The Auk 112(3):613-632, 1995
LONG-TERM
POPULATION
BIRDS IN THE SOUTHERN
TRENDS
UNITED
OF COLONIAL
WADING
THE IMPACT
OF
STATES:
CRAYFISH AQUACULTURE ON LOUISIANA POPULATIONS
BRUCE E. FLEURY AND THOMAS W. SHERRY
Department
of Ecology,
Evolution,
andOrganismal
Biology,TulaneUniversity,
New Orleans, Louisiana 70118, USA
Al•sTl•C•r.--Long-term
populationdynamicsof colonialwadingbirds(Ciconiiformes)
were
examinedusingdata from Audubon ChristmasBird Counts(CBC,1949-1988)and Breeding
Bird Surveys(BBS,1966-1989).Winter populationsof Louisianawading birds increased
dramaticallyover the 40-yearperiod,with the sharpestincreases
occurringduring the last
20 years.Severalspeciespopulationsgrew exponentiallyfrom 1968to 1988.High overall
positivecovariance
wasfoundin the abundanceof the variousspecies
overtime,andcluster
analysisshowedthat the specieswith similardietaryrequirements
and foraginghabitscovariedmoststronglyand positivelywith eachother.Trendanalysisof CBCandBBSdatafrom
1966-1989and 1980-1989showeda closecorrespondence
betweenCBC and BBStrendsin
Louisiana.Most speciesincreasedin Louisianaat the sametime as they declinedin Florida
andTexas.Severalfactorsmightexplainincreases
in populations
of wadingbirdsin Louisiana,
includinglong-termrecoveryfromthe effectsof humanexploitation,
expansion
of breeding
populationsin more northernstates,changesin weather,recoveryfrom DDT and similar
pesticides,
and regionalmovements
due to habitatlossin othercoastalstates.Thesehypothesesarenot mutuallyexclusive
andmeritfurtherstudy.However,basedonseveralinferential
linesof evidence,increasedacreagedevotedto crayfish(Procambarus)
aquaculturein Louisiana
appearsto be the mostsignificantfactorexplainingobservedpopulationtrendsin the state.
First,populationsof colonialwading-birdspeciesthatusecrayfishwerecorrelatedpositively
with the wild crayfishharvestin Louisiana,and even morestronglywith commercialcrayfish
pond acreage.Second,the regularitywith which thesepondsare managedprovidesa more
predictableforaginghabitatthan do correspondingnatural areas.Third, the useof crayfish
pondsby wading birds peaksduring pond drawdowns,which may increasereproductive
success
by concentratingprey availableto wading birdsduring their nestingseason.Fourth,
thosespeciesof wading birds that specializeon crayfishshowedthe greatestpopulation
increasesand the strongestcorrelationwith crayfishpond acreage.These findings have
importantimplicationsfor conservation
and managementof Louisiana'swading-birdpopulations. Received16 May 1994, accepted
5 September
1994.
ALTHOUGH WE LACK reliable
nineteenth
cen-
tury data on colonial wading-bird populations
(order Ciconiiformes), accounts from pioneer
naturalists
like Audubon
document
enormous
coloniesthroughout the southernswampsand
estuaries(Bent 1926). By the end of the nineteenth century, plume hunting had reduced
many speciesto near extinction. Legal protection began in 1900 with the Lacey Act, which
prohibited interstate commerce in illegally
killed animals,followed by the Migratory Bird
Treaty in 1916. The establishmentof wildlife
refuges also helped protect wading-bird colonies (Parnell et al. 1988).
Since their protection from plume hunters,
many speciesof colonial wading birds have
steadily increasedtheir North American populations, and expanded their rangesnorthward
613
and inland (Ogden 1978, Ryder 1978). However, wading-bird increaseshave not occurred
in all regions. Whereas populations have increasedandrangesexpandedin the past30 years
in someregionsof the United Statesfor species
suchas Snowy Egrets,Great Blue Herons, and
Cattle Egrets(Bockand Lepthien 1976, Ryder
1978, McCrimmon 1982, Larson 1982; Table 1),
regionaldeclinescharacterize
otherspecies,
such
as the Reddish Egret and RoseateSpoonbill
(Powellet al. 1989,but seeOgden1991).Florida,
for example,despitehaving a high historical
populationdensityof mostspeciesof colonial
wading birds (Root 1988), has witnessed reduced wading-bird populations due to prolongeddrought,estuarinedegradation,and human-induced hydrological changes in and
around the Everglades (Bancroft et al. 1988,
614
FLœUR¾
ANDSHERRY
Bancroft 1989). Ogden (1994) estimated that
populationsof five speciesof wading birds in
the Florida Evergladesdeclined by 93% between
1931-1946
and 1982-1989.
[Auk, Vol. 112
TABLE1. Speciesof Ciconiiformesregularly seenon
343 Louisiana Christmas Bird Counts 1949-1988,
and averagenumber observedper count.
Such conflict-
Aver-
ing trends in populationsof colonial wading
birdsmay even indicatelarge-scaleshiftsof re-
age per
Species
count
gional population centers.
This diversity of population trends among
regions and speciesindicatesthe need for in-
American Bittern (Botaurus
lentiginosus)
Least Bittern (Ixobrychus
exilis)
Great Blue Heron (Ardeaherodias)
albus)
creasedmonitoring, and for in-depth analyses Great Egret (Casmerodius
Snowy Egret (Egrettathula)
at national and regional scales.This need is par- Little Blue Heron (E. caerulea)
ticularly apparentin Louisiana,where wadingbird populationtrends have been poorly docmented in the past,with breeding-colonydata
only availablefor 1977,1984,and 1990(Portnoy
1977, 1978, Keller et al. 1984, Martin and Lester
1990, R. P. Martin 1991), and where wetland
acreageis more abundant than in any other
state.
We review population trends of 15 speciesof
colonial wading birds in Louisianaover the past
40 years, using data extractedfrom Audubon
ChristmasBird Counts(CBC).We compareand
contrastthese trends with state, regional, and
nationaltrendsfrom BreedingBirdSurvey(BBS)
data. We usepublished data to discussbriefly
and evaluateseveralsuchfactors,namely longterm recovery from the effects of the plume
1.5
0.1
41.3
122.8
118.3
81.3
50.4
0.2
100.9
1.1
Tricolored Heron (E. tricolor)
ReddishEgret (E. rufescens)
Cattle Egret (Bubulcus
ibis)
Green-backed Heron (Butoridesstriatus)
Black-crownedNight-Heron (Nycticoraxnycticorax)
13.0
Yellow-crowned Night-Heron (N. violaceus) 0.3
White Ibis (Eudocimus
albus)
256.5
Glossyand White-facedibises(Plegadis
falcinellusand P. chihi)
432.1
RoseateSpoonbill(Ajaiaajaja)
21.2
involves: tropical-fishfarms in Florida; catfish
farms in California, Arkansas,and Mississippi;
and fish farms in Europe(Draulans 1987b,Stickley andAndrews 1989,Boyd1991,Cezilly 1992,
Stickleyet al. 1992).Wadingbirdscurrentlyare
trade and other human exploitation, increased treatedas agriculturalpestsunder the Departwinter populationsdue to expandedrangesof ment of Agriculture'sAnimal DamageControl
migratorywading birdsin morenorthern states, program. Depredation permits have been is-
effectsof weather, recoveryfrom heavy pesti-
sued in several
states for lethal
control
of sev-
cide use, competition, predation, habitat loss,
and land-use changesin Louisiana due to the
eral migratory bird species(including herons,
egrets, pelicans, and cormorants)that feed at
growth of crayfish (Procambarus)
aquaculture catfish farms and other aquaculture facilities.
(note: in Louisiana, the common name "crawIncreasedpopulationsof White Ibisesand Yelfish" is usedinstead of "crayfish").
low-crowned Night-Herons, which feed heaviContinueddeclinesin the quality and quan- ly on crayfish,are of particularconcernto Loutity of foraging habitat in natural wetlands due isiana wildlife biologists and farmers. Sound
to drainage,pollution, and other human influ- managementpolicy for colonial wading birds
enceshighlight the importanceof understand- in Louisianawill require detailed understanding how and why birds use artificial wetlands ing of the relationship between wading-bird
suchasfloodedfieldsor aquacultureponds.The populationsand crayfishaquaculture,towards
exponentialgrowth of commercialcrayfishpond which our analysesof wading bird population
acreagein Louisianamay have directly contrib- dynamicsare an important first step.
uted to increasesin Louisianawading-bird populationsby creatingnew high-qualityforaging
METHODS
habitats.Maddock and Baxter(1991) suggested
that food limitation is a critical factor in egret
CBC.--We
used data from the annual Audubon
reproductive success.
The useof aquaculturepondsby wadingbirds
alsoraisesimportant legal and conservationissues, both locally and internationally. The
problemof bird predationin aquacultureponds
ChristmasBird Count (CBC) to analyze winter ciconiiform populationtrendsin Louisiana.CBCsare held
on a singleday between15 Decemberand 5 January.
All birds seen within a 24.1-km (15-mi) diameter cir-
cleare tabulated.In addition,observereffortis gauged
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
615
of birds countedas stronglyas would the addition of
new countcirclesin wetland areas(Raynor1975,Morrison and Slack 1977, Bock and Root 1981). Therefore,
we calculated the total number
of birds observed di-
vided by the number of countsheld in that year.
There are several methodologicalproblems with
CBC data. CBC data are not truly random samples,
sinceregionsof high speciesdensityare favoredin
the selection of count areas (Drennan 1981). CBC
counts also are subject to error due to year-to-year
differencesin weather and timing of the count (Falk
1979, Smith 1979, Rollfinke and Yahner 1990), and
0
1949
due to observer-reliabilityerrorsand volunteer turn1959
1969
1979
1989
over (Stewart 1954, Confer et al. 1979, Arbib 1981,
Butcher1990). CBC data are most meaningful when
they covermanyyearsand includea broadgeographic area because annual variations
in weather
and ob-
server reliability become relatively less important
(Bockand Root 1981, Ferner 1984, Butcher et al. 1990).
The potential problem of speciesidentificationis
not seriousfor wading birds, exceptfor the difficulty
of discriminating between Glossy and White-faced
ibises.We pooled thesespeciesinto a single category.
Most CBC observations
1949
1959
1969
1979
989
Year
Fig. 1. Number of Christmas Bird Counts held and
total colonial wading birds counted, 1949-1988 (see
Table I for completelist of species).
by recordingthe total mileagecoveredand the total
observationhoursby all parties(party milesand party
hours, respectively;Butcher 1990). We used abundancedatafor the 16 speciesof ciconiiformsregularly
seenin Louisiana(Table 1), alongwith observereffort
recordedfor each count location.Analysesspanned
the 40-yearperiod from 1949,the first year in which
three or more countswere regularly held statewide,
through 1988, for a total of 343 individual counts
(number of countsper year are summarizedin Fig.
Florida, Louisiana, Texas, and California, states which
consistentlyrecordedthe highest wading-bird population densities in CBC counts (Root 1988).
1).
Several methods have been used to normalize
data in order to minimize
of dark ibises in Louisiana
were reported as White-facedIbises.
To provide an independenttest of observedCBC
trends,we analyzedBreedingBird Survey(BBS)data
for the same speciesfor the periods 1966-1989 and
1980-1989(1966 is the first year in which BBStrends
were reported).BBSsurveysare held in the summer,
andreflectthe populationsizeof breedingbirds,while
winter populations (CBC) reflect a combination of
remaining (resident) birds plus wintering migrants.
Several studieshave found closeagreement between
CBC and BBSdata for a variety of species(Robbins
and Bystrak 1974, Dunn 1986, Burdick et al. 1989,
Butcher et al. 1990). BecauseBB$ data are compiled
by roadsidecensus,they tend to underestimatepopulation sizesof colonialwading birds (Bystrak1981,
Butcher1990).Wading-birdcoloniesoftenarelocated
in remotewetlandsfar from highways.Nonetheless,
the BB$samplesizesfor eachspeciesof wading bird
usually exceededthe BBSminimum recommended
size for dataanalysis,often by severalordersof magnitude (10 routes minimum per species;S. Droege
pers. comm.). We analyzed BBSsurvey results for
CBC
variation due to differences
in observereffort.Initially, we examinedtrendsstandardized per 10 party-hours,per 100 party-miles, and
per countfor all wading bird speciesover the 40-year
sampleperiod, and found that the method of normalization had surprisinglylittle effect on the observedtrends(unpubl. data).Given that colonialwading birdsaggregate,changesin observereffortwithin
a countcircle would probablynot influencenumbers
Dataanalysis.--Wecalculatedlinear regressions
and
exponentialcurvesfor eachspeciesbasedon the annual number of birds per CBC count. Data serieswith
zero values were transformedby adding 0.1 to each
datapoint for testingwith an exponentialmodel.The
use of regressionto test hypothesesconcerningstatisticalsignificancein autocorrelateddata sets,such
as annual censuses,violates the assumptionof independence(Bealand Khamis 1990).Therefore,we use
the regressiondata only to demonstratethe obvious
616
F•.msa¾
tu'4DS•r•a¾
[Auk, Vol. 112
TABLE
2. Slopeand error (R2)valuesfor linear and exponentialbest-fitregressionmodelsof Louisianawinter
wading-bird populations,basedon per-count ChristmasBird Count data, 1949-1988 (343 counts).Trends
were not testedfor significance(seetext).
1949-1988
1968-1988
linear
linear
exponential
-0.04 (0.09)
0.00 (0.00)
1.15 (0.34)
-0.05 (0.06)
0.00 (0.02)
3.87 (0.83)
6.86 (0.40)
8.61 (0.40)
3.56 (0.09)
1.93 (0.19)
0.02 (0.26)
8.71 (0.45)
0.06 (0.35)
0.80 (0.20)
0.01 (0.10)
14.47(0.39)
18.55(0.37)
-8.65 (0.09)
2.90 (0.09)
0.03 (0.15)
12.03(0.20)
0.06 (0.07)
3.02 (0.47)
0.04 (0.36)
0.00 (0.02)
0.00 (0.02)
0.01 (0.21)
0.03 (0.36)
0.04 (0.28)
0.05 (0.46)
0.02 (0.18)
0.02 (0.32)
0.11 (0.84)
0.04 (0.59)
0.00 (0.00)
0.00 (0.01)
0.04 (0.79)
0.04 (0.64)
0.05 (0.62)
-0.01 (0.02)
0.02 (0.11)
0.03 (0.22)
0.05 (0.45)
0.02 (0.24)
0.02 (0.09)
0.01 (0.14)
0.10 (0.62)
0.04 (0.41)
White Ibis
23.22 (0.49)
56.21 (0.59)
0.07 (0.48)
0.07 (0.66)
Glossyand White-facedibises
RoseateSpoonbill
22.92 (0.30)
1.35 (0.29)
55.21 (0.46)
1.37 (0.07)
0.05 (0.21)
0.08 (0.56)
0.06 (0.41)
0.06 (0.24)
Species
American Bittern
Least Bittern
Great Blue Heron
Great Egret
Snowy Egret
Little Blue Heron
Tricolored Heron
ReddishEgret
Cattle Egret
Green-backedHeron
Black-crownedNight-Heron
Yellow-crowned Night-Heron
long-rangetrend in the populationdensitiesof these
species,rather than to test hypothesesof statistical
significanceof trends.We smoothedpopulation data
with a three-yearsliding averagefor graphicpresentation only (Raynor 1975). The squaredcorrelation
coefficientof the fitted curve (R2) for linear regressionsof nonsmootheddata approachedor exceeded
0.50 for the most abundantand rapidly increasing
species(Table 2).
To allow direct comparisonof BBSwith CBC data,
we calculated
CBCtrends(annualpercentchangeper
year) for each speciesfor 1966-1989 and 1980-1989.
These two periods representedthe most recent BBS
data available.
BBS trends are calculated with a route-
1949-1988
1968-1988
exponential
pressedthe resultingtrendaspercentchangeper year
(100[trend-
1]).
We assessed
covariationof temporaltrendsamong
speciesfor the 40-yearCBCdataset using Schluter's
(1984) covariance test. This test is based on the ratio
(V) of the variance in the total number of individuals
per sample(ST2, all speciescombined)to the sum of
the variancesof the individual speciesdensities(•.s,2).
Bythe null hypothesisof no covariationin population
trendsamongspecies,the conditionalexpectedvalue
of ST2is equalto 2i s,2,giving V the value of 1. In this
case,the variance in total abundancefor all species
from year-to-yearcanbe completelyexplainedby the
sum of the variancesin density of each individual
regressiontechnique,which estimatesmediantrends
using a bootstrapanalysis to estimate trends from
species.
We calculated Schluter's index of association(W),
random subset samples (Geissler and Noon 1981,
Geissler 1984, Geissler and Sauer 1990). Our trend
which is equal to VN (N equalsthe number of sampies) and
analysisof CBCdatais basedon one estimateof species abundancefor each year (per-count data). We
fitted a simple linear regression
log•(count,+ 0.5) = year,(log•[b])
+ log•(a), (1)
where e is the baseof the natural logarithms,count•
is the number of birds observedper count in year/,
b is the slope of the fitted regression,and a is the
y-interceptof the fitted regression.
We addedthe constant0.5 to insurea defined logarithmfor countsof
zero.We calculatedthe trend fromthe antilogof the
slopein this regression(Holmesand Sherry 1988):
Trend = e(løs'l'-ø-ssE),
(2)
where b is the slopeand SE the standarderror of the
slopeof the regressionon logarithmicper countdata.
The SE term correctsfor the asymmetryof the lognormal distribution (Geissler 1984). We then ex-
W=VN=ST2N/.•.
s?,
(3)
where
ST
2=(l/N)• (Tj- t)2,
(4)
s,
2=(l/N)• (X,j
- t,)•.
(5)
J
and
]
Tj is the total densityof birdsin yearj, t is'the mean
densityof totalbirdsacross
all j years,X•jequalsthe
densityof speciesi in yearj, and t•equalsthe observed
meandensityof speciesi amongall j years.If the ratio
V isgreaterthanone,species
tendto covarypositively
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
and, if V is lessthan one, speciestend to covarynegatively. W is approximately chi-squaredistributed,
with n degreesof freedom(Schluter1984).
We calculated
Pearson correlation
coefficients
for
each possiblepair of speciesover the 40-year CBC
data set.We then clusteredall specieson the basisof
their correlationcoefficients,using a maximum-linkagealgorithm to summarizesimilaritiesin abundance
patterns.
For example,from 1966 to 1989,breeding populations (BBStrends) showed a higher rate of
increase than did winter populations (CBC
trends) in White-faced Ibises, RoseateSpoonbills, Snowy Egrets,Great Egrets,and Yellowcrowned Night-Herons, while CBC data indicated greater increasesthan BBSdata for BlackcrownedNight-Herons and Great BlueHerons.
BBStrendsby state.--A comparisonof Louisiana BBS trends with
RESULTS
Sixteencolonialwading bird specieswere observedregularly on LouisianaChristmascounts
(Table 1). Winter population sizesfor mostspeciesincreased(Table2) basedon per count CBC
datasmoothedwith a three-yearsliding average
(Fig. 2). Slopesof linear regressions(abundance
per year) from 1949 to 1988, based on nonsmoothedper-countdata, were positive for 13
617
those of other states con-
taining large wading bird populations (Florida,
Texas,and California) revealed regional differencesin population trendsduring the pasttwo
decades(Table 3). Florida'swading-bird populations, unlike those in Louisiana,appear to
be declining. Only the Snowy Egret,Little Blue
Heron, and Green-backed Heron populations
increasedfrom 1980-1989.Colony censusdata,
however, indicate that Florida'sSnowy Egret
population may have declined sharply during
of the 16species
(Table2). Species
with highest this period (Ogden 1994).Furthermore,popuslopeswere White Ibis,Glossyand White-faced lationsof 10 Florida speciesdeclinedmore rapibises, Cattle Egret, Snowy Egret, and Great idly from 1980-1989than 1966-1989.Most speEgret; other specieswith positive slopeswere cies of Texas wading birds also declined, esLittle Blue Heron, Tricolored Heron, Roseate peciallyin the lastdecade.Severalof the species
Spoonbill,GreatBlueHeron, and Black-crowned that remained stable or declined from 1980Night-Heron. LeastBitternsand American Bit- 1989 in Florida or Texas simultaneously interns had a slopenear zero. The relative rarity creasedin Louisiana,namely the White Ibises,
of the Green-backedHeron, ReddishEgret,Yel- Glossy Ibises, White-faced Ibises, Yellowlow-crowned Night-Heron, American Bittern, crowned Night-Herons, Black-crownedNightand Least Bittern precludes drawing any firm Herons, and Tricolored Herons. California
wading-bird populations showed the largest
conclusionsregarding these species.
When we restricted analysesto the last 20 number of significantincreasesof the statesexyears (1969-1988), most species(11 of 16) in- amined,particularlyin 1980-1989for the Whitecreasedexponentially, based on a somewhat facedIbises,Cattle Egrets,Snowy Egrets,Great
better fit to an exponential than a linear re- Egrets,and Black-crownedNight-Herons.
Community-level
analyses.-The Schluter(1984)
gression(i.e. larger R2;Table 2). In particular,
populationsof Great Blue Herons, White Ibises, covariance test assesses covariance within a
community or assemblage.This test revealed a
Great Egrets, Black-crowned Night-Herons,
Snowy Egrets, Cattle Egrets, Yellow-crowned high degreeof positive covariancein 40-year
Night-Herons, and Glossy and White-faced CBC population trends in Louisianaamong all
ibisesincreaseddramaticallyfrom 1968to 1988 16 species(W = 127.15,df = 40, P < 0.001). We
believe that this covariance
reflects a similar
(Fig. 3).
response
among
species
pairs
to changing ecoComparison
of CBC and BBStrendsin Louisiana.--Most Louisiana BBS trends closely par- logical conditions, particularly the quality of
alleled LouisianaCBC trends during the 1966- foraging habitat (seeDiscussion).
We clusteredspeciesbased on speciespair1989 time period in terms of their magnitude
and direction (Table 3). Both BBSand CBC trends wise correlation coefficientsof population sizes
showthat mostLouisianapopulationsincreased over time, using a complete-linkage method
from 1966to 1989,especiallyWhite Ibises,Glossy (Sneath and Sokal 1973, Bock and Root 1981,
and White-faced ibises, Roseate Spoonbills, Holmes et al. 1986).The resulting tree diagram
Snowy Egrets,Cattle Egrets,and Great Egrets. (Fig. 4) showsthat the Great and Snowy egrets
Not all Louisiana BBS trends corresponded (diurnal visual feeders on primarily fish and
with the CBC trendsin thesetwo time periods. crayfish) clustered most closely with one an-
618
FLEURY AND SHERRY
a. Great
Blue
[Auk, Vol. 112
Heron
b.
Green-backed
Heron
4
100
3
2
1
194919•9 1969 19•79
'1'989
c. Little
Blue
1949 19'•919'6919'79'1'989
d. Cattle Egret
Heron
400
4OO
3OO
300-
200
0
0
1949
1959
1969
1979
1949
1989
1959
1969
1979
1989
1979
1989
f. Great Egret
e. Reddish Egret
1.5
5OO
1.0.
4OO
300
200
0.5.
100
0.0
........
, ....
,..
194919'591969 1979'1'989
Year
0
1949
1959
1969
Year
Fig.2. Numberof individualsof 16speciesof colonialwadingbirdsper LouisianaChristmasBirdCount,
1949-1988.Curvesare least-squares
exponentialfunctionsfit to smoothedper-countdata.
other, indicatinggreatestsimilarity of population trends. The ibises (diurnal tactile feeders
on primarily crayfish)alsowere closelycorrelated with one another,aswere the two nightheron species(nocturnal visual feederson pri-
marily crayfish).The Great Blue Heron clustered most closelywith the ibisesand egrets.
The Little Blue Heron and the two bitterns were
dissimilar from the other speciesin population
changeswithin Louisiana.
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
h.
g. Snowy Egret
619
Tricolored
Heron
2OO
6OO
4OO
loo
0
1949
i.
1959
1969
1979
1989
1949
Black-crowned Night-Heron
j.
1959
1969
1979
Yellow-crowned
1989
Night-Heron
1.2
0.8
0.6
0A
0.2
1949
k.
1959
Least
1969
1979
1989
1949
Bittern
0.5
1.
1959
American
1969
1979
1989
Bittern
4
3
2
1
Year
Year
Fig. 2. Continued.
DISCUSSION
es demonstratedby CBC data were not an artifact of data-collection
methods. The use of the
Louisianawading-birdpopulationtrends.--CBC per-countmethodfor normalizingCBCdatafor
datashowincreasingwinter populationsof most colonial wading birds only removed the exspeciesof Louisianawading birds. The increas- podential trend shown in observer effort over
620
FLEUR¾
AIqDSHERRY
m. Glossy and Whim-facedibises
2OO0
[Auk,Vol. 112
density species,however, in two individual
countsat oppositeends of the state (New Orleans and Sabine NWR) from 1954 to 1988
showedthe sametrendsasin the completedata
set (Fleury unpubl. data). Furthermore,Louisiana BBS trends generally support the observed CBC trends for the same time periods
(Table 3).
The Schluter covariance test suggeststhat
someecologicalfactorswere affectingmostspeciessimilarly,resultingin a strongand positive
covariancebetweenthem. Closelyrelatedspecies,however,need not covaryin their popu-
0
1949
1959
n. White
1969
1979
1989
lation fluctuations, and BBS data from areas out-
side of Louisianasuggestdifferent population
trendseven in congenericpairsof species(Table 3). Interspecificcompetitionis one causeof
Ibis
negative values of W in some communities
(Schluter 1984), but the positive value of W in
our data indicate that interspecificcompetition
1200
was not an important factor influencing population trends in these wading-bird species.
Moreover, these speciesclustersare similar to
thoseproposedby Kushlan(1978)basedon resourcepartitioning in South Florida wadingbird communities.Many environmental factors
could account
1949
1959
1969
1979
1989
o. Roseate Spoonbill
for the trends we have docu-
mented.As we arguebelow, however, the most
likely single factoris variation in food supply.
Stateandregionalpopulation
trends.--Analyses
of BBSdataprovedusefulherein corroborating
many CBC trends (Table 3), and suggesteda
complexregionalpatternof shiftingrangesand
population centers for colonial wading birds.
Our studysuggests
the possibilityof large-scale
regionalchangesin populationcentersof wading birds along the Gulf Coast.Many of the
decreases
in wading-birdpopulationsin Texas
and Florida indicatedby BBSdatawere contemporaneouswith increasesin Louisianapopu-
lationsof thesespecies.Frederickand Spalding
(1994)describeda generalpatternof population
shifts of White Ibises from Florida to other areas
o
1949
1959
1969
Year
1979
1989
Fig. 2. Continued.
in the Southeast.A decline in nesting White
Ibis in the Carolinasalsoseemsto parallelgains
in Louisiana'sWhite Ibis population(Frederick
et al. in press).R. P. Martin (1991) noted that
for 8 of the 11 wading-birdspeciesknown to
nest in the Louisiana coastal zone, the state now
accounts for over 25% of the known
U.S. coastal
breeding populations.Further study will be
the sametime period(Fig. 1). SomeCBCcounts neededto determinewhether the observedpataddedin the last decadeare in areasof high terns are causedby regional populationshifts,
bird densityand,therefore,mightaffectthecal- or are the result of complexlocal population
culatedtrends.A separateanalysisof sixhigh-
changes.
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
a. Great
Blue
Heron
621
b. Great Egret
500
100
4OO
300
200
100
0
0
1968
1978
1968
1988
'19s78
1988
d. Yellow-crownedNight-Heron
c. Black-crownedNight-Heron
1.2
8O
1.0-
o
•)
0.80.60.4-
0.20.0
0
1968
1978
1968
1988
e. Glossyand White-faced•ises
'19•8
1988
L White Ibis
2000
1200
6
2OO
0
1968
0
1978
1988
1968
1978
1988
Year
Year
Fig.3. Exponential-growth
models
of selected
species
of colonial
wadingbirdsin Louisiana
Christmas
BirdCounts,1968-1987(numberper count,smoothed).
Wadingbirdsandcrayfishponds.--Wefound a
strongpositivecorrelationbetweenseveralspeciespopulationsof wadingbirdsand the avail-
between
1968 and 1987. We list several lines of
evidencebelow to support the hypothesisthat
increasedacreagedevoted to crayfishaquaculability of both wild and pond-raisedcrayfish ture is a dominant ecologicalfactorinfluencing
622
FLEURY
ANDSHERRY
[Auk,Vol. 112
TABLE3. ChristmasBird Count (CBC) trends for Louisiana, and Breeding Bird Survey (BBS) trends for
Louisiana,Florida, Texas,and California (percentannual change).Dash (--) indicatesno data reported.
Louisiana
Species
Florida
CBC
BBS
BBS
Texas
BBS
California
BBS
1966-1989
American Bittern
Least Bittern
Great Blue Heron
GreatEgret
SnowyEgret
Little Blue Heron
Tricolored
Heron
ReddishEgret
CattleEgret
Green-backed
Heron
0.4
0.4
8.8
-1.7
1.7
9.7
15.1
15.5'
24.8
1.2
7.7
-2.2
8.4
3.0
11.1
-0.2
0.9
-1.1
1.1
0.6
3.6**
-9.4
17.0
0.4
0.7
-2.0
- 1.2
-12.3'**
-0.8
1.2
-2.3
-0.1
0.0
2.7
12.4'**
11.1'**
---
-19.1'
4.0
2.0
- 2.7
Black-crownedNight-Heron
Yellow-crownedNight-Heron
20.3
4.0
3.3
9.1
- 11.6
0.2
- 0.2
-0.2
11.1
--
White Ibis
White-faced
25.4
24.9 a
18.5
58.8
-1.2
--
10.1'*
- 0.9
-26.8 * * *
24.9a
17.5
-36.4
-8.9
132.3
Ibis
GlossyIbis
RoseateSpoonbill
1.0
3.2
-1.9
-1.3
5.7 * * *
---
1980-1989
American Bittern
Least Bittern
Great Blue Heron
GreatEgret
SnowyEgret
3.7
7.5
Little Blue Heron
Tricolored Heron
ReddishEgret
CattleEgret
Green-backed
-3.3
-0.6
4.8
Heron
---2.4
10.6
49.6
-9.7
6.9
6.6
6.1
3.4
-3.2
-0.7
-2.0
- 11.4
- 11.0
1.0
-- 15.0
-7.0*
-1.1
-5.6
- 16.8'* *
- 1.0
3.2
-6.5
-10.8'*
-- 3.0
-4.7
-3.9
2.5
12.3'**
13.0'**
---
-25.4*
2.6
18.3
7.7
White Ibis
White-faced
13.5
8.0 a
2.2
4.7
-3.3
--
0.9
- 21.2* * *
-44.9* * *
8.0a
16.6
-14.6
-20.5
0.9
0.9
- 16.9'*
---
GlossyIbis
RoseateSpoonbill
- 15.1
- 1.2
10.1
8.9*
1.0
--3.1
Black-crownedNight-Heron
Yellow-crownedNight-Heron
Ibis
5.8
21.2'*
--2.7
4.2
21.9
--
*, P < 0.10; **, P < 0.05; ***, P < 0.01.
Combineddata for Glossyand White-facedibises.Most birds reportedas White-facedIbis.
the strongrecentgrowth trendsof many Louisianapopulationsof wading birds.
The Atchafalayabasinprovidesabundantfish,
crayfish,frogs,and otherprey for wadingbirds
(Blades1974,Gary 1974a).This rich food supply, however,fluctuatessharplyfrom year to
year dependingon weather (McSherry 1982,
Huner 1990).The crayfish-aquaculture
industry
providesfarmerswith a reliable alternativeto
an unpredictableswampharvest(Gary 1974b,
McSherry1982),and the regularitywith which
pondsare managedalsomakescrayfishponds
a morepredictablefoodsourcefor wadingbirds
than comparablenatural foragingareas.Wellmanagedpondsmimic the naturalhydrologic
patternsthat promotecrayfishgrowth in the
extensive
marshes of southern
Louisiana.
The
hydrologicalcyclein crayfishpondsis managed
slightly out of phasewith, and ahead of, the
naturalwetlandshydrologicalcycle(Fig. 5). This
minimizes the overlap between the pond harvestand the wild harvest,and preventsan overabundanceof marketablecrayfish,which would
significantlyreduce the market price (Martin
1979,McSherry1982,Huner and Romaire1990).
Louisianacrayfishlandingsfrom 1949to 1987
were significantlyand positivelycorrelatedwith
populations of Great Blue Herons, Yellowcrowned Night-Herons, Black-crownedNightHerons,GreatEgrets,Snowy Egrets,White Ibises,and Glossyand White-facedibises(Table4).
All of thesespeciesare known to feed on crayfish (Table 5).
We found correlationsnearly twice as high,
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
Little
Y-c
Blue
623
Heron
Night-Heron
B-c Night-Heron
Roseate Spoonbill
Great Egret
Snowy Egret
Great
Blue
White
Ibis
Heron
Glossy and Whitefaced ibiseS
Green-backed
Heron
Trlcolored
Heron
Reddish Egret
Cattle Egret
Least
Bittern
American
Bittern
i
1.0
I
I
0.8
I
I
I
I
0.6
I
0,4
Pearson
I
0,2
r
Fig. 4. Clusteranalysisof speciespairwisecorrelationcoefficients
basedon per-countCBC data, 19491988.
however,betweenthesesamespeciesof wading
birdsand the total acreagedevotedto commercial crayfishpondsin Louisiana.Crayfishfarm
pondsincreasedfrom 99 ha in 1949to over 284,
165 ha in 1991 (W. Lotto pets. comm.), but reliabledatafor crayfishpondsareonly available
from 1968 to date (Table 4). Many speciesof
wading birds feed regularly in crayfishponds,
especially Yellow-crowned Night-Herons,
White Ibises,Great Egrets,Snowy Egrets,and
on, Black-crowned Night-Heron, Yellowcrowned Night-Heron, White Ibis, Glossyand
White-facedibises,Great Egrets,and Snowy
Egrets(Table 4). These same specieshad the
highestproportionof crayfishin their adult or
nestling diets, and were among those increasing mostrapidly in LouisianaCBCor BBStrends
fledglng
Wading-bird
stage
J
Little Blue Herons (Table 6; Huner 1994, Martin
and Hamilton 1985; Huner 1976 presentation
Wading.birdI
at EcologicalSocietyof America;Fleury pers.
inenbatlon
stage]
obs.).
More importantly, species-specificdietary
patternsof Louisianawading birdssupportthe
hypothesisthat crayfishaquacultureand wading-bird populationincreasesare causallyrelated (Table 5). Populationgrowth of species
that rarely feed on crayfish,including Cattle
Egretsand RoseateSpoonbills,are poorly correlatedwith crayfishpondacreage(Tables4 and
5). Wading-bird specieswhose population in-
creases
from 1969-1989correlatedmosthighly
with crayfishacreagewere the GreatBlueHer-
I Wild
crayfish
I
harvest
Pond-raised
crayf".b
]
[Pond-rai.d
Drawdown
or ]
crayfishponds]
Jan Feb Mar
Apl May June July Ang Sept Oct Nov •
Fig.5. Temporalrelationshipbetweenwading-bird
breedingcycle, crayfishpond harvest,and crayfish
pond drawdown.
624
FLL•.rR¾
ANDSHERRY
[Auk, Vol. 112
T^BLE4. Pearsonproduct-momentcorrelationof wading-bird populations(per count CBC, 1968-1987)with
crayfishaquaculturepond acreage,wild crayfishharvest(in pounds),and rice field acreage(harvested).
a
1968-1987
crayfish
(df= 18)
Species
American Bittern
Least Bittern
Great Blue Heron
Great Egret
SnowyEgret
Pond
-0.20
0.10
0.90'*
0.63**
0.63'*
Little Blue Heron
Tricolored Heron
-0.26
0.44
ReddishEgret
Cattle Egret
0.46*
0.38
Green-backed
Heron
Wild
1949-1987
wild crayfish
(df = 37)
1968-1987
rice
-0.12
-0.01
0.45*
-0.21
0.08
0.52* *
-0.14
-0.02
- 0.55
0.35
0.44
0.56**
0.61**
-0.30
- 0.37
-0.17
0.25
0.18
0.44'*
0.33
- 0.18
0.02
- 0.02
0.29
0.39'
0.19
- 0.21
0.40
0.32
0.55' *
- 0.24
Black-crownedNight-Heron
Yellow-crownedNight-Heron
0.89'*
0.68**
0.56'
0.45*
0.47'*
0.25
- 0.59
-0.07
White
-0.45
0.72**
0.42
0.44**
Glossyand White-facedibises
Ibis
0.75**
0.34
0.60**
-0.40
RoseateSpoonbill
0.35
0.31
0.53**
-0.14
*, P < 0.05; **, P < 0.01.
ßCrayfishpond acreagedatafrom W. Lorio (pets.comm.)and J. Huner (pets.comm.).Data for wild-crayfishlanding datafrom the National
Marine FisheriesService.Rice-acreage
datafrom Fielderand Nelson(1982)and Universityof New Orleans(1990).
from 1966to 1989(Tables3 and 5). A few species
are exceptionsto this pattern. Great Blue Herons,for example,consumerelatively few crayfish, but their numberscorrelatestrongly with
of prey in shallowwater, governedby the seasonaldrying rate, is an important requirement
for efficient wading bird foraging in the Everglades,particularlyfor tactilefeeders(Kushlan
crayfishpond acreage.They may benefitfrom et al. 1975). A rapid drying rate ensuresthat
otherprey,suchastheabundantsmallfishfound prey items will be concentratedin sufficient
density in small pools (Bancroft et al. 1988).
in mostfarm ponds.
Seasonalpatternsof crayfish-pondusageby While this relationshipmay not hold generally
Louisianawading birds also strongly support for wading birds outsidethe Everglades(Fredthe hypothesisthat crayfishpondscontribute erick and Spalding 1994),the timing of breedto population growth by increasingreproduc- ing cycleand ponddrawdown in Louisianasugtive success(Fig. 5). Wading birds use crayfish gestsa similarpattern,particularlyfor the White
ponds throughout the year, but usage peaks Ibis. White Ibisesrely heavily on crayfishfor
their nestling diet (Bildstein et al. 1990, Bildtwice annually (Huner 1983,Martin and Hamilton 1985).The firstpeakoccursin winter,when stein 1993), and abandonment and failure of
the pondsare filled and adult crayfishemerge White Ibis nestsis closely related to low prey
from their burrows with broodstock. A much
density and excessiverainfall (Frederick and
higher peak occurs in May and June, when Collopy 1989b).
Because drawdowns
occur at different
times
ponds are drained following the commercial
harvestand wading birds are establishingnests on different farms throughout southern Louiand feeding nestlings(Martin 1979;Fig. 5 and siana, crayfish farms create a landscape-level
Table 6).
seriesof patchesof high prey density. These
Crayfishpond drawdown simulatesthe dry- landscape-levelpatchesare analogousto those
ing of surfacewater in natural wetlands,and tracked by wading birds foraging in natural
consequentconcentrationof prey,althoughthe wetlands.Colonialwading birdsare well adaptdrying period in crayfish ponds is relatively ed behaviorally to track spatially and temposhort.During the pond-drawdownphase,cray- rally patchyfood supplies(Kushlan1976,Erwin
fish that have not yet burrowed are concen- 1983, Kushlan 1986,Bancroftet al. 1994). If wadtrated in shallow water, along with other crus- ing-bird populations are ultimately food limtaceans,fish, amphibians,and aquatic insects ited, the addition of a new sourceof prey that
(Huner 1983, 1994). The natural concentration
is readily availableaswell asspatiallyand tern-
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
625
T^nI,E 5. Percentageof crayfishand other crustaceans
in diets of colonial wading birds. Where range is
given, first referencecited correspondsto lower value, with "ad" referring to adult bird diet, and "j"
referringto juvenilebird diet. Percentvaluesrepresentratiosof numbersof crayfishto total numberof
prey items.Valuesmarked"mass"representdietary percentageby massor volume.
Percentcrayfishin diet of
Species
American
Adult
Bittern
19-25
Least Bittern
Great Blue Heron
I0 a
9-14'
Great Egret
Juvenile
Source
Palmer 1962 (ad), Sprunt and Chamberlain
1949 (aS)
Palmer 1962 (ad)
Palmer 1962 (ad), Sprunt and Chamberlain
3O
9 (mass)a,b
1949 (aS)
Recher and Recher 1980 (ad), Howell 1932
(j)
Snowy Egret
16 (mass)-39
0.7-3
Recher and Recher 1980 (ad), Telfair 1981
(ad), Jenni 1969(j), Howell 1932(j)
Little
Blue Heron
11-73
5-8
Niethammer and Kaiser 1983 (ad), Telfair
1981(ad), Howell 1932(j), Rodgers1982
(j)
Tricolored
Heron
2a-20
0-2
Sprunt and Chamberlain 1949 (ad), Palmer
1962(ad), Jenni 1969(j), Howell 1932(j)
ReddishEgret
Cattle Egret
2 (mass)c
0
0
Recher and Recher 1980 (ad)
Palmer 1962 (ad), Telfair 1981 (ad), Jenni
1969,1973(j)
Green-backed
Heron
Black-crownedNight-Heron
Yellow-crowned Night-Heron
1-20
Niethammer and Kaiser 1983 (ad), Palmer
1962 (aS)
8
22
74-97'
White Ibis
52-72 (mass)
GlossyIbis
_a
White-faced
Ibis
RoseateSpoonbill
46
37-40
Palmer 1962(ad), Howell 1932(j)
Niethammer and Kaiser 1983 (ad), Riegner
1982 (ad)
Kushlan and Kushlan 1975 (ad), Kushlan
1979 (ad), Howell 1932 (j)
Sprunt and Chamberlain1949(ad), Baynard 1913(j), Howell 1932(ad,j)
35
Palmer 1962 (ad)
_c
Palmer
1962
ßDietary reports include other crustaceansin total.
bDiet reportedas 100%prawns.
cFor thesespecies,Palmer (1962)describeddiet as mainly fish, with a few crayfishor other crustaceans.
dSpruntand Chamberlain(1949)describeddiet as"for the mostpart'' crayfishand grasshoppers.
Howell (1932)notedthat crayfishcomprised
a "considerable
percentage"of the diet.
porally staggered from natural food sources for example,winter in ever-increasingnumbers
should have a substantialimpact on reproduc- to feed in 222,390ha of catfishfarm pondsscattive successof speciesusing this resource.Dou- teredacrossMississippi's
Delta region.Thisnew
ble-crested Cormorants (Phalacrocoraxauritus), foraginghabitat may have contributedto a 25%
TABLE
6. Wading birds observedduring drawdownof crayfishpondsat the Universityof Southwestern
Louisiana's Crawfish Research Center, 1992-1995. Annual census is taken near end of drawdown when
pondbottomspartlyexposed,
andrepresents
peakannualbird countrecordedon 20-hafarm-pondcomplex.
Species
1992
Great Blue Heron
Great Egret
Snowy Egret
0
211
445
Little Blue Heron
Tricolored Heron
4
2
1993
1994
2
26
192
4
2
1995
I
0
499
273
84
439
18
42
5
--
Black-crownedNight-Heron
Yellow-crownedNight-Heron
---
2
I
-8
-1
White Ibis
104
0
225
85
Total
766
229
1,024
656
626
FLEUR¾
m,•VS•-mRR¾
[Auk, Vol. 112
increase in the species' northern Great Lakes and other pesticides,competition, predation, a
breedingpopulation(Conniff 1991,A. May pers. responseto changingweather, and changesin
comm.). The explosive growth of California's habitat.
The AOU calculated that some 5,000,000 birds
White-faced Ibis population (Table 3) may be
attributable to its ability to forage successfully of all specieswere killed each year by plume
in the irrigatedfieldsthat havelargelyreplaced
its former foraginghabitatof backwatersloughs
and flooded meadows (Bent 1926, Small 1975,
Bray and Klebenow 1988).
Foodlimitation may be especiallycriticalduring the nestling phase of the annual breeding
cycle(Rodgersand Nesbitt 1979,Newton 1980,
T. E. Martin 1987, 1991). Mortality in wading
birds peaks in the first two weeks of life, and
remainshigh for fledglings(Kushlan1978).Kahl
(1963) reviewed mortality in several wadingbird species,and found that first year mortality
ranged from 61 to 78%. Abundant food supply
may affectpopulationgrowth by improving the
survivorship of both nestlings and newly
fledged young (Powell 1983, Mock et al. 1987).
Crayfishpondsalsomay increasefledglingsurvival by assuring inexperienced fledglings a
steadysupplyof easilycapturedprey while they
learn to forage. Juvenile wading birds are less
effective foragers than adults in the same hab-
hunters
for the fashion
trade
from
about
1875
through the early 1900s(Buchheisterand Graham 1973),althoughthe questionableaccuracy
of early population estimatesmakesit difficult
to judge the true impact of the plume trade on
wading birds (Robertson and Kushlan 1974,
Frohring et al. 1988,Ogden 1994).Exploitation
of wading birds for plumes, eggs,and food was
an important source of mortality throughout
the southern
United
States as late as the 1930s
for severalspecies(Powell et al. 1989),and some
illegal harvestingprobablycontinuesto this day.
Most wading-bird species,at least in the Gulf
coaststates,had probablyrecoveredfrom plume
hunting by the 1930s (Ogden 1978, Bancroft
1989). Therefore, it is unlikely that the post1940s increaseswe have documented are primarily the result of recovery from the plume
trade or other human exploitation.
Could the observedpopulation increasesreflect expanding ranges of migratory wading
itat (Recher and Recher 1969, Bildstein 1983,
birds outside of Louisiana?Great and Snowy
1993, Draulans 1987a, Cezilly and Boy 1988, egretsare expandingtheir rangesto the north,
Frederickand Spalding 1994). Farm pondsalso possibly reoccupyingportions of their range
have fewer alligators (Alligatormississippiensis)occupiedprior to plume hunting (Ogden 1978,
and other avian predators than natural wetRyder 1978).Great Blue Herons have increased
lands,perhapsbecausethere is little or no cover in northern New York, perhaps in responseto
reforestation of abandoned farmland (Mcaround farm ponds where predators can hide
(Fleury pers.obs.).As a result,birdsrequire less Crimmon 1982).The GlossyIbis and White Ibis
vigilance when foraging in farm ponds, and also have expandedtheir rangesalong the Atmayexperiencelower mortality from predation lantic coast, and the White-faced Ibis has been
while foraging.
extending its range steadily eastwardin LouiOur data supportthe hypothesisof food limsiana.Many wading birds that breed in more
itation for populationsof colonialwading birds northern regions are known to winter in Loubreedingin Louisiana,but further work is need- isianaand elsewherealong the Gulf coast(Byrd
ed to establish
the
mechanism
cause we cannot determine
involved.
the extent
Be-
1978, Hancock and Kushlan 1984, Root 1988).
to which
Detailed banding studies will be required to
local population increasesare due to immigra- determine what proportion of Louisiana'swintion at the regional level versus local recruit- ter populations are resident birds asopposedto
ment, we cannot unambiguouslydemonstrate migrants. Emigration from neighboring states
the role of food limitation in Louisianawading into Louisiana may explain some of the obbirds.
served increases, but in the absence of historic
Possiblealternativehypotheses.--Webriefly data and relevant data from band recoveries,
considerbelow several alternative hypotheses we cannotfully evaluatethis hypothesis(Fredfor the observedtrends in populationsof Lou- erick et al. in press).
Could population increasesbe due to a reisiana wading birds. These hypotheses,which
leasefrom heavypesticidecontaminationin the
are not mutually exclusive,include a long-term
recovery from the effectsof the plume trade, 1950sand 1960s?Per-count data for all species
showrelativelyflat populationcurvesfrom 1949
range expansion of wading-bird species in
to 1960, followed by a general decline beginnorthernstates,recoveryfrom the effectsof DDT
July1995]
Crayfish
Aquaculture
andWading-bird
Populations
ning about 1960and continuing through about
1970 (Fig. 2). CBC efforts during this period
generally were stableor increasing(exceptfor
1963) in number of counts, party-hours, and
party-miles (Fig. 1). The timing of the declines
in Louisiana wading-bird populations corresponds with similar pesticide-related declines
noted for cormorants(Morrison and Slack 1977),
Brown Pelicans(Pelecanus
occidentalis;
Schreiber
and Mock 1987), and raptors (Bednarz et al.
1990).Recoverycoincideswith the banning of
DDT in 1972. We conclude, therefore, that some
of the population increaseswe have documented (Fig. 2) could have resulted from recoveryof populationsfollowing the banningof
DDT and other pesticides.However, not all the
post-1970population changesare attributable
to release from pesticide effects, becausethis
hypothesis predicts rangewide increases,and
these are not observedin Florida and Texas(Table 3).
Competition does not appear to be a major
limiting factor for colonial wading-bird popu-
lations,at leastat presentpopulation densities.
The high positive covariance of wading-bird
populationsin our study arguesagainsta strong
competitive interaction among speciesof Louisiana wading birds (Schluter 1984). The birds
are highly social,breeding and feeding together in large numbers,with a well-defined partitioning of resources(Burger 1978, Kushlan
1978).
Predation is an important limiting factor for
627
success
of many speciesin a particular year, and
alsoaffectCBC countsby depressingvolunteer
efforts. Severe winter weather in 1962, 1963,
and 1968,for example,causedCattle Egretsto
decline in both Florida and national
CBC counts
(Bockand Lepthien 1976,Larson1982),and 1963
winter
storms reduced CBC efforts in Louisiana
to a single count. Although weather fluctuations may explain at least part of the annual
variation in population size, they cannot accountfor the long-termexponentialgrowth of
someLouisianawading-bird populations.
Habitat destructionand degradationdue to
water management, urban development, impoundment, and wetland drainage, coupled
with seve•ral
yearsof drought,havecontributed
to the precipitousdecline of wading-bird populations
in
southern
Florida
and
California
(Small 1975, Kushlan and White 1977, Bancroft
1989, Ogden 1994). Louisianaalso has experienceda significantlossof inland wetland habitatsduring the past40 years(R. P. Martin 1991),
but an abundanceof suitable wading-bird habitat
remains.
Forested
wetlands
in
Louisiana
compriseabout 7,400,000ha, with an additional
8,400,000 ha of nonforested wetlands (Williams
and Chabreck 1986).Louisianawading birds do
not seem to be limited by the availability of
suitable nesting sites, and a recent survey of
wading-bird colonies in southern and coastal
Louisiana found little change in the number
and turnover rate of coloniesfrom the previous
census (Keller et al. 1984, Martin and Lester
1990).Aquaculturepondshavenot significantly
alteredthe quantityof foraginghabitat,but have
sufficientlyto releasewading-birdpopulations. provided a new and higher-quality habitat in
Frederickand Collopy(1989a)observedthat 43% termsof predictabilityin spaceand time, aswell
of wading-birdnestlossin the Evergladeswas as high densitiesof preferred prey.
Not all flooded fields provide high-quality
due primarily to predation by snakesor raccoons(Procyonlotor). Both these predatorsare foraginghabitats.Crayfishpondsfrequentlyare
abundant in Louisianaas well, and probably reflooded after drawdown and seeded with rice
(Huner 1990, Huner and Romaire 1990). Wadaccountfor a significantproportion of nestfailing birdsfrequently foragein thesefloodedrice
ure. We have seenno evidencethat thesepredatorshavedecreasedsignificantlyover the study fields (Cardiff and Sinalley 1989, Reinsen et al.
1991).Although rice fields in Italy provide critperiod. Snake populations have declined locally in areas of severely disturbed habitat, but ical foraging habitat for Black-crownedNightthere has been no noticeable change in total Herons and Little Egrets(Egrettagarzetta;Fasola
populations(B. Prima pers. comm.).Crowsalso 1983, 1986), we found no significant positive
correlation between any wading-bird populaprey on wading-bird nests(Frederick and Collopy 1989a).Analyses of 1949-1988 CBC data tions and changesin rice-field acreagein Lousome avian populations, but there is no evidence to suggestthat predation has decreased
for FishCrows(Corvus
ossifragus)
andAmerican
isiana (Table 4; Fielder and Nelson 1982, Uni-
Crows(C. brachyrhynchos)
show increasingpopulationsthat peakin 1982and declinethereafter
(B. Fleury unpubl. data).
Weather conditions can affect reproductive
versity of New Orleans 1990). Use of a wide
variety of powerful pesticidesin Louisianarice
fieldsmaydepresspopulationsof fish,crayfish,
and other invertebrate prey species(Louisiana
628
FLEI/R¾^Nr) SHERRY
T^BLE7. Total United StatesAnimal Damage Control depredation permit kills authorized and reported in 1991(compiledby U.S. Fish and Wildlife
Service).
Group
Authorized
Reported
Egrets
2,766
1,174
Herons
4,300
2,400
Night-herons
Cormorants
Pelicans
Kingfishers
175
10,814
243
6,789
50
43
216
126
Gulls
1,338
561
Coots
200
134
Anhingas
Grebes
Ducks
Miscellaneous
Total
0
19
70
110
336
20
4,665
24,704
1,483
13,328
StateUniversityAgriculturalCenter 1987).Pesticide use in French rice fields adversely af-
fectedLittle Egretforaging(Hafner et al. 1986)
To summarize our evidence for the hypoth-
[Auk, Vol. 112
and stateand federal wildlife officialsis likely
to increase (Boyd 1991). Current aquaculture
farm-managementpoliciesare focusedon scaring birds from pondswith a variety of nonlethal
methodssuchaspyrotechnics(butanecannons,
"screamer"shotgunshells,etc.), and screening
pondsto limit accessby birds. When nonlethal
methodsfail to reduce bird predation, U.S. Department of Agriculture (USDA) permits allow
limited taking of wading birds (Stickley and
Andrews 1989, Littauer 1990a, 1990b, Boyd
1991).Reportedbird kills in 1991 under USDA
depredation permits, issuedmainly to Mississippi catfishfarmers,included 2,400heronsand
1,174egrets(Table 7), and the National Audubon Society also has documented extensive illegal killing of wading birds (Williams 1992).
Avian predatorson crayfishfarmsactually may
have a minimal effect on the final harvest (Hunet 1976, 1983). Martin (1979) estimated that
wading birds consumedonly about 1% of the
total commercial harvest, and Martin and Ham-
ilton (1985) estimatedconsumptionat lessthan
esisthat crayfishaquacultureis the primaryfac- 2% of the commercial harvest. Becausehigh
tor causingthe dramaticincreasesin wading- crayfishdensityresultsin reducedaveragecraybird populationsin Louisiana,the highestwad- fish size, limited predation by birds actually
ing-bird populationincreasesoccurredduring may benefit crayfish farmers by reducing the
the 20-year period in which the crayfishin- level of intraspecificcompetition among craydustrywasexpanding.Increasedpond acreage fish. The timing and duration of crayfishfarm
correlates best with the abundances of those
drawdownand floodingperiodsacrossthe state,
species
knownto prey mostheavilyon crayfish. and their relation to the reproductive cycle of
Pond drawdowns may influence population colonial wading birds need further investigagrowth by providinga predictableand readily tion, as do the effectson bird populations of
available food resourceduring the incubation, farmerseliminatingaccess
to aquacultureponds.
nestlingand fledging periods.Finally, the al- Gooddocumentationof avianpopulationtrends,
ternative hypothesesare not compelling.
predationrates,andthe actualimpactof wading
Conservation
andmanagement
implications.--Our birds on commercialcropsare needed to estabfindings have important implicationsfor the lish sound management practices.Our study
conservationand managementof wading birds. representsan early step towardsthat goal.
The increasesin several specieswintering in
Louisiana and corresponding increases in
breeding-birdpopulationssuggestthat Louisianawill continue to provide critically important habitat for wintering and breeding North
American colonial wading birds. Current wet-
ACKNOWLEDGMENTS
We thank JayHuner for his invaluableassistance,
and Tom Bancroft, Peter Frederick, Fran James, Jim
Kushlan,JohnOgden, and Allan Strongfor review-
lands managementoften is aimed exclusively ing variousdraftsof thismanuscript.Our researchis
at enhancingduck habitat,particularlyin na- presentlysupportedby the LouisianaCrawfishProtional wildlife refuges,and thesemanagement
motion
and Research Board.
policiesneed to be reevaluatedfor their possibleimpacton colonialwading-birdhabitatre-
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