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Received
28 April 1997,accepted
2 February1998.
Associate
Editor:M. E. Murphy
TheAuk 115(3):786-791,1998
AssortativeMating and Sexual Size Dimorphism in Westernand
SemipalmatedSandpipers
BRETT K. SANDERCOCK •
Department
of Biological
Sciences,
SimonFraserUniversity,
Burnaby,BritishColumbia
V5A 1S6,Canada
Sexualdimorphismin body size is widespread
amonganimals,and mostexplanationsfor the evolution of dimorphismcanbe groupedinto two categories:(1) sexual selection,and (2) intraspecific
nichedifferentiation(Jehland Murray 1986,Shine
1989).Sexualselectionclearlyhasbeenimportantin
shorebirds(suborderCharadrii),becausethe direction and magnitudeof size dimorphismare related
nogamously(JehlandMurray 1986,OlsenandCockburn 1993).
Female-biasedsexual size dimorphism could
evolveif the sexesbecomeadaptedto differentecological conditions(Shine 1989). Alternatively,female-biaseddimorphismmay result if sexualselection actssuchthat fitnesscovariespositivelywith
body size for femalesbut negativelyfor males.En-
to both matingsystemand the durationof parental ergy storagecapacityincreaseswith structuralsize,
care (J6nssonand Alerstam 1990, Reynolds and andlarge-bodied
femalesandpipers
mayhaveanadSz6kely1997).Typically,the sex that competesfor vantageif they are able carry resourcesthat allow
mating opportunitiesis larger, and this is true for them to breedearlieror lay largereggs(Erckmann
mostpolygynousshorebirds(maleslarger)andspe- 1981,Jehland Murray 1986,J6nsson1987).On the
cieswith sex-rolereversalwherefemalesarelarger otherhand,energyefficiencydecreases
with increas(JehlandMurray1986,J6nsson
andAlerstam1990). ing body size(J6nssonand Alerstam1990).Male calIt is moredifficultto explain,however,why females idrine sandpipersoftengive complexdisplayflights
are the largersexin manyshorebirds
that matemo- duringcourtship(Miller 1979),and smallbody size
may allow for increasedagility in theseaerial displays(Jehland Murray 1986,Blomqvistet al. 1997).
• Presentaddress:University of California, Eco- Negativeassortativemating could further increase
system SciencesDivision, Department of Environ- selectionon body size if birds pair with respectto
mentalScience,
PolicyandManagement,151Hilgard morphology.Examplesof negativeassortativematHall, Berkeley,California94720,USA. E-mail:
ing are rare in birds,and theyusuallyare basedon
bsanderc@nature'berkeley'edu
color-morphpreferences
(Partridge1983,Houtman
July1998]
ShortCommunications
and Falls 1994).Nevertheless,
negativeassortative
matingwith respectto body sizehasbeenreported
in at leastthreespeciesof monogamous
sandpipers:
LeastSandpipers(Calidrisminutilla;Jehl1970),Stilt
Sandpipers(C. himantopus;
Jehl1970),and Dunlins
(C. alpina;J6nsson1987).
WesternSandpipers(Calidrismauri)havethehigh-
estfemale-biased
sizedimorphismamongall of the
monogamoussandpipers(Jehland Murray 1986:50).
If sexualselectionmaintainssexualsizedimorphism
in monogamous
shorebirds,
the processes
mightbe
mostpronounced
in thisspecies.
Toevaluatethepotential importance of sexual selection,I examined
787
tion); thesemales had smaller culmensthan their
matesin almostall of theirpairings(96.2%,n = 26).
Moreover,individuals that paired with different
birdsin separateyearsusuallyhad mateswith culmensthat consistently
were eitherlongeror shorter
than their own (88%,n = 32 possiblecases).Therefore,I assumedthatthebird with thelongestculmen
in eachpair wasthe female.Errorsin sexingshould
havehad little effecton my conclusions
becausethe
few exceptions(3 of 4) usuallywere due to pairs of
Semipalmated
Sandpipersthat had culmensof similar length(i.e. <0.8 mm difference).
Statisticswere calculatedusingSAS(SASInstitute
1990);all testswere two-tailed and consideredsignificantat probabilitylevelslessthan 0.05.Paramet-
whethertherewas:(1) assortativematingfor body
size, and/or (2) a relationshipbetween fecundity
and body size in WesternSandpipersand Semipal- ric tests were used because all of the variables were
matedSandpipers(C. pusilia).
normallydistributed(Shapiro-Wilk's
test,P > 0.05),
Methods.--I studied Western and Semipalmated with theexception
of timingof laying.Timingof laysandpipersat a 4-km2 study site located21 km east ing variedamongyears,but eggvolumeand modal
of Nome,Alaska(64ø20'N,164ø56'W)during May to clutchsizedid not (Sandercock1997).I standardized
July, 1993 to 1995. Like most monogamousshore- clutchinitiationdatesby subtractingthemedianlaybirds, malesare territorial and give display flights ing date (for eachspeciesin a givenyear) and then
duringcourtship.Femaleslay a modalclutchsizeof rank-transforming
the adjusteddate.I calculateda
four eggs,bothsexesincubatethe clutch,and males meaneggvolumefor eachclutch,andpooledclutchprovide most of the parental care after hatching esof two andthreeeggsin theanalysisof clutchsize.
(Gratto-Trevor 1991, Sandercock 1997). Nests were Somebirds and pairs were observedin more than
locatedby observingsandpipers
thatflushedor gave oneyear,but I includedreproductivedataonly from
distractionbehaviors.Egg-laying rates were 0.8 the firstyearthata bird wascaptured.In theanalysis
eggs/day, and duration of incubationwas 20 (Semi- of assortativemating,eachuniquepair wasincluded
palmatedSandpiper)or 21 days(WesternSandpiper; only onceto avoidpseudoreplication.
Sandercock1998). Date of clutch initiation was cal-
Univariate
measures
often are used to describe
culatedby backdatingfrom egg laying, stageof incubation(determinedby floatingeggsin water), or
dateof hatching.Clutchsizewasdeterminedby revisitingnestsfoundduring layinguntil eggnumber
wasunchanged.
Egglength(L) andbreadth(B)were
recordedwith calipers,and eggvolume(V) wasestimated using:
avian morphology,but such variablesmay not be
representativeof overall body size (Freemanand
Jackson1990). I used principal componentanalysis
to createan indexof body sizebasedon culmen,tarsus,and wing lengths(Risingand Somers1989).I
used the morphometrics
from the first captureoccasionand treatedeachspeciesseparately.
All of the
eigenvectors
of principalcomponent
1 (PC1)showed
V = 0.47LB2
(1)
positive loadings,and PC1 explained70.4% and
68.5%of the variationin body size of Westernand
(B. Sandercockunpubl. data).
Incubatingbirds were capturedin walk-in traps Semipalmatedsandpipers,respectively.
placedoverthe clutch,and both parentswere capResults.--Femalesandpipers were significantly
tured on most nests. Each bird was individually largerthanmalesin all univariatemeasures
of body
markedwith coloredleg bands,and I measuredex- size (Table1). The sexualsizedimorphismof Westposedculmenlength,tarsuslength,and lengthof ern Sandpipers(culmen 16.3%, tarsus 6.2%, wing
flattenedwing. Plumageis sexuallymonochromatic 3.6%)was greaterthan that of Semipalmated
Sandin both species.WesternSandpiperswere sexedby pipers(culmen9.0%, tarsus3.4%,wing 2.0%).Uniculmenlength(<24.2 mm = male, >24.8 mm = fe- variate measuresof body size were not significantly
male) becausepreviouswork with sacrificedbirds correlatedwithin matedpairsof WesternSandpipers
hasshownthat this technique
is 95%reliable(Page (culmen, r = 0.14, P = 0.10; tarsus, r = 0.10, P - 0.27;
and Fearis1971,Cartar 1984).A smallproportionof wing, r = 0.17,P = 0.07,n = 126)or Semipalmated
WesternSandpipers(3.6%,n = 196)had a culmenof
intermediatelength(24.2to 24.8mm); I inferredthe
sex of these birds from the culmen measurements of
Sandpipers(culmen,r = -0.05, P = 0.60;tarsus,r =
-0.14, P = 0.11;wing, r = -0.08, P = 0.40, n = 118).
Multivariatetechniquesgave the sameresults:fetheir mates.SemipalmatedSandpiperswere more male body size and male body size (PC1) were not
difficultto sexby externalmorphologyalone.A sub- correlatedin either sandpiperspecies(Western
sampleof birds (n = 15) was sexedasmalesby their Sandpiper,r = 0.13, P = 0.16;SemipalmatedSandbehavior(i.e. courtshipdisplays,copulatoryposi- piper, r = -0.14, P = 0.13).The powerto detecta
788
ShortCommunications
[Auk, Vol. 115
TABLE1. Univariatemeasurements
(• _+SD) of WesternSandpipersandSemipalmated
Sandpiperscaptured
near Nome, Alaska.
Variable
Culmenlength(mm)
Tarsuslength(mm)
Wing length(mm)
n
Culmenlength(mm)
Tarsuslength(mm)
Wing length(mm)
n
Females
26.4 _+1.1
23.7 -+0.8
101.1_+2.6
Males
Western Sandpipers
22.4 _+1.0
98
22.2 _+0.8
97.5 _+2.5
98
SemipalmatedSandpipers
18.7 +_0.9
17.1 _+0.8
22.4 _+0.8
21.6 _+0.8
99.1 _+2.7
97.0 _+2.5
106
95
t
P
27.3
12.9
9.7
<0.001
<0.001
<0.001
--
13.1
7.5
5.5
--
--
<0.001
<0.001
<0.001
--
correlationwashigh,becausean r-valuehigherthan
0.18would havebeensignificantin eitherspecies.
Therewasa significantrelationshipbetweenbody
size and date of clutchinitiationin male Semipal-
bius;Hedenstr6m 1987) and SemipalmatedPlovers
(C. semipalmatus;
Teatherand Nol 1997),but older,
Less than 7% of the variation in date of clutch initi-
causesample sizes (118 to 126 pairs) were greater
than thoseof Jehl(29 to 41 pairs) and J6nsson(33
pairs). I note, however,that differencesin method-
largerbirdsmayhavepairedtogether.Negativeassortativematingfor body sizehasbeenreportedin
mated Sandpipers(y = 72.3 - 9.74x;r2 = 0.04, t =
Dunlins, Least Sandpipers,and Stilt Sandpipers
-1.19, P = 0.049)andbetweenbodysizeand dateof (Jehl1970,J6nsson1987).Differencesin reproducclutchinitiation in femaleWesternSandpipers(y = tivebiologycannotexplainwhyWestern
Sandpipers
52.8 + 10.3x; r2 = 0.05, t = 2.17, P = 0.03). Large and Semipalmated
Sandpipersdid not showassortmalesand smallfemalesnestedsignificantlyearlier, ative mating. All five calidrine speciesare small,
trendscontraryto the predictionthat early nesting male-territorialshorebirds
thatbreedmonogamousactsas a disruptiveselectivepressureon body size. ly in the arctic.My conclusions
shouldbe robustbeation was explainedby body size in either species.
Body size did not differ betweenfemalesthat laid
two or three versusfour eggs (t-tests,P > 0.40).
Thus,largeand smallfemalesdid not differ in clutch
size.Finally,therewasa significantrelationshipbetween mean egg volume and female body size in
bothWesternSandpipers
(y = 6.86+ 0.22x;r2= 0.15,
t = 3.96,P < 0.0005)and SemipalmatedSandpipers
(y = 6.23 + 0.11x;r2 = 0.09, t = 3.21, P < 0.005). Fe-
malebody size accountedfor 9 to 15% of the variationin eggvolumein bothspecies.Meaneggvolume
increasedby 17% and 9% over the range of female
body sizes in Westernand Semipalmatedsandpipers, respectively.
Discussion.--Western
and SemipalmatedSandpiperswere sexuallydimorphicin body size,and the
greatest degree of dimorphism was in culmen
length.This is consistentwith previousreportsfor
theseandothershorebirdspecies
(Ouelletet al. 1973,
Cartar 1984,Mueller1989).Despitethe pronounced
differencein culmen length betweenfemale and
maleWesternSandpipers(16.3%;range2 to 16%for
other monogamoussandpipers;Jehl and Murray
ologymayhavebeenimportant.Jehl(1970)andJ6nsson (1987) calculatedmultiple correlationsbetween
univariate measuresof body size and fecundity.
Their resultsmighthavebeennonsignificant
if they
had adjustedthe degreesof freedomfor thenumber
of tests(Rice1989)or describedbody sizewith multivariatestatistics(Risingand Somers1989,Freeman
and Jackson1990). Moreover, neither Jehl nor J6nsson comparedthe morphologyof mated pairs di-
rectly.Bothauthorsregressedan indexof intrapair
dimorphism(a differenceor ratio betweenfemale
and malebody size)on timing of breeding(dateof
hatchingor egg-laying),andfoundthatearlybreeding pairshad greaterdimorphism.However,a relationshipbetweentiming of breedingand body size
in one sex could producethe sameresult without
negativeassortativemating.Therefore,the evidence
for negativeassortativemating in calidrine sandpipers seemsweak.
The relationshipbetweentiming of breedingand
bodysizewassignificantin thisandin otherstudies
1986, J6nssonand Alerstam 1990), I found no evi- of sandpipers(Jehl1970,Erckmann1981:228,J6nsdencefor assortativemating in this speciesor in son1987).Unlikepreviousstudies,however,I found
SemipalmatedSandpipers.
no evidencethat small malesor large femalesbred
Assortativematingfor body sizecanbe produced earlier.My resultsdo not supporttheideathat sizeby passivesize-dependent
variationin the availabil- dependentvariationin timing of laying has led to
ity of mates,or by activematechoice(CookeandDa- sexual size dimorphism. Moreover, it is unclear
vies 1983).Positiveassortativemating for body size whetherearlynestingconfersa reproductive
advanhasbeenreportedin RingedPlovers(Charadrius
du- tagein sandpipers.
Predationon shorebird
nestscan
July1998]
ShortCommunications
789
behighearlyin thebreedingseason(Byrkjeda11980, sourcesfor egg laying, particularly in specieslike
Pienkowski1984).Low natalphilopatrymakesit dif- Western Sandpipers and Semipalmated Sandpificultto measuresurvivalratesofjuvenileshorebirds pers,wherethe short-billedmalesprecedethe fe(Thompsonet al. 1994),but Lanket al. (1985)found malesnorth during spring migration(Harrington
no evidence of seasonal declines in the recruitment
1982,Butleret al. 1987).Male sandpipersoften are
of SpottedSandpipers(Actitismacularia).
the soleprovidersof parentalcareafter the young
In my study, large females laid large eggs, as hatch (Gratto-Trevor1991), and a short bill could
foundfor severalotherspecies
of shorebirds
(Miller help males to glean prey in terrestrial habitats
1979,J6nsson1987,BlomqvistandJohansson
1995). (J6nsson1987). Future researchshould examine
Egg sizehas a residualpositiveeffect(controlling whethersexualdimorphismin bill morphologyaffor parentquality) on the growth and survivalof fects foraging rates and/or microhabitatpreferyoungin other precocialbirds (e.g. Dawsonand encesof femaleand male sandpipers.
Clark 1996),and large body size may confera fitAcknowledgments.--I
am gratefulto the manyfield
nessadvantageto femalesandpipers.The relation- assistants
who workedhard to find sandpipernests
shipbetweenegg sizeand body sizecouldbe con- and catchbirds duringthis project.SitnasaukNafoundedby femaleage(Coochet al. 1992),but year- tive Corporationkindly permittedaccessto their
ling breeders were relatively uncommon at Nome,
and egg size did not increasewith relative age(San-
land for this research,and R. E. Gill, Jr.(Alaska ScienceCenter, U.S. GeologicalService)and R. Harris
dercock1997).The mostparsimonious
explanation (NationalParkService)providedlogisticalsupport.
for the lackof a relationshipbetweenbodysizeand
egg number is the low variability in clutch size;
most females (66.7 to 91.7%) laid four eggs (San-
The manuscriptwas improvedby commentsfrom
S. R. Beissinger, E Cooke, C. Gratto-Trevor, D. B.
Lank, K. Martin, T. D. Williams, and an anonymous
dercock 1997).
reviewer.I wassupportedby graduatefellowships
and teachingassistantships
from Queen'sUniversexualsizedimorphism
throughvariationin mating sity and SimonFraserUniversity.Fundingfor this
success.Erckmann (1981) and Jehl and Murray researchwas providedby the CanadianWildlife
(1986)arguedthatmalesarethesmallersexbecause Service, the CWS/NSERC Research Chair in Wildtheygivedisplayflightsduringcourtship,andaerial life Ecology,a Dean'sGrant for Doctoral Field Travagilityis predictedto increase
with smallbodysize el (Queen'sUniversity),theNorthernStudiesTrainIntrasexual competition for mates could lead to
(Anderssonand Norberg 1981).Recentstudiessup-
ing Program, an NSERC Operating Grant (to E
port this hypothesis:
the rate and durationof aerial Cooke),and awardsfrom the FrankM. Chapman,
displayis negativelycorrelatedwith body size in JohnK. Cooper,and JenniferRobinsonmemorial
male Dunlins (Blomqvist et al. 1997), and male
funds.
NorthernLapwings(Vanellus
vanellus)
thatgivecomplex aerial displaysobtain more mates (Gronstol
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Received
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2 February1998.
Associate Editor: K. Martin
TheAuk 115(3):791-797,1998
SubadultMovement Patternsof the EndangeredHawaiian Stilt
(Himantopusmexicanusknudseni)
J. MICHAEL REED,•'6MICHAEL D. SILBERNAGLE,
2 KAREN EVANS,3
ANDREW ENGILIS, JR.,4 AND LEWIS W. ORING5
•Department
of Biology,
TuftsUniversity,
Medford,Massachusetts
02155,USA;
2U.S.FishandWildlifeService,66-590Kamehameha
Highway,Room26, Haleiwa,Hawaii96712,USA;
3U.S.FishandWildlifeService,
P.O.50167,Honolulu,
Hawaii96850,USA;
4DucksUnlimited,3074GoldCanalDrive,Rancho
Cordova,
California95670,USA;and
sProgram
in Ecology,
Evolution,
andConservation
Biology,
UniversityofNevada,
1000 ValleyRoad,
Reno,Nevada 89512, USA
Data on individual movementpatternsare important for understandingforagingpatterns,mate acquisition,and dispersal(Baker1978,Krebsand Inman 1992,Colwell and Oring 1989,Reedet al. 1999).
Morerecently,requirements
for conservation
biology
pendspartly on agriculturaland aquaculturalpractices(e.g.runoff from taro, rice,and sugarcanefarm-
ing)thatprovidebreedingandforaginghabitat(Bro-
shears1979,Griffin et al. 1989).Dependenceon agriculture, coupled with habitat conversionfor
have resulted in increased interest in the movements
housingand business,has resultedin a fragmented
of individual(abilitiesandpatterns)because
of their andreducedwetlandlandscape(Shallenberger
1977,
relationships
to population
persistence
in fragment- Coleman1981,Griffin et al. 1989)and disjunctdised landscapes.
Immigrationis necessary
to maintain tributionsof waterbirds(Reed and Oring 1993, Enlocal componentsof metapopulations(Brown and gilis and Reid 1994,Reed et al. 1994).Hawaiian Stilt
Kodric-Brown1977),and the parameterthat deter- populationsizedecreased
substantially
earlyin this
minesthe amountof interactionamongcomponents century until the 1940s(Munro 1944) but increased
of a metapopulationis dispersal(Hansson1991,Wu during the last 50 years(Reedand Oring 1993)to its
et al. 1993).
In this paper,we presentdata on movementsof
current population size of around 1,300. Hawaiian
Stiltsappearto be habitatlimited (Reedet al. 1998)
subadult Hawaiian Stilts (Himantopusmexicanus
and are threatenedconstantlyby exoticpredators
knudseni),
an endangeredsubspecies
confinedto the
and exoticwetland plants that make wetlandsunmain Hawaiian Islands. Most of the data are from the
suitablefor breedingand foraging(Engilisand Reid
islandof Oahu,but we alsopresentinformationon
1994).This shorebirdstudyis unusualin that it fomovements
amongislandsacrossthe rangeof the
subspecies.Hawaiian Stilts forage in shallow water cuseson short-term,predispersalmovements.Alexistfor some
and neston adjacentflatsand embankments(Cole- thoughdataon shorebirdmovements
theytypicallyfocuson dispersal,migration,
man 1981). Current wetland area in Hawaii is less species,
than 30% of its original extent (Dahl 1990, Engilis or foraging(e.g.Oring andLank 1984,Warnocket al.
and Pratt1993),andthepopulationsizeof stiltsde- 1995, Butler et al. 1997).
Studysystem.--HawaiianStilts inhabit sevenislands(Hawaii, Kauai,Maui, Molokai,Oahu,Niihau,
E-mail: mreed@tufts.edu
Lanai), althoughonly the Oahu, Maui, and Kauai