftencontentthe•nselves
with very modestfreeholds. Whateverthe extentof the domain,
the birdswho occupyit as a summerhomeevidentlyregard it as
exclusivelytheir own. The readinessand celeritywith which
trespassing
birds are accustomed
to retire when attackedor even
merelythreatenedby the established
tenants,has seemedto me to
indicatethat the claims of temporaryownershipare respected
by all right-mindedbirds.... In my opinionthe desirefor exclusive
possession
so conspicuously
shownby the •nale,and oftenby him
alone,is usuallythe directresultof sexualjealousy. This, as is
natural, •nakeshim intolerant,duringthe breedingseason,of the
nearpresence
of rival males. If his concernwerechieflyin respect
to the foodsupply,it wouldbe equallymanifestedat everyseason
and towards all birds who subsist on the same food that he and his
materequire- whichis certainlynot the case."
The tendencyor pairs and speciesto ,spacethemseh,
es and to
becomerelativelysedentary
is thus a characteristic
conditionin an
association,
and is an importantclementin an understanding
of
succession
becauseit showsthe internal organizationand habit
with which an invaderor pioneerfrom anotherassociation
has to
contend. As Dixon ('97, p. 91) haspointedout, this spacing
tendencyis an i•nportantfactorin the extension
of rangeof species
and
is inti•natelyrelatedto the locationof nestingsites. These facts
clearlyshowthat both theseinternalinfluencesand the environmentalonesmustbe distinguished
if we wishto determinethe relative influenceof eachand their bearingon succession.The above
quotationfrom Brewsterclearlyshowsthat in generalnot only a
142
AD^•Is,Ecological
Succession
o[ Birds.
[Auk
LApril
greaternumberof birdscanlivein a givenarea,but alsothat they
can live closertogether,if they vary in kind. Then again,within
the association
therearc markeddifferences
in habitatpreference.
Thus in the forest there are those birds which nest in the trunks
or amongthe topmostbranchesof the trees,or even upon the
ground; and theseare differences
largelydistinctfromthe spacing
of the pairsof the samespecies.Theseinfluences
mustbe recognized amongthe dominantinfluences
within the association,
and
uponwhichmuch
emphasis
mus]c
beplaced.
4.
Environmet•tal Factors.
Then in addition to these internal
factors,therearethe dominantphysicalfactors. In the following
discussion
primary emphasiswill be placedupon succession
as
found in the NortheasternBiotic or Conifer Center,becausesucces-
sionsat othercenterswith differentbioticcomponents
and other
dominantphysicalconditionsmust possess
a certainamountof
individuality,in additionto thosefeaturescommonto succession
in
general. The dmninantbiotictendencyor dynamictrend of this
center,as a resultanto[ all itdernaland environmental
influences,
is for the coniferbioticassociation
to encroach
uponall othersocietiesandhabitats
andtobecome
thedomitmt•t
oruniversally
distributed
association.
Thus,in general,
all habitats
.produced
by localinfluences tend to become transformed into the dominant biotic
association
or formation. In generalalso, small bodiesof water
arerapidlyencroached
uponby inwash,vegetation
or drainage,and
tend to tiecomeextinctand forested. All otheropenings,
as the
rocky ledgesand ridgesor burns,are encroached
upon as soil
accumulates
or firesareprevented,andthe forestbioticassociation
spreadsoverthe entirearea.
From suchrelationsit will be seenthat our knowledgeof the
causesand conditionsof succession
mustlargelyresultfrom the
studyof theselocalenvironments
or habitatsand their bioticsuccession, because,where dominanceis establishedthe successionis
almostcompletely
obliterated.Eachminorhabitatandsociety
is
to belooked
uponas dinply a stage,moreor lesstemporary,
in the
onward wave toward the dominant or elimax association. Thus in
themarshes,
birchor aspen
woods,
rockopenings
andpondsmaybe
"orginal"conditions
whicharebecoming
cumulatively
transformed
in thedirectionof thefinaldominance
of the climaxbiotictype.
Vol.
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1908
J
ADA•S,Ecological
Succession
o/ Birds.
143
The relativelyslowrateof changein manyenvironmental
processesand the relativestabilityof the climax biota,• is doubtless
the basisfor the currentview that suchconditions
arc rclatively
constantor fixed; but that changeand not constancy
is the normal
and usual conditionin nature is quite evidentupon a moment's
reflection. Ahnosteveryonenoticesthesechanges
afteran absence
of a few yearsfrom a region. Thus intimacytendsto blind us to
changes
unlessa habitof givingattentionto themis deliberately
cultivated. For thisreasonsomefindit ahnostimpossible
to recognize environmental
changesor to comprehend
their significance.
It is thereforeof practicalvalue to clearlyrecognize
underwhat
conditionschangesmay be mostreadilyperceived. Thereforethe
importanceof the studyof localinfiue•cesis emphasizcd,
and the
necessity
recognized
of distinguishing
the dominance
of geographic
and relativelystableconditionsor formationsas contrastedwith
thosedueto localandoftenrelativelychangeable
conditions. Then
amongthesechangeswe must distinguish
thosewhich are mere
fluctuations
andthose
whichareindicative
of thetrueprogresslye
succession.This is mainlyaccomplished
by attentionto general
relations and the subordination of minor details.
5. Environmental
and Associational
Converger•ce.
At thepres-
ent'imperfect
stageof ecological
development,
comparison
must
furnishus the most importantand generalcluesto the processes
of succession;
and undoubtedlythis methodmust long remainas
our main guideon accountof its comprehensive
application
and
the magnitudeof the problemto be solved. It is thereforedesirable that the limitationsof the methodshouldbe deafly bornein
mind. It is oftenasstuned
that the impliedsuccessions
of a given
placeare thesameasthosewhichhave developed
at that placein
the evolutionof thepresent
clinmx. Butaswepositively
knowthat
mar•ydifferentcausesare able to producethe sameor very similar
results,such conclusions
must be receivedwith due caution. That
thedominant
geographic
conditions
tendto override
localinfluences
seemsvery fairly established
becausediverselocalor originalcondi-
tionsaretransformed
i, totheclimaxordomi,•ant
type. This clearly
showsthat in timediverse
localinfluences
haveflowedintothegeneral environmental
trend or currentand havebecomea part of it.
For the migrations of climax societies, el. Adams, '05.
144
ADx•s, Ecological
Succession
o] Birds.
[Auk
]_April
There is thus a very strongconvergent
tendency. By convergence
is meantthe independent
productionof the samekind of association
from diversestartingpointsor habitatsand associations.Quite
minorecological
unitsmay showsimilarbut temporaryconvergent
tendencies
in their succession.
It is thereforenot surprising
that
any marked emqronmentaldominancewill tend to producesimilar or convergentresults,even in local areas. Under such circumstances
similarassociations
or societies
may be independently
and repeatedlyformedby the selectingenvirm•mentalinfluences,
suchas, for example,are foundin the numeroussmalllakesseatteredthroughoutthe coniferous
forests. This convergent
phenomenon is certainlya fertile sourceof confusionthroughoutall phases
of science. Perhapsthe bestguidethroughsucha labyrinthwill
be to deafly bear in mind the relativevalueof generaland local
influences,
and watchwith an "eternalvigilance"for convergent
resultsdue to diversecauses. This convergentphenomenon
is
particularlyliableto occurin the easeof en•Sronments
producedby
reversiblephysicalconditions.It shouldfurtherbe statedthat a
studyof theseproblemsfrom a geneticand dynamicpointof view
will aid in recognizingsuchresults. Under suchcircumstances
attentionis primarily directedtoward the dominant causesand
conditionsof changerather than to the stages,products,and
resultsproducedby them. Convergence
thus viewedis the result
of severalcausesand shouldbe considered
a productrather than
a process. This samedistinctionmay be made for all societies,
associations
and formations. Convergent
phenomena
are thusparticularly liable to con]use
whereverproductsrather ttmn genetic
processes
receive
primaryemphasis.
6.
Succession and Environmental
Evolution.
The
relation
of
succession
to generalbiologicalproblemsis very intimate. This
opensup a veryextensive
fieldwhichis onlymentionedto indicate
its generalrelation to succession.The facts of succession
and
evolutionmust ever remainfar in advanceof our knowledgeof
their causes. If, however,one turns to the standardevolutionary
treatises and searches for a discussion of the evolution of the environ-
ment, as correlatedwith animal evolution,only the mostgeneral,
or the elementary
and superficial
phases,are as a rule discussed.
To be sure,certainpapersand treatisestake up specialphasesof
Vol.
XXV'I
1908
3
ADA•xS,
Ecological
Succession
o/ Birds.
145
the problem,andthe broadestphasesaretreatedby the geologists;
but noneof themseemadequateasa comprehensive
treatmentof so
importanta subject. Succession,
broadlyand geneticallyconsidered(dynamicratherthanstatic),is a phaseof environmental
evolution.
7. The Relationo[ Succession
to OrganicEvolution. Mention
has been made of the relation of succession to environmental
evolu-
tion, but its relationto the organicevolutionof birdsshouldalsobe
indicated. The mutual relationsof organicand environmental
evolutionhave been and will continueto be the battlegroundof
biologicalthoughtfor an indefinitelengthof time. Here lies the
tensionline betweenthe two main schools
of biologicalinterpretation.
One schoolmaintains that all causesof evolution are internal,
and that the environment
is only a condition,
not a cause. From
this point of view the fundamentalcausesare internaland therefore environmentalconditionscan only indirectlyinfluenceevolution throughthe weedingout of thoseformsnot in harmonywith
the conditions;and henceit has a selective
ratherthan an originativeinfluence. From thispoint of viewsuccession
and environmentalevolutioncan contributenothingto the elucidationof the
causes
of organicevolution,
thoughtheymay to an understanding
of the selectionproducedby the succession
of conditionsin which
organicevolutionhastakenand is takingplace. In harmonywith
this point of view, succession,
broadlytreated, shouldfurnish a
fundamentalmethodof treatmentfor the processof selection,and
the detailedprinciplesof itsworking. This wouldcertainlybe an
importantadvancebecause
naturalselection
has frequentlybeen
reproachedfor its indefinitemethodsand lack of definite treatment. Succession
fromthispointof viewisprimarilyrelatedto the
Darwinianfactorsof evolution. No doubtthis is onereasonwhy
Dal•vin himselfput suchhigh valueuponthe studyof ecological
relationsof animals,i.e., their relationto their completeenvironment,or theirstrugglefor existence.
If, however,all causesare internal and not directlysubjectto
external influences,they nmst be beyondexperimentationto a
corresponding
degree. Under suchconditions
evolutionbecomes
a descriptive
ratherthan a causalscience,and all that investigation
146
Ama.•s,Ecological
Succession
o] Birds.
[Auk
LApri!
can do is to describethe succession
of formsproducedby these
internal
causes.
On the other hand the rival school maintains that both internal
and externalconditions
may be real causesof organicevolution.
This isthoughtto bebroughtaboutbythedirectorindirectinfluence
of theenvironment
uponthegermcells,by environmental
selection,
or evenby bothcombined. Frmnsucha pointof viewtheenvironmeritmaythusbe eithera cause
or a condition
of organicevolution,
or both. Fromsucha standpoint
the evolution
of theenvironment
receivesincreasedimportance,
as undersuchconditions
organic
and cnviromncntalevolutionare causallyrelated,and thus intimatelycorrelated. Viewedthus,environmental
evolutionis more
than the description
of the succession
of conditions,
but may be
explanatoryaswell.
The particularlysignificant
feattireis that environmental
evolution and biotic succession
are of great value and can contribute
eitherto the causesor conditions,
or to both, of evolutionary
advancement.
VI.
SoME ADVANTAGES OF A ]KNowLEDGE
OF THE LAWS OF
SUCCESSION.
The studyof succession
impliesa detailedknowledgeof the field
relations of birds, and as this has received so little attention as a
subjectof specialstudy,it isperhaps
worthwhileto brieflymention
stoneofthepracticalandscientific
advantages
whichwemayreasonablyexpectwill resultfromthedevelopment
of thisphaseof investigation.
The currentdiscussions
of environments
are generallyveryfragmentaryand chaotic,and the carefillstudyof bird habitatsand
succession
will greatlyilnprovethis phaseof ecology.Here is a
fieldof studyin needof distinctrecognition
asa subjectworthyof
detailedinvestigation,
in additionto thoselines alreadycurrent.
Whenoncethis fieldis developed,
thenand onlythenwill it be
possible
to intelligently
discuss
theevolution
of arianenvironments
and to correlate them with the evolution of birds themselves. It is
quiteprobablethat oneof the main conditions
whichpreventsa
Vol.
XXVq
190S
J
A•,•s, Ecological
Succession
o] Birds.
147
morerapid advancealongevolutionary
linesis in a largemeasure
dueto the almostutterfailureto analyzedynamically
enviromnental complexes.Succession,
studiedin its broaderaspects,
should
greatlyaid in the for•nulation
of the lawsgoverningthe "struggle
for existence,"which is frequentlycondemnedfor its indefinite
character.
From anotherpointof viewthereareveryimportantreasons
for
urgingextensive
studiesof this ehm'acter
at a relativelyem'ly(late,
because
the encroachments
of civilization,
whichby the destruction
of the forests,the drainageof the land, irrigation,farnfingand
grazingof the grasslands,
are rapidlydestroying
originalenviron•nentalconditions
beforethey are studiedecologically.Much of'
Europehas alreadygonethroughthis stageof de•nolition,and it
is only to new and relativelyunlnodifiedcountriesthat we can look
for an adequatestatementof these problemsand their relations.
in their originaland primarilyevolutionaryand developmental
forin. It is not improbablethat the next generationnmy wonder
why somesubjects,the investigation
of which might have been
delayed,havereceiveddetailedattention,while othersequallyor
perhapsevenmoreimportanthavebeenalmostignoredand nmst
foreverremainunknownbecause
of thisneglectto sccurethe "vanishingdata." (C[. Haddon, '03.)
Suchecological
studiesmaybe expected
to }lavea valuablereflex
influenceupon the naturalisthimself. Wc may hope that the
futurerevisorof a groupo['birdswill considera knowledge
of the
field relationsof his specimens
as an essentialqualification,
just
asat the presenttime a largeseriesof specimens
is heldnecessary.
Fifty yearsagoa limitedserieswasconsidered
no disqualification,
just as to-daytile lack of a knowledge
of their ecological
relations
is not so considered.Perhapsour ideasof relativevaluesmust
change. In thisconnection
a statement
fromTristra•n('94,p. 472)
is to the point:-"The closetsystematist
is very apt to overlook
or take no count of habits, voice, modification and other features
of life which have an importantbearingon the modificationof
species.To take one instance,the short-toedlark (Cala•drella
bracltydactyla)
is spreadoverthe countries
borderingon the Mediterranean;but alongwith it, in Andulnsiaaloneis foundanother
species,
Cal. bactida,of a rather darkercolor,andwith the second-
148
A•)A•lS,Ecological
Succession
o[ Birds.
[.auk
[April
ariesgenerallysomewhat
shorter. Withoutfurtherknowledgethan
that obtainedfrom a comparison
of skins,it mightbe put downas
an accidentalvariety. But the field naturalistsoonrecognizes
it
as a most distinct species. It has a differentvoice,a differently
shapednest; and, while the comnlonspecies
breedsin the plains,
this one alwaysresortsto the hills. The Spanishshepherds
on
the spot recognizetheir distinctness,
and have a name for each
species."
Many examples
of similarcharacter
mightbe citedto showthe
scientificvalue of a knowledgeof the environmental
relationsof
birds, and a moment'sreflectionwill show that the problemof
succession
is onlya smallpart of tile generalproblemof environmental relationsof plants and animals. Attention has already
been directedto the relationwhich this generalsubjectbearsto
evolutionaryproblems.
It is not at all unlikelythat succession
is very closelyrelatedto
someof the causes
of bird migration,and that with advancein this
subjectmuchlightwouldbethrownuponnfigration. Migrationis
doubtlessanother illustrationof convergentphenomena. In all
probability,
migrationhasoriginated
notonlyindependently
in very
diverse kinds of birds, but perhapsrepeatedly,from different
causes,evenin the samegroup. The causesof nfigrationmustbe
nmnerous,varyingwith differentecological
groups,whichappear
to be the true natural unitsfor studyand comparison.Thus the
comparative
studyof migrationsof differentkindsof associations,
as formationsand societies,
shouldleadnot onlyto a betterunderstandingof the variousassociations,
but shouldalsocontributeto
the generalsubjectof migrationwhich seemsto have showna
tendencytoward stability in the current methodsof study. It
.scarcely
seemsprobablethat with the diverseformations
inhabited
by birds,and with their ecological
diversities
thereshouldbe only
a few causesof the phenomena.
To keeppacewith successions
anhnalsmusteitheradjustthemselves,changetheir habitat,or migrate. From suchrelationsit is
evidentthat varioussupposed
environmental
responses
nmstbe
tested
primarilywithin liteassociation
andenvironment
towhichthe
animal normallybelongs.To this classbelongsprotectivecoloration and allied phenomena.To be of fimdamentalvalue, the
Vol.
XXV-I
1908
/
AD•s, Ecological
Succession
of Birds.
149
influencemusthavesomepermanence
and this may be soughtin
the dyrmmictrend and dominantinfluencesof differentassociations.
It is diffleult to eoneelve of other more reliable tacthods of
approachto suchproblelns.
In additionto the scientificvalueof this line of investigation,
there are importanteconomicapplicationsof the laws of avian
environment.This is particularlyh'ueof forestryand agriculture.
The forestryproblemis continually
becoming
moreimportant,but
the relation of bird life to forests and forest succession has received
little attention. As agentsfor scatteringseedsof treesand shrubs,
birdsareveryimportant. Here iswherethe interests
of the arian
ecologistand forest ecologistoverlap. The studentof bird life
xvillwishto knowhow a regionis to be reforested,
andwiNatsuccession of bird life will attend the successionof the forcst as reforesta-
tion progresses.On the otherhand,the foresterwill wishto know
bow birds will aid or retard him in the processof reforestation.
Then, in guardingor protectingthe forest,what helt) can be securedfrom birdswith regardto insectpests? Theseare only
samplesto shoxv
that here is a fieldwhich,as time advances,
will
becomeof more and more importance,and that theseproblems
will eventuallycall for speciallytrainedmen to handlethem.
In connection
with forestryand agriculturewe have quite exceptionalconditionsfor extendedexperimentalstudiesin bird
succession
as relatedto forestsuccession,
croprotation,etc. The
relationof birdsto agricultureappealsto a nmchlargernmnbcrof
peoplethan doestheir relationto forestry. There are severalreasonsfor this; first,becausemorepersonsare interestedin farm and
horticulturalcropsthan in forests;and second,because
birdsare
soonattractedin suchlargenumbersby the foodsupplyof grains
and fruitswhichthesecropssogreatlyincrease,
that the extensive
destruction
by birdsreadilyattractsattention. And whilewe hear
lnuchof the greatreductionof certainspecies
of birdsin partsof
the country,it is not at all improbablethat with the destruction
of
theforests(whichweredenseand dominantandtendedto limit the
abundanceof •nanyspecies
frequcntlngthe open),and the increase
of food in cultivated fields, there has been an increasein the total
numberof birds,evenin spiteof the grcatnumbers
killedby man.
But to the phaseof succession
with whichwe are primarily
150
ADAMS,
Ecological
Succession
o/ Birds.
[Auk
[_April
concerned,ahnost no attention has been given, in spite of its
fundamentalrelationto croprotationand the corresponding
avian
succession
attendingthis. Indeedthereseemsto be a very decided need of a thorough investigationand discussionof the
general principlesunderlyingall theseeconomicproblems,that
they may be broughtinto harmonywith the advancesmade in
someotherphasesof ecology.
Hull ZoologicalLaboratory, University of Chicago.
October 21, 1907.
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THE
BIRD
COLONIES
BY
WILLIAM
OF
LEON
THE
OLYMPIADES.
DAWSON.
TlaE recentcreationby executiveorder of three reservesamong
the islandswhichlie off the westcoastof Washingtonhas served
to callattentionfor the firsttime,in an ornithological
way at least,
to thishithertolittle-knowncoast. The reasonsfor previousneglect are not far to seek. No really safe harborsoffer,for even
the smallestcraft,betweenthemouthof Gray'sHarbor,Lat. 46ø 56'
N., and the Straitsof Juande Fuca,Lat. 48ø 24' N. The prevailing windsarewesterly,and the aspectof the coastsomenacing,as
viewedfromthe seawardside,that marinershavealwaysgivenit a
wideberth,savewhenseizedby somehapless
mischance
of reckoning or tackle.
No commercialexploitationof the tributarycountryhas been
attempted,beyondthe preemptingof timber lands sometwenty
yearsago, and the occupationby hardy settlersof a few small
prairiesand rich alluvial bottoms. Only recentlya spur of the
NorthernPacificRailwayhasbeenbuiltfromHoquiam,on Gray's
Harbor,to Mocllps,onthecoast;and'thislatterpointmarksnearly
the northernextensionof a splendidsea-beach,
hard as macadam,
whichstretches
south,practicallyto the ColumbiaRiver. To the
north of Moclipsthe beginningof the roughway is markedby
Point Grenville, and the oceandrive becomesa tradition.