J. Field Ornithol., 62(2):195-199
BIRD
DENSITY
AND
PRODUCTIVITY
CATTAIL
MARSH
WILLIAM
IN
AN
IMPOUNDED
POST AND CAROL A. SEALS
The Charleston
Museum
350 Meeting Street
Charleston, South Carolina 29,103 USA
Abstract.--In an impoundedSouthCarolina cattail (Typha spp.) marshduring 1986-1988,
averagebird populationdensitywas 16.7 nestsper ha. Productivitywas 3.8 kg per ha. BoattailedGrackles(Quiscalus
major)represented
71% of the population,and accounted
for 60%
of the productivity.Population density was about the same as that reported for cattail
marshesin northeasternNorth America, and approachedthat reported for a northeastern
salt marsh.Populationdensityand productivityvariedlittle betweenyears,and variation
was relatedmainly to changesin water level.With low water levels,nestsbecameaccessible
to groundpredators.However,productivityappearedto belittle affectedby predationbecause
Boat-tailed
Grackles
that lost nests renested within
DENSIDAD
Y PRODUCTIVIDAD
TYPHA
SPP.
DE
AVES
the marsh.
EN UN
ANEGADO
DE
Sinopsis.--Durante un estudioque se11ev6a caboentre 1986-1988 en un anegadode eneas
(Typhaspp.)en Carolinadel Sur, se encontr6una densidadpoblacionalpromediode 16.7
nidosde aves/hectfirea.
La productividad
result6ser de 3.8 kg/hectfirea.Quiscalus
major
represent6
el 71% de la poblaci6nde avesy rue responsable
del 60% de la productividad.
La densidadpoblacionalrue similar a la informadapara anegados
de eneasen la parte
norestede Norte Amgricay se aproxim6bastantea la informadapara anegados
salobres
del noreste.La densidadpoblacionaly productividadvariaron muy pocoentre aftos;la
variaci6nserelacion6a cambiosen losnivelesde agua.La mayorlade losnidossehicieron
accesibles
a depredadores
cuandohubo nivelesbajosde agua. Sin embargo,la depredaci6n
afect6muy pocola productividad,
debidoa que los individuosde Q. majorque perdieron
suscamadasreanidaronen el anegado.
Althoughcattail (Typha spp.)marshesand wetland impoundments
are
widespreadon the coastalplain of southeasternNorth America, there are
few data on the populationdensityor productivityof birds nestingin
these habitats in this region. Impoundmentsand similar low-salinity
marshesare often managed for ducks and geese,few speciesof which
breed in the Southeast.Becauseof their importanceto waterfowl, impounded wetlands have been the focusof extensiveresearch (see review
by Fredericksonand Taylor 1982). Other than severaldescriptivestudies
from North Carolina (Lewis 1970, Weaver 1975), little attention has
beenpaid to speciesthat nestin thesehabitats.The ecologicaleffectsof
impoundingsalt marshesare not well known. One step toward making
meaningfulassessments
would be to measurethe densityand productivity
of birds nestingin impoundedand open marshes.
STUDY
SITE
AND
METHODS
We conductedthe study at Magnolia Gardens,CharlestonCounty,
South Carolina, from 1986-1988. The 13-ha studysite forms part of a
30.5-ha marsh. The area was createdin 1954 by the impoundmentof a
195
196]
W.PostandC.A.Seals
J.Field
Ornithol.
Spring 1991
TABLE
1.
Bird populationdensitiesin an impoundedcattail marsh.
Number
of nests
3-yr mean
(no. per ha)
Species
1986
1987
1988
Pied-billed Grebe
Least Bittern
Green-backed Heron
Common Moorhen
Red-winged Blackbird
Boat-tailed Grackle
1
37
1
7
10
177
0
42
0
14
16
141
1
34
1
14
12
141
0.7 (0.1)
37.7 (2.9)
0.7 (0.1)
11.7 (0.9)
12.7 (1.0)
153.0 (11.8)
233 (17.9)
213 (16.4)
203 (15.6)
216.5 (16.7)
Total (# per ha)
salt marsh peninsulaon the Ashley River. The outer part of the marsh
is surroundedby a channel(_<3 m deep)that was excavatedduring diking.
Away from this channel,water depthin open-waterareasaveragesabout
1 m. Open water covers75% of the site, and cattails (Typha ar•gustifolia
and T. domir•ger•sis)
coverthe remaining 25%. Averagevegetationheight
in early July was 124 + 42.4 (SD) cm. Extent of vegetationand water
coverwas determinedby measuringaerial photographs.
Vegetationheight
was determinedby tallying contactsthat vegetationmade with eachdm
of a slendermetal rod placedvertically(r• = 100 points).
We establisheda 25 x 25 m grid with numberedwoodenstakes,placed
over the vegetatedsectionsof the marsh. The censuswas based on the
number of nestsfound. Nestswere found by searchingall cattail stands
as the observerpoleda boat. Once nestswere located,the worker walked
through the cattails and searchedthe area for additional nests.We also
locatednestsby following birds as we watched them from a distance
through binocularsor a telescope.We marked all nestswith numbered
plasticflagging.In all 3 yr, the site was visitedan averageof five times
per week, from 1 April to 1 July.
Productivityestimatesare basedon biomassof unsuccessful
eggsand
young, as well as the number of young successfully
reared in the area
per year. For Boat-tailed Grackles (Quiscalusmajor) and Red-winged
Blackbirds (Agelaiusphoerdceus),the estimatesare based on the total
numberof youngfledged,multipliedby the averagebiomassof fledglings.
Fledglingweight was usedbecauseyoungof thesespeciesleft the marsh
shortly after leaving the nest. For the other species,Common Moorhen
(Gallir•ulachloropus),Least Bittern (Ixobrychus
exilis)and Green-backed
Heron (Butoridesstriatus),biomasswas basedon adult weight because
thesespeciesremainedin the studyarea after leavingthe nest.The biomass
of blackbird,grackleand bittern youngthat did not fledgewas calculated
as F5the fledglingweight.
RESULTS
For the six specieslisted in Table 1, we found 16.7 nestsper ha. If
only vegetatedportionsof the marsh are considered,
the densityper unit
[197
Densityand Productivityin a CattailMarsh
Vol. 62, No. 2
T•,BLE
2.
Avian productivity(kg/ha) in an impoundedcattail marsh.
1986
1987
1988
Least Bittern a
Green-backed
Heron b
Common Moorhen c
Species
0.592
0
0.502
0.566
0
0.891
0.939
0.047
0.915
0.699
0.016
0.769
Red-wingedBlackbirdd
0.029
0.040
0.039
0.036
Boat-tailed
2.301
2.921
1.704
2.309
3.424
4.418
3.644
3.829
Grackle e
Total
3-yr mean
Adult weight = 115.5 g (Clenchand Leberman1978); eggweight= 10.0 g (this study).
Adult weight = 204.1 g (Clench and Leberman 1978).
Adult weight = 332.0 g (Ripley 1977); eggweight = 23.3 g (this study).
Fledglingweight = 38.3 g (Haigh 1968); eggweight= 3.9 g (Haigh 1968).
Fledglingweight = 77.2 g (Bancroft 1983); eggweight = 7.7 g (this study).
area of available habitat ("ecologicaldensity";Kale 1965) was 66.6 nests
per ha.
Crude populationdensityin the South Carolina site exceededthat of
three other cattail study areas, and was exceededby one study area, in
Haldimand-Norfolk County,Ontario (McCracken 1982). However,when
we computeddensityfor only vegetatedportionsof the respectivestudy
areas,ecologicaldensityin the South Carolina site was 35% higher than
that reportedfrom the Ontario site (Table 3).
As notedby Bancroft(1987) in Florida cattails,nestingbirds were not
randomly distributedthrough the marsh. Nests were usually aggregated
on predator-freeislandsor peninsulas.Nesting coloniesthat were invaded
by rat snakes(Elaphe obsoleta)were usually desertedby female grackles
and by Least Bitterns within 10 d. Over 95% of grackle nestswere in
colonies.Least Bitternsand CommonMoorhensnestedwithin the grackle
colonies.The largestnumber of Least Bittern nests,13, was in a Boattailed Grackle colonywith 32 nestsactiveat the sametime.
Averageannual productivityof all specieswas 3.8 kg per ha per year
(Table 2). Productionvaried little betweenyears, the greatestvariation
(1 kg) being 22% of the mostproductiveyear'sbiomass.The main factor
contributingto variation in productivitywas changein water level. When
water levelswere low, as during the droughtof 1988, rat snakesand rice
rats (Oryzomyspalustris)were able to reachbird neststhat were usually
protectedby water barriers containingAmericanalligators(Alligatormississippiensis).
Alligatorsdid not appear to depredatebird nests.In several
colonies,however, alligators that basked along channel edgesknocked
down cattails
that had nests.
Avian predationwas low in all 3 yr of the study, and accountedfor
lessthan 5% of nestlosses.Male and female gracklesmobbedFish Crows
(Corvusossifragus),
Black-crownedNight-Herons (Nycticoraxnycticorax)
and Northern Harriers (Circuscyaneus).We foundno evidencethat Boattailed Gracklestook the eggsor young of their own or of other species,
suchas has been reportedfrom Louisiana (Mcllhenny 1937).
198]
W.PostandC. A. Seals
J.Field
Ornithol.
Spring 1991
TABLE3.
Breeding bird densitiesin cattail marshesof easternNorth America.
Locationof study
NJ a
NY b
Duration of study(yr)
2
3
Size of area (ha)
Size of non-water area (ha)
No. of species
5.9
5.9
4
No. of territorial
males or females
Crude densityg
Ecologicaldensityh
44.8
7.63
7.63
9.7
4.9
15.3
48.2
6.09
12.18
ONTc
1
7.5
4.9
9
241.0
3.22
49.52
ONT d
SC•
1
3
5.5
--f
10
13.0
3.3
6
81.0
14.72
--f
216.5
16.65
66.62
aMorris County, New Jersey(Laskowski1981, 1982).
bSt. LawrenceCounty, New York (Crowell 1981, 1982; Harper and Crowell 1979).
cHaldimand-Norfolk County, Ontario (McCracken 1982).
dKent County, Ontario (Veenstraand Haggeman1981).
• This study.
f No data.
• Number of breedingmales or femalesper ha.
hNumber of breedingmalesor femalesper ha of availablehabitat (water areasexcluded;
Kale 1965).
A secondaryfactor that affectedproductivitywas heavy wind, usually
that associatedwith storms occurring in June and July. During this
period, many nests,exceptthoseof Pied-billed Grebes (Podilyrnbus
podiceps),were higher than thosebuilt in April, and were usually in new
vegetation.More nestsbuilt in this substratewere overturnedby storms
than were those built lower, in dead cattails (see also Bancroft 1987).
DISCUSSION
The populationestimatesthat we obtainedin this studyare similar to
thosefrom other studiesof similar habitatsand are alsocloseto population
densitiesfound in salt marshes. For example, Post (1970) found 27.6
pairs per ha in a New York salt marsh. No estimatesof breeding-bird
population densitiesare available for salt marshesin South Carolina.
The lack of annual variation in population densityor in productivity
may be attributableto the behaviorof the mostcommonspecies,the Boattailed Grackle. Although gracklecolonies(<36 nests)were occasionally
destroyedby predators,female gracklesmovedto new nest siteswithin
the marsh. Speciesnumberswere lower in our studysitethan in northern
cattail marshes,exceptfor the New Jersey census.As in the South Carolina marsh, most of the others had one or two numerically dominant
species.In all exceptthe New Jerseycensus,thesewere blackbirds(Icterinae). The numerically dominant speciesin the New Jersey marsh
was the Marsh Wren (Cistothorus
palustris).In our censusarea, the most
commonspecies,Boat-tailed Grackles, made up 71% of the population.
A similar patternof dominanceby Icterinaewas foundat the HaldimandNorfolk County site, where the two most commonspecies,Red-winged
Blackbirdsand CommonGrackles(Quiscalus
quiscula),madeup 79% of
the bird population.
Vol.62,No.2
Density
andProductivity
in a CattailMarsh
[ 199
The resultsof this study cannotbe used as evidencethat impounded
wetlands are more productiveof birds than are open (unimpounded)
marshes.The data providedhere may be viewedas a first steptoward
makingmeaningfulcomparisons.
As speciescomposition
may differ significantlybetweenevenstructurallysimilarfresh-and saltwatermarshes,
researchersshoulduse a commoncurrencysuchas biomassfor comparison. Further researchis neededon the avian productivityof wetlands,
particularly in the Southeast.Such researchshouldconsiderall species,
not just waterbirds.
ACKNOWLEDGMENTS
We were aided in the field by Dianne and Tom Howell, William Hutcheson,Kathy Post
and RobbieTietge. We appreciatethe very helpful critiquesof Richard Brewer and R. J.
Craig. We are especiallygrateful to Drayton Hastie and the Magnolia Gardens for their
support and encouragement.
LITERATURE
CITED
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Naturalist
15:1-8.
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Received7 Sep. 1988; accepted25 Sep. 1990.