Migrating Shorebirds and Habitat Dynamics at a Prairie Wetland Complex

15 Pages·1994·6.3 MB·English

by  Susan K Skagen

WilsonBull., 106(1), 1994, pp. 91-105
MIGRATING SHOREBIRDS AND HABITAT DYNAMICS
AT A PRAIRIE WETLAND COMPLEX
Susan K. Skagen' and Fritz L. Knopf'
Abstract.—We examined the responses ofmigrating shorebirds to habitatdynamics in
a wetland complex on the Great Plains during 1989-1992. Availability of habitat was
variable within and between seasons, but fluctuations in habitat were dampened when
wetlands were considered as a complex rather than individually. Shorebirds exhibited an
ability to colonize available habitat opportunistically, to occupy wet mud/shallow water
habitats that became available duringtheirresidency period regardless ofwetland history,
and to use wet mud/shallow water habitat almost immediately upon its appearance. We
foundasignificantrelationbetweennumberofshorebirdsandtheareaofwetmed/shallow
waterhabitat, regardlessofdramaticchangesin habitat. Managementforcontinental stop-
oversitesforshorebirdsrequiresthemaintenanceofcomplexesofpotentialhabitattoassure
resourcealternativesforbirdsaslocalconditionsvacillate.Received11 Jan. 1993, accepted
20July 1993.
During migration, several species of Arctic-breeding shorebirds use
freshwaterwetlandsintheNorthAmerican interioras stagingorstopover
sites for replenishing fat reserves. Without food resources to “refuel”,
these birds would be unable to complete their journeys to breeding or
wintering grounds. The protection of stopover resources for migrating
shorebirdsiscriticaltothesurvivalofmanyofthesespecies(Myers 1983).
The first step in this protection effort, the identification of sites that
traditionally support large populations during migration and the protec-
tion ofthese sites as a network (Myers et al. 1987), is being undertaken
specifically by the Western Hemisphere Shorebird Reserve Network and,
ingeneral, byotherwetlandconservation programs(Bildsteinetal. 1991).
Shorebirds migrating across continental wetland habitats encounter
temporally and spatially dynamic wetlands (Fredrickson and Reid 1990,
Szaro 1990, Skagen and Knopf 1993). The dynamic and unpredictable
nature of interior wetlands and the rapid rate of loss and alteration of
wetlands in these regions (Tiner 1984, Dahl 1990) combine to make the
above “reserve” management approach problematic. Species that use
disjunct patches of changing habitat in an irregular fashion, as seen es-
pecially during migration, may be the most difficult to protect (Takekawa
and Beissinger 1989).
In this paper, we evaluate the predictability of stopover sites in the
Great Plains and responses ofmigrating shorebirds to habitat dynamics.
We hypothesized that when resource availability changes rapidly, iran-
' National Biological Survey. National Fxology Research Center. 4512 McMuriy Avenue. F'ort Collins.
Colorado 80525-3400.
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92 THE WILSON BULLETIN • Vol. 106, No. 1. March 1994
sitory populations respond to wetland dynamics opportunistically rather
than exhibit strong annual site fidelity. More specifically, if migrating
shorebirds use habitats opportunistically, we expectthem to use available
habitat regardless of the recent wetland history. Thus, although some
wetlandsprovidenowetmud/shallowwaterhabitatforshorebirdforaging
during all or parts ofsequential migration periods, we expect shorebirds
to find and use these wetlands when wet mud/shallowwaterhabitat again
becomesavailableduringmigration. Second, wehypothesizedthatifhab-
itats are constantly fluctuating and birds are opportunistic, a positive
correlation between birds and wet mud/shallow water habitat would oc-
cur. Alternatively, shorebirds that use sites traditionally would be tied to
habitatsthat maybe marginal in someyears, and no relationshipbetween
numbers ofbirds and area ofwet mud/shallow water habitat would be
apparent.
STUDY AREA AND METHODS
QuiviraNationalWildlifeRefuge,StaffordCounty,Kansas(38°10'N,98°40'W),isa8830-
harefugeoftheU.S.FishandWildlifeService(Fig. 1).Forestsandcroplandsareinterspersed
with 30 water units ranging in size from 1 to 600 ha and Rattlesnake Creek that flows
intermittently. Vegetation in and surrounding the wetlands includes wetland plant species
in thegenera Distichlis, Spartina, Typha, Carex, andJuncus.
Shorebirds occurring in water units and in extensive mudflats and marshes throughout
therefugeweresurveyedfrom avehicleandonfoot 1-2 timesweeklyduringlate summer-
fallmigration(Augustthroughmid-October) 1989-1991 andspringmigration(Apriltoearly
June)in 1990-1992. Becausethesurveyareasarerelativelyopenandunvegetated,wewere
ableto makecompletecountsofshorebirds(seealso Colwelland Oring 1988, Handsetal.
1991). When feasible, we identifiedall individuals. When largenumbersoccurred orwhen
birds were too distant to identify individually, we estimated total numbers ofbirds cate-
gorized by relative body size. We extrapolated to the largergroup based on subsamples of
birds.
Weestimateddimensionsofwetlandunitsfrom mapsandbypacing. Duringsurveys,we
estimatedthe percentage ofeach unitthatcomprisedthefollowinghabitattypes: dry mud,
wetmud,mud-waterfilm(1-2cmofwaterinterspersedwithmud),shallowwater(2-8cm),
and deep water (>8 cm) and noted presence ofvegetation. We collected information on
NWR
habitatavailabilityatQuivira onten small(<5 ha) waterunitsthat werepresentall
six seasons and on eight additional ephemeral wetlands during one or more seasons (Fig.
1). Wequantifiedhabitatavailabilityonlyinthesmalldiscretewaterunits,notinthemore
extensive mudflats and marshes on the refuge (Fig. 1).
We operationally define the terms “suitable habitat” and “suitable wetland” to refer to
wet mud-shallow water habitats with little or no vegetation, habitats that are generally
Fig. 1. MapofstudysiteatQuiviraNationalWildlifeRefuge, centralKansas. Eighteen
individualwaterunits are identifiedby number, and the ten unitswith extensivecoverage
are darkened.
Skagen andKnopf* SHOREBIRD MIGRATION
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94 THE WILSON BULLETIN • Vol. 106, No. 1, March 1994
attractive to most shorebird species (Colwell and Oring 1988). Below we use these terms
interchangeablywith “shorebird habitat” and “wet mud/shallow water.”
We documented the sequential pattern of habitat availability in wetlands during six
migrationseasons,andnotedshorebirduseofwetlandsthathadnowetmud/shallowwater
habitat earlierin a season or in previous seasons. We also examined bird responses to the
relativedistributionofhabitatsinseasonswhenwetmud/shallowwaterhabitatwasavailable
in water units. First, we selected five surveys in each season that corresponded with large
increases in numbers ofbirds, suggesting the presence ofnew arrivals, and large overall
counts on the refuge. Because many birds had recently arrived and because time intervals
between these selected surveys averaged 12 days, we considered the surveys independent
ofeach other. We formed threecategoriesofshorebirdsbasedontheirprimary patternsof
habitatuse (Table 1). All statistical analyseswere performed using SYSTAT 5.0.
RESULTS
QuiviraNational Wildlife Refuge provided important stopoverhabitat
for a total of 35 species during spring and late-summer/fall migrations
(Table 1). During spring 1990, the season ofheaviest shorebird use, peak
counts totalled 15,633 birds. Peak numbers of individual species were
generally higher in spring than in fall, and species composition varied
between seasons. In both spring and fall. Long-billed Dowitchers (Lim-
nodromus scolopaceus). Stilt Sandpipers {Calidris himantopus), Semipal-
mated Sandpipers (C. pusilla), and Wilson’s Phalaropes {Phalaropus tri-
color) were among the most common birds. White-rumped Sandpipers
(C.fuscicollis) and Baird’s Sandpipers (C. bairdii) were common only in
spring, while Western Sandpipers (C. mauri) and Least Sandpipers (C.
minutilld) were common only in fall. Shorebirds commonly associated
with wet mud/shallow water habitat comprised more than 80% ofpeak
numbers ofbirds (Table 1).
Habitat variability within and between seasons.— condition ofthe
wetlands varied considerably between the six migration seasons. During
spring 1990, all units were full of water and had no wet mud/shallow
water habitat suitable for shorebird foraging. In late summer-fall 1991,
most small units were dry. Collectively, in a total of74 unit-seasons (one
water unit for one migration season), 36 had no wet mud-shallow water
habitat. In 16 unit-seasons, wet mud/shallow water habitat was present
initially but eventually disappeared, and in 18 unit-seasons, wet mud/
shallowwaterhabitatwasabsentinitiallybutappearedlaterinthe season.
Only four unit-seasons had available habitat throughout a migration sea-
son.
The amount ofwet mud/shallow water habitat in the water units often
fluctuated during the 2-3 month migration season, as illustrated by 1
wetlands in fall 1990 (Fig 2). Because the amount of wet mud/shallow
water habitat in individual wetlands was dependent on wetland topog-
raphyandwaterlevels, thepresenceofhabitatacrossthevariouswetlands
Skagen andKnopf* SHOREBIRD MIGRATION 95
was not always synchronized (Fig. 2). At times, deeper wetlands had wet
mud/shallowwaterhabitat onlyduringdryingcycleswhen shallowerwet-
lands completely dried. The patterns ofwater level fluctuations in indi-
vidual wetlands were dramatically different between the six migration
seasons (Fig 3).
However, fluctuations in the amount ofwet mud/shallowwaterhabitat
were dampened at a larger geographical scale (Fig 4). As a result, there
was high likelihood that wet mud/shallow water habitat was available in
at least one wetland in the complex at a given point in time. Wet mud/
shallow water habitat was generally available within the complex of 10
wetlands throughout four ofthe six migration seasons in this study (Fig.
5). When also considering the extensive mudflats and marshes on the
refuge, suitable habitat occurred somewhere on the refuge in all six sea-
sons.
—
Birddistribution relative to wetlandhistory. \n general, shorebirds re-
sponded quicklyto the first appearance ofwet mud/shallowwaterhabitat
in a wetland in a given season (Table 2). In 15 ofthe 18 unit-seasons in
which suitable habitat was absent initially but later appeared during that
season, some shorebirds responded immediately (were present in the first
survey after the appearance ofwet mud/shallow water habitat). In only
two cases, one survey elapsed before any shorebirds appeared, and in one
case, shorebirdsdidnotusethewetlandatall, possiblybecausethehabitat
appeared late in the season when few birds remained in the area. Some
species responded more consistently than others. Baird’s Sandpipers,
White-rumpedSandpipers, LesserYellowlegs{Tringaflavipes), andLong-
billed Dowitchers appeared immediately in more than halfthe new suit-
able habitats in all seasons (Table 2), whereas Semipalmated Sandpipers
and Least Sandpipers did not.
On average, only four days elapsed between the two surveys bracketing
the appearance ofhabitat and birds. There was a broad range in numbers
of individual birds that responded within the first few days of habitat
availability (median = 28, range 3-1 122). The species that occurred in
the largest assemblages were Long-billed Dowitchers, Lesser Yellowlegs,
Semipalmated Sandpipers, and Baird’s Sandpipers.
Shorebirds also responded quickly to the first appearance ofwet mud/
shallow water habitat in a given wetland in several seasons (Table 2).
During spring of 1992, three suitable wetlands had no wet mud/shallow
waterhabitatduringthe precedingspringmigration season, and fourother
suitable wetlands had no wet mud/shallow water habitat during two pre-
ceding spring seasons. In fall 1990, nine suitable wetlands had no wet
mud/shallow water habitat during one preceding fall migration period.
Even though no shorebirds had used these wetlands during spring (or fall)
1
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96 THE WILSON BULLETIN • Vol. 106, No. 1, March 1994
to — — — O O — O —
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Skagen andKnopf* SHOREBIRD MIGRATION 97
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98 THE WILSON BULLETIN • Vol. 106, No. 1, March 1994
<
I
GO
<
X
AUG SEPT OCT AUG SEPT OCT AUG SEPT OCT
DRY MUD WET MUD SHALLOWWATER DEEPWATER
Fig. 2. Changesinavailabilityofmicrohabitats(drymud, wetmud, shallowwater[<8
cm],anddeepwater[>8cm])in 11 wetlandsthroughthelatesummer-fallmigrationseason
of 1990.
UNIT 14A
AUG SEP OCT APR MAY
UNIT 7
AUG SEP OCT APR MAY
DRY MUD E3 WET MUD SHALLOW WATER DEEP WATER
Fig. 3. Changesinavailabilityofmicrohabitats(drymud, wetmud, shallowwater[<8
cm], anddeepwater [>8 cm]) in two wetlandsthroughoutthe six migration seasonsofthe
study.
Skagen andKnopf* SHOREBIRD MIGRATION 99
AUG SEPT OCT
DRYMUD WETMUD ED SHALLOWWATER ED DEEPWATER
Fig. 4. (a) Amount ofwet mud/shallow water(<8 cm) habitat in 11 wetlands through
the 1990 fall season, (b) Total amount ofwet mud/shallow water habitat across the 11
wetlands.
migration for 1-2 years, the first occurrence ofwet mud/shallow water
habitat coincided with the appearance of shorebirds (Table 2). As pre-
dicted, nearly all ofthe common species appeared in these wetlands once
wet mud/shallowwaterhabitat appeared, regardless ofthe recentwetland
history.
<
X
CD
<
X
AUG SEPT OCT APR MAY
Fig. 5. Total amount ofwet mud/shallow water habitat in 10 wetlands throughout the
six migration seasons ofthe study.
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100 THE WILSON BULLETIN • Vol. 106, No. I, March 1994
Table 2
Shorebird Responsesto First AppearanceofWet Mud/Shallow Water Habitat
WITHIN AND BETWEEN SEASONS IN SEVERALWETLANDS
Presenceofshorebirdspeciesinwetland
SESA WESA LESA WRSA BASA GRYE LEYE STSA LBDO
Within season^
Fall 90 (N = 4) + - + + + +
Spring 91 (N = 5) - + + + - + + +
Spring 92 (N = 9) - - + + + + - +
Between seasons*’
Spring: no habitat available during
two precedingsprings (N = 4) ++ ++ + + ++ ++ ++
Spring: no habitatavailable during
one precedingspring (N = 3) ++ + ++ + ++ ++ ++
Fall: no habitat available during
one precedingfall (N = 9) ++ ++ ++ ++ ++ ++ ++ ++
wa“teSrpehcaibeistacto.deBsetawseiennTsaebalseon1s.:Wsiptehciienssperaessoenn:tsipneci>e5s0p%resoefntwe(t+la)nodrsa(b+s+en)t;(s—pe)caitesfiprsrtesaepnpteairnan1c-e5o0f%woeftwmeutdl/asndhsal(l+o)w;
spe^cWieestimnurde/gsihonalbluotwnwoatteprrehsaebnitta(t—)f;irbstlaonckcucrerlledinidnicNatewsetslpaencdiessdnuortinignmrieggriaontiaotntsiemaesoonf.survey.
Bird distribution relative to habitat.—'Wt examined the relationship
between numbers of birds and the amount of wet mud/shallow water
habitat in wetlands on days selected according to total numbers ofbirds
on the refuge and time in the season (see Methods). During spring, 71%
ofthe selected wetland-days had no suitable habitat and no birds, 8% of
Table 3
Relation between NumbersofShorebirdsandAreaofWet Mud-ShallowWater
Habitat during Eight FallSurveys
Shorebirdsassociatedwith
Allshorebirds wetmud/shallowwater
Date wetlands r r p
14 Aug. 89 12 0.639 0.0125 0.615 0.017
31 Aug. 89 6 0.991 <0.0001 0.996 <0.0001
7 Sept. 89 7 0.982 <0.0001 0.977 <0.0001
10 Aug. 90 10 0.584 0.038 0.599 0.034
22 Aug. 90 1 0.723 0.006 0.720 0.006
4 Sept. 90 1 0.761 0.003 0.764 0.003
14 Sept. 90 1 0.300 0.370 0.360 0.138
26 Sept. 90 10 0.915 <0.0001 0.928 <0.0001
'Pvaluesareone-tailed.