THE
IMPACT
AT
OF
SOME
FRUITING
NORTH
TREES
AMERICAN
IN
MIGRANTS
PANAMA
CHARLES F. LEC•<
A•ONG the major seasonalphenomenapotentially affecting neotropical
birds is the winter influx of North Americanmigrants. The subjecthas
been much neglected, except for brief general discussions(Morel and
Bourliere, 1962; Miller, 1963) or studiesrestricted to specializedgroups
(e.g. Willis, 1966). To gain a new quantified estimation of the effect
of migrants I studied their use of local food sources,especiallyfruiting
trees, relative to use by residents.
STUDY AREAS AND METIIODS
During March and April 1968, I studied bird use of certain fruits in a highland
semi-agricultural area of western Panama (82 ø W, 8 ø N), near E1 Hato (4,000
feet elevation) and Cerro Punta (6,000 feet elevation). Data included the numbers
of feeding visits by each speciesof migrant and resident, and banding population
estimates of all speciesexploiting a particular resource.
From September 1968 through April 1969, I gathered similar data at fruiting
and flowering plants in the Panama Canal Zone, particularly at a clearing surrounded
by a mature lowland rain forest on Barro Colorado Island (79 ø W, 9 ø N).
Monthly sightings of all migrants were tabulated and the numbers of common
species (including residents) estimated for the study area through color-banding.
During the fall wet season I compared the effects of various rain intensities on
the foraging of migrants and residents.
Specific names are from Eisenmann (1955).
RESULTS
Barro Colorado Island and the Canal Zone.--Figure 1 showsthe occurrenceof the commonermigrants at the clearing on Barro Colorado
Island. Octoberand early November formed the peak of migrant activity,
with the numbersof individualshigher during the fall than in the spring
(Table 1). Willis (1966) similarly found migration in the forest of
the island to be greatestduring the fall and this appearsto be true through
most of Panama (Galindo and Mendez, 1965; Loftin, pers. comm.). We
may then predict any competitiveimportanceof m/grants to be greatest
in the fall.
Populationestimatesof winteringmigrantsin the clearingwere similar
(Table 2) to those of Moynihan (1962). For two migrants (Summer
Tanager and Bay-breastedWarbler) all individualsseemedto be banded.
For several residentsand two migrants (Red-eyed Vireo and Yellow
Warbler) with no bandings, the populations were estimated from the
maximum numbersof individuals seen at any one time.
842
The Auk, 89: 842-850. October 1972
October 1972]
Migrants in Panama
OCT.
EASTERN
NO•
DEC.
JAN.
843
FEB.
MAR.
KINGBIRD
OLIVE-S.
N
--
67
FLYCATCHER
5
SWAINSON•S THRUSH
WOOD THRUSH
RED-EYED
VIREO
TENNESSEE
YELLOW
36
WARBLER
56
WARBLER
56
CHESTNUT- S. WARBLER
BAY-BREAST.
81
WARBLER
35
N. WATERTHRUSH
KENTUCKY
CANADA
50
WARBLER
9
WARBLER
BALTIMORE
9
ORIOLE
SUMMER
TANAGER
SCARLET
TANAGER
ROSE-BR.
GROSBEAK
19
89
7
23
TOTAL
572
Figure 1. Occurrence of migrants on Barro Colorado Island, at the clearing
study area from 21 September 1968 to 13 April 1969. N:total
number of
sightings(each sighting equals one day per individual bird). Solid lines • periods
of regular occurrence,dots m periods of irregular occurrenceand individual records.
TABLE
1
COMMON TRANSIENT MIGRANTS ]DURING FALL AND SPRING J/q TltE BARRO COLORADO
ISLAND CLEARING
1
Fall (1968)
migration
Species
Eastern Kingbird
Olive-sided Flycatcher
Oct.
Spring (1969)
migration
Nov.
4
2
1
Swainsoh's Thrush
21
1
Red-eyed Vireo
Feb.
23
5
Tennessee Warbler
8
14
13
Bay-breastedWarbler
8
11
5
Kentucky Warbler
3
2
3
Canada Warbler
Baltimore
Oriole
5
5
2
1
3
19
24
17
SummerTanager
Scarlet Tanager
Rose-breasted
Subtotals
GRAND TOTALS
Grosbeak
6
1
16
6
1
68
42
120
188
Mar.
15
57
• Data gatheredas in Figure 1, with approximately 100 hours in the field for each month. The
numbersrepresentthe total numbersof sightings(each sighting equalsone day per individual bird).
844
CI-IARLES
F. L•cx
TABLE
[Auk, Vol. 89
2
POPULATIONESTIIVfATES
FOR CO1VF2ViON
TANAGERS•
HONEYCREEPERS•
AND WARBLERS
AT BARROCOLORADO
ISLANDx
Moynihan's
1959
estimates
2
2
10
2
4
4
4
2
4
Species
Fulvous-vented Euphonia
Plain-colored Tanager
Golden-maskedTanager
Blue-gray Tanager
Palm Tanager
Crimson-backedTanager
Summer Tanager
Bananaquit
Winter 1968-69a
8
10
3
3
4
1
4
3
(6)
(6)
(1)
(2)
(1)
(4)
( 1)
74
Blue Dacnis
8
-
6
5
?
?
Red-legged Honeycreeper
Green Honeycreeper
Red-eyed Vireo
TennesseeWarbler
5
2
3
3
(2)
2
Yellow Warbler
3
-
?
?
Chestnut-sidedWarbler
Bay-breastedWarbler
3
2
(1)
(2)
?
NorthernWaterthrush
2
(1)
• Basedon daily observations
of the speciesand notes on the approximateproportionsof each
speciesthat were banded. North American migrantsare in italics.
SMoynihan (1962: 15) made theseestimateson a subjectivebasisafter severalweeksof daily
censusingand observationin the clearing, from 26 October to 20 December 1959.
a Numbers of color bandingsin parentheses.
4 ?: no data given.
Many of the migrant specieswere primarily insectivorous.Even the
commonfrugivorousSummerTanagersspent much time in flycatching
for insects.Thus in 100 hoursof censusing
at a Cecropiatree in fall,
three residenttanagers(Palm, Blue-gray,and Plain-colored)made 585
fruit-feeding
visits,versus10 from the SummerTanagers.Similarlyat
Oryctanthus
plantsthe mostfrequentmigrant,the Red-eyed
Vireo,made
only 49 of the 797 berry-feeding
visitsrecordedduringthe fall. At all
other studyplants (more than 50 species)on the island,the migrants
contributedeven lessthan at the Cecropiaand Oryctanthus.Hence at
thislowlandlocalityit appears
that migrantsare not likely to compete
seriously
withtheresidents
for fruit resources,
especially
duringthe spring
dry season
whenfruit is superabundant
(Leck, 1970, 1972).
Elsewhere
in the Canal Zone migrantflycatchers(Fork-tailedFlycatcher,Easternand Gray Kingbirds) fed with numerousresidentsat
Ficus and Roystoneatrees (Leck, 1969), but the fruit productionexceededexploitationin every month. In fact, migrantsoften congregated
about the unnaturalfood surplusesof many ornamentaltrees and shrubs.
Even then, migrantsonly comprised12 percent (maximum) of all the
individualsfeedingat specifictrees (e.g. ornamentalFicus in Balboa).
October 1972]
Migrants in Panama
TABLE
845
3
EXPLOITATION
O1*A FIG TREE (FICuS TURCKHEIMII) IN PANAMA
•
Species
As
B•
C•
D*
17
13
7
17.5
1.8
3.0
Robin
10
1.75
•Elaenia sp.
Clay-colored Robin
Red-crowned Woodpecker
12
6
7
1.2
2.0
1.25
297.5
23.4
21.0
17.5
14.4
12.0
8.75
63
5
4
4
3
3
2
RESIDENTS
Sulphur-winged Parakeet
Flame-colored Tanager
Long-tailed Silky Flycatcher
Mountain
Yellow-throated
White-throated
Brush-Finch
Robin
8
6
1.0
1.0
Blue-gray Tanager
3
2.0
Streaked Saltator
5
1.0
Silver-throated Tanager
Acorn Woodpecker
2
3
2.0
1.0
Emerald
Toucanet
2
1.0
SwainsoWs Thrush
Baltimore
Oriole
8
6
2.7
2.3
Scarlet Tanager
Summer Tanager
2
2
3.0
1.5
8.0
6.0
6.0
2
1
1
5.O
4.0
3.0
2.0
1
1
0.5
-
MIGRANTS
21.6
13.8
6.O
3.0
5
3
1
0.5
472.95
100%
•Near E1 Hato deI Volcan, northwest Panama, 16 March-22 April 1968. Observation periods:
16 March, 07:00-10:00; 27 March, 07:00-10:00; 1 April, 10:30-11:30; 4 ApriI, 14:0018:00; 15 April, 11:00-14:00; and 22 April, 07:00-10:00. For each hour recordswere maintained
of all speciesfeeding on the fruits at the tree.
eNumber of 1-hour censusperiods specieswere recorded (total n •-18).
a Average maximum number of individuals hourly.
• Product
of A and B.
• Percentage of total usage.
El Hato and Cerro Punta.---At higher elevationsthe importanceof
migrantsis reportedto be much greater (Willis, 1966), but no such
effect was noted at a fig tree at 4,000 feet elevationin westernPanama,
where migrantscontributedonly about 10 percentof the total exploitation (Table 3 and Figure 2). In Costa Rica at 4,000 feet elevation,
Hespenheide
(in litt.) alsofoundmigrantsratherunimportantat several
Cecropia trees in March. But at 6,000 feet elevation near Cerro Punta,
bandingshowedSwainson'sThrushesrepresenting45 percent (n-----110)
of all the individuals (n= 245) eating berries at Leandra shrubs by
April (Leck and Hilty, 1968, and Figure 2). The thrusheswere not so
plentiful in winter as during this migration month.
In each case (Ficus, Cecropia, and Leandra), no competition was
apparent between the residentsand migrants as the fruits were superabundant (moderate to heavy fruit drop, few fruit stalks eaten, and
many overripe berries, respectively). Thus while limited to the spring,
the data again suggestthat migrantsare not important competitors.
846
C•R•s
F. L•c•:
[Auk, Vol. 89
65%
ISULPHUR•
Figure 2. The exploitation of (A) a fig tree (Ficua turckheimii) and (B)
melastome shrubs (Leandra subseriata) in the highlands of western Panama. The
large Ficus was near E1 Hato, N. W. Panama; elevation 4,000 feet. The figure is
based on the Exploitation Index of Table 3, representing 16 March-22 April 1968.
The Leandra shrubs were near Cerro Punta, N. W. Panama; elevation 6,000 feet
in early April 1968. The figure is based on the number of individuals of each
species,as determined through extensive banding.
Foragingduringrainy periods.--I studiedthe foragingbehaviorof lowland migrants and residentsto determineif migrants foraged more than
residentsin inclement weather. Birds under greater feeding pressuresof
the wet seasonwouldbe expectedto continueforagingeven in unfavorable
rainy periods.
Activity of birds at the Barro Colorado Island clearing dependedon
the intensity of precipitation (Table 4). With a light rain many species
continued feeding or foraging, though at a much reduced rate. With
moderate rains most of the residents decreasedactivity further, while
the migrants regularly became more active. In the heavy rains both
residents and migrants suspendedactivity and sought shelter. The
advantagesor requirementsinvolved in the rain feedingmust be important
for the maintenanceenergyduring suchforagingis considerable.Migrants
(Eastern Wood Pewee, Chestnut-sidedWarbler, Summer Tanager) frequently flicked wings and tail and body-shaked. In moderate rains the
wing-flickingrate was as high as 30-35 per minute,and yet birdswould
get conspicuously
wet, contributingfurther energylosses.
Social behavior o/ migrants.--Within the migrant passerinesof the
lowlandsonly isolatedindividualsor intraspecificassociations
were regularly observed.These associations
includedmale-femalepair bonds (for
someSummerTanagers,BaltimoreOrioles,Rose-breasted
Grosbeaks)and
intraspecificflocks (for certain icteridsand fringillids). The passerines
that formed intraspecificfeeding flocks were often associatedwith concentrated local food sourcesin man-disturbed areas. These foods included
October 1972]
Migrants in Panama
TABLE
847
4
INDIVIDUAL FORAGING OR FEEDING RECORDS ON BARRO COLORADODURING RAIN •
Light'ø
(5 hours)
Moderate
(3• hours)
Heavy
(lI• hours)
RESIDENTS
Orange-chinnedParakeet
Collared Aracari
2
7
Tropical Kingbird
Social Flycatcher
Short-crested Flycatcher
Green Honeycreeper
Fulvous-vented Euphonia
Plain-colored Tanager
Golden-masked Tanager
Blue-Gray Tanager
Palm Tanager
Crimson-backed Tanager
1
1
2
4
26
1
3
9
1
TOTALS
53
1
1
4
3
4
8
25
MIGRANTS
Olive-sided Flycatcher
1
Eastern Wood Pewee
Tennessee Warbler
Yellow Warbler
Chestnut-sided Warbler
7
1
2
8
2
5
Bay-breasted Warbler
2
1
Northern
1
Waterthrush
Summer Tanager
Rose-breasted
1
2
4
14
27
Grosbeak
TOTALS
4
x All food sources (mostly October and November 1968). A 2 X 2 Chl-square test of residents
and migrantsin light and moderaterains showeda highly significant differencebetween the activity
of the two avifaunas (p (0.001).
.oDefinition of intensitiesof rainfall: Light, a drizzle or fine rain, does not require one to seek
shelter; moderate, requires one to seek shelter or to wear rain gear; heavy, a torrential downpour,
usually of short duration.
cultivated fruiting and flowering trees for icterids and grain fields for
fringillids. At such superabundantresourcesthe competition, intraspecificallyor with residents,was negligible.
Migrants also formed interspecificassociations
with the mixed flocks
of residents.In mostcases,the migrantswereattractedby the residents;
the migrantswere the active joiners. Moynihan (1962) made the same
observationbut did not directly discussthe basisof this one-way relationshipbetweenmigrantsand residents.As the joining behaviorcauses
migrants and residentsto share foods, competitionmight result when
resourcesare scarce. In aggressiveencountersat food sources,migrants
were usually subordinateto residents(Leck, 1972) through dominance
hierarchies
basedon sizewith the largerspecies
dominant(Leck, 1970;
848
CttARLESF. L•CX
[Auk, Vol. 89
Wolf, 1970). Since migrantsare often small (e.g. Parulidae) or belong
to families that have larger representatives
in the tropics (e.g. orioles
and thrushes),they are more likely to loseaggressive
encounters.Their
small biomasswould also reducetheir competitiveimpact.
Territorialbehaviorwasnotedin only two migrants,the Northernand
LouisianaWaterthrushes,which defend individual interspecificfeeding
areas along small montane streams. Where allopatric they also exhibit
territorial tendencies(Eaton, 1953; Schwartz,1964). The territoriality
may be necessary
partly becausefew streamsare availablein the wintering
areas.
DISCUSSION
In Panamathe impactof migrantsincreases
at 6,000 feet elevation.
Thisincrease
hasbeensubjectively
appreciated
by previous
workers,
with
severalsuggestions
of its significance.
Willis (1966) notedthat in the
man-disturbed
habitatssuchas the highlandcoffeefincas the resident
birdshave"practicallydisappeared,"
leavinguntilledareasfor migrants.
In thelast 50 years,agriculture
in the highlands
hashad manydetrimental
effectsonmanynativebirdsandmammals(Bennett,1968),but migrants
might benefit from the resources
availablein the open habitats (Slud,
1960: 144). Widely-spacednatural habitat patchesof highlandsmight
alsoprovideareastoo smallto be filled by residents,
and the mobile
migrantscouldselectsuchplaceswhere residentcompetitorsare fewer
thanin a largecontinuous
habitat,suchas a lowlandrainforest.Highlands
may alsobe attractivebecausethey are morelike the breedinghabitats
of the migrants(Brosset,1968). Workersin other tropicalregionsalso
describethe migrantpreferencefor higherelevations(e.g. in Thailand,
B. King, pets. comm.) and concomitantlytheir apparent avoidanceof
maturelowlandforests,as in Malaysia (Ward, 1969).
A reviewof the data suggests
that the migrantsgenerallydo not
competestrongly with residentsin Panama, at least within natural
habitats.Thesedata include: (1) the comparatively
low fruit exploitation ratesof migrantsin the clearingon Barro ColoradoIsland, (2) the
preferenceof frugivorousmigrantsfor areas of food superabundance,
eitherin man-disturbed
areasor in thehighlands,
and (3) thelowpositions
of migrantsin dominancehierarchies.The impactof migrantexploitation
might sometimes
be importantduringmarked food shortages,
as in the
wet season(Leck, 1970) or duringpeak migrationperiods. Differences
in migrant and residentfeeding efficienciesmay also be reflected in
the dichotomyof rain foragingin the wet season.More efficient residents
maybe ableto avoidforagingduringthe rainyspells,whilethe migrants,
October 1972]
Migrants in Panama
849
beinglessefficientand/orof reduced
weightsfrommigration,are obliged
to continuefeedingthroughthe day, evenduringunfavorable
weather.
Low efficiencycouldresultfromproblems
in foodrecognition,
especially
for young migrants.
Considering
the foodrecognition
problems
of migrants,their partial
dependence
on residents,
and their low positionin dominance
hierarchies,
it is likely that they, ratherthan nativespecies,
wouldsufferduringany
foodshortage
competition.
In recentyearsthe migrantsare moreable
to avoidsuchcompetition
by exploitingthe superabundance
of fruit in
man-disturbed
areas.
ACKZVOW•EDO•EZVTS
This article is based on part of doctoral dissertationprepared at Cornell
University under the chairmanshipof Stephen T. Emlen. At Barro Colorado
Island I was associatedwith the SmithsonianTropical Research Institute, with
financialsupportfrom CornellUniversity.Transportation
costswerekindly covered
by the Public Health Service and a Sigma Xi Grant-In-Aid of Research.
SUMMARY
The relative populationsizes and exploitationlevels of frugivorous
migrantwerestudiedduringseveralseasons
in Panamaand the Canal
Zone. In the lowlandsmigrantswere responsible
for lessthan 10 percent
of the total avian-fruit utilization. Their importance increasedto 45
percentat 6,000 feet elevation.In both regionsmigrantsappearedto
frequentsuchareasof fruit superabundance
as man-disturbed
areasand
natural habitat patches. During rainy periodsmigrants remainedmore
active in foragingthan residents.
LITERATURE
BEZVZVETT,
C.F.
CITED
1968. Human influenceson the zoogeographyof Panama. Ibero-
Americana, 51: 1-112.
BROSSET,
A. 1968. Localisationecologiquedes Oiseauxmigrateurs dans la foret
equatoriale du Gabon. Biol. Gabonica, 4:
211-216.
EATON,S.W. 1953. Wood warblerswinteringin Cuba. Wilson Bull., 65: 169-175.
EISEIVlVrAIVlV,
E. 1955. The speciesof Middle American birds. Trans. Linnacan
Soc. New York, vol. 7.
GAnI•DO,P., A•D E. M•D•Z.
1965. Banding thrushesand catbirdsat Almirante,
Panama. Secondyear of observations.Bird-Banding, 36: 233-239.
L•cx, C. F. 1969. Palmae: hic et ubique. Principes, 13: 80.
L•cx, C. F. 1970. The seasonalecologyof fruit and nectar eating birds in
lower Middle America. Unpublished Ph.D. dissertation,Ithaca, New York,
Cornell
Univ.
LEcx, C. F.
1972. Seasonalchangesin feeding pressuresof fruit and nectar
eating birds in the neotropics. Condor, 74: 54-60.
LECX,C. F., A•D S. HI•T¾. 1968. A feedingcongregation
of local and migratory
birds in the mountains of Panama.
Bird-Banding,
34:
318.
850
C•ARLESF. LEC•:
[Auk, Vol. 89
M•LLER, A. H. 1963. Seasonal activity and ecology of the avifauna of an
American equatorial cloud forest. Univ. California Publ. Zool., 66: 1-78.
MOREL,G., ANDF. BOURLIERE. 1962. Relations ecologiquesdes avifaunas sedentaire
et migratrice dans une savane saheliennedu bas Senegal. La Terre et la Vie,
4:
371-393.
MOYN•aAN, M. 1962. The organization and probable evolution of some mixed
speciesflocks of neotropical birds. Smithsonian Misc. Coil., 143.
SCaWARTZ,P. 1964. The northern waterthrush in Venezuela. Living Bird, 3:
169-184.
SL•rD,P. 1960. The birds of finca "La Selva", Costa Rica: a tropical wet forest
locality. Bull. Amer. Mus. Nat. Hist., 121.
WARD,P. 1969. The annual cycle of the Yellow-vented Bulbul Pycnonotus goiavier
in a humid equatorial environment. J. Zool., 157: 25-45.
W•LL•S, E. O. 1966. The role of migrant birds at swarms of army ants. Living
Bird, 5: 187-231.
WOLF, L. L. 1970. The impact of seasonal flowering on the biology of some
tropical hummingbirds, Condor, 72: 1-14.
Sectionof Neurobiologyand Behavior, Cornell University, Ithaca, New
York. Presentaddress: Departmentof Zoology,Rutgers University,New
Brunswick, New Jersey 08903. Accepted 14 October 1971.
APPENDIX:
SCIENTIFIC
I•AMES
Sulphur-winged Parakeet (Pyrrhura hoffmannO
OF THE
SPECIES
MENTIONED
Blue Dacnis (Dacnls cayana)
Orange-chinnedParakeet (Brotogerisjugularis)
Bananaquit (Coerebafiaveola)
Emerald Toucanet (Aulacorhynchusprasinus)
TennesseeWarbler (Vermivora peregrina)
Collared Aracari (Pteroglossus torquatus)
Yellow Warbler (Dendroica aestiva)
Acorn Woodpecker(Melanerpes formlcivorus)
Chestnut-sidedWarbler (Dendro•ca pensylvanica)
Red-crownedWoodpecker(Centurusrubrlcapillus)
Bay-breastedWarbler (Dendroica castanea)
Fork-tailed Flycatcher (Muscivora tyrannus)
Northern Waterthrush (Sdurus noveboracensis)
Eastern Kingbird (Tyrannus tyrannus)
LouisianaWaterthrush (Seiurus motac•lla)
TropicaI Kingbird (Tyrannus melancholicus)
Kentucky Warbler (Oporornis•ormosus)
Gray Kingbird (Tyrannus dominicensls)
CanadaWarbler (Wilsonla canadensls)
Short-crestedFlycatcher (Myiarchus ferox)
BaRimore Oriole (Icterus galbula)
Social Flycatcher (Myiozetetes simills)
Fulvous-ventedEuphonia (Tanagra •ulv•crissa)
Olive-sidedFlycatcher (Nuttallornis borealis)
Plain-coloredTanager (Tangara inornata)
Eastern Wood Pewee (Contopus virens)
Golden-masked Tanager (Tangara larvata)
Mountain Elaenia (Elaenia frantzii)
Silver-throatedTanager (Tangara •cterocephala)
White-throated Robin (Turdus assimilis)
Blue-gray Tanager (Thraupis episcopus)
Clay-colored Robin (Turdus grayi)
Palm Tanager (Thraupis palmarum)
Mountain Robin (Turdus plebejus)
Crimson-backed Tanager
Wood Thrush (Hylocichla mustelina)
(Rampttocelus dimidiatus)
Swainson'sThrush (Hylocichla ustulata)
Summer Tanager ( Piranga rubra)
Long-tailed Silky-flycatcher (Ptilogonys caudatus)
Scarlet Tanager (Piranga olivacea)
Red-eyed Vireo (Vireo olivaceus)
Flame-coloredTanager (Piranga bidentata)
Green Honeycreeper (Chlorophanes spiza)
Streaked Saltator (Saltator albicollis)
Red-leggedHoneycreeper (Cyanerpcs cyaneus)
Rose-breasted
Grosbeak(Pheucticusludovlcianus)
Yellow-throatedBrush-Finch (Ariaperesgutturalis)