AGE-RELATED MORTALITY IN A WINTERING POPULATION OF DUNLIN

BARBARAE. KUS) PHILIP ASHMAN, 2 GARY W. PAGE,3 AND LYNNE E. STENZEL 3 •Divisionof EnvironmentalStudies, University of California,Davis, California 95616 LISA; 21242East Dayton Street,Madison, Wisconsin,53703 USA; and 3PointReyes Observatory, 4990 ShorelineHighway, Stinson Beach, California 94970 USA

ABSTRACT.--Despiteconsiderable evidence that juvenile shorebirdsexperience signifi- cantly higher annual mortality rates than adults, identification and quantificationof the sourcesof mortalityhave received little attention.We found that the proportionof juvenile Dunlins (Calidrisalpina) in the kills of a Merlin (Falcocolumbarius) one winter at Bolinas Lagoon,California was greaterthan the proportionof juvenilesin the lagoon'swinter pop- ulation.This is evidencethat raptorpredation may be one of the factorscontributing to the age differencesin annual mortality ratesof shorebirds.We suggestthat the greatervulner- ability of juvenilesto predationby the Merlin may be causedby age-relateddifferences in Dunlin flocking behavior. Received23 March 1983,accepted 6 September1983.

STUDIESof the populationdynamics of shore- Lagoon, reducing the population by as much (Charadrii) indicate that, in general, ju- as 21%over a 5-month period (Pageand Whir- veniles experience higher annual mortality acre 1975). Northern Harriers (Circuscyaneus), rates than adults (Goss-Custard1980). This dif- AmericanKestrels (Falco sparverius), Short-eared ferential mortality hasbeen reportedfor a wide (Asiofiammeus), Peregrine Falcons(Falco range of European(Boyd 1962 and references peregrinus),and Merlins (Falcocolumbarius) are therein) and North American species (e.g. among the more frequent predators of small Holmes 1966, Myers 1980,Page et al. 1983).Al- shorebirds at the lagoon, and attacks by Red- though a difference in survivorship between tailed Hawks (Buteo jamaicensis), Cooper's age-classesappears to be widespread among Hawks (Accipitercooperii), and Great Horned shorebirdpopulations, little attention hasbeen Owls (Bubovirginianus) also occur occasionally. directed toward identifying and quantifying We confined our analysisof the effect of pred- specific sourcesof mortality and evaluating ators on mortality rates to an examination of their contribution to this pattern. Heavy mor- Dunlins killed by a female Merlin present at tality among first-year birds is generally the lagoon during the fall and winter of 1979- thought to be the result of their relative inex- 1980. Merlins are the most significant of the perience in dealing with such selectivepres- Dunlins' diurnal predators at Bolinas Lagoon; suresas predation, feeding efficiency,and ex- during one winter (1972-1973) a single Merlin tremes in environmental conditions (see was responsiblefor approximatelyhalf of the referencesabove); these hypotheseshave not observedpredation on Dunlins by all raptors been tested empirically in shorebirds,how- (Page and Whiracre 1975). Unlike many other ever. shorebird predators at the lagoon, the Merlin Here, we report the results of a study in we studied was relatively predictable in her which we attempted to determine the extent to hunting patterns. This enabled us regularly to which predatorsare responsiblefor age-related observeher hunting and eating sandpipersand differencesin the mortality rates of Dunlins to collect the remains from a large portion of (Calidrisalpina). Specifically, we comparethe age her kills. We were therefore able to base our ratio of Dunlins in predator kills with the age examination of age-related mortality rates on ratio of Dunlins in a population wintering at direct observation of the effect of a single, BolinasLagoon, California in order to evaluate known predator. whether or not the mortality rate due to pre- dation is higher for juvenilesthan for adults. STUDY SITE AND METHODS Predationby raptorsis one of the majorcaus- Studysite.--Bolinas Lagoon is a shallow 570-ha es- es of winter mortality for Dunlins at Bolinas tuary on the central California coast,approximately

69 The Auk 101: 69-73. January 1984 70 Ku$ •T ^•. [Auk, Vol. 101

24 km northeastof San Francisco.This study area and 3000- its shorebirdpopulation have been described by Page et al. (1979).It is a majorover-wintering site for large numbersof shorebirdsthat forage on the expansive mudflats.Dunlins are presentat the lagoonfrom late Septemberthrough early May. During 1979-1980, Dunlin numbersincreased to approximately2,000 by November and remained at this level until the onset of migration in early April (Fig. 1). Age-compositionof the population.--We trapped Dun- linsat approximately2-week intervals throughout the winter in order to determine the ratio of juveniles to adults in the BolinasLagoon population. For each period,our estimateof the proportionof juvenilesin the lagoonpopulation is derivedthrough direct ex- trapolationfrom the age ratio of the corresponding sampleof trappedbirds. We capturedbirds by two OCT NOV DEC JAN FEB MAR APR MAY methods and comparedthe results to evaluate the possibilitythat our samplingtechnique, and there- MONTH fore our populationestimates, were biasedtoward a particularage-class. Although it is difficult to deter- Fig. 1. Number of Dunlins at BolinasLagoon, Oc- mine the nature and extent of biases associated with tober 1979-May 1980. differenttrapping techniques, there is someevidence (Pienkowskiand Dick 1976,Goss-Custard et al. 1981) that the compositionof catchesmade by mist-nets, which we used extensively,may be biased in favor primary, the juvenile being more pointed, of juveniles.In Novemberand December,we caught worn, and narrower than the more bluntly tipped four sampleswith mist-netsand four with noose- feather of the adult. Because this characteristic is more mats. The mist-netswere positionedat dusk over variable than the first, we testedthe reliability of the channelsthat the Dunlins followed as they flew into primary-shapemethod by determiningthe age of the salt marsh to roost for the night. The noose-mats eachbird we capturedindependently by both criteria were hidden beneaththe sandduring the day in areas and comparingthe results.We found that we were where large flocksof Dunlins were feeding or roost- able to age birds by primary feather characteristics ing. Noose-mattraps have the advantageof elimi- alone with 97% accuracy.Although it was not nec- nating any age-relateddifferences in flying expertise, essaryfor aginglive birds,we relied heavilyon the which hasbeen suggestedas a reasonfor the poten- feather-shapetechnique for agingprey from feather tial bias associatedwith the use of mist-nets (Pien- remains, which often did not include the key coverts kowski and Dick 1976).We regressedthe proportion (see below). of juvenilesin eachtrapped sampleagainst date for Age compositionof Merlin kills.--A female Merlin eachmethod, compared the resultantlines, and found was observedhunting sandpipersat BolinasLagoon no statisticaldifferences in either the slopes(P = 0.29) between 12 October 1979 and 14 March 1980. The or the Y-intercepts(P = 0.18). While this evidence Merlin generallyspent the entire day on the lagoon, suggeststhat our estimatesof the agestructure of the restingor hunting birds on the mudflatsby launch- populationwere not differentiallybiased by the two ingattacks from low perches in theadjacent salt marsh. methods,it doesnot refute the possibilitythat both Followinga successfulhunt, shereturned to one of trapping techniquesmight be similarly biasedto- theseperches where she ate the entire prey except ward catchingjuveniles. However, any biasthat re- for the body feathersand parts of the wings, which suited in our overestimatingthe proportion of juve- were discardedat the baseof the perch.We checked niles in the population would only make the the Merlin's perches regularly and collected the experimenta conservativetest of our hypothesisby feather remains of any kills we found. We aged all making a selective preference by Merlins for juve- Dunlin kills in the manner describedabove, except niles more difficult to detect. for seven for which there were insufficient remains. Eachbird was banded with a unique color combi- Statisticalanalyses.--We used a multiple linear nation for individual recognitionand was aged by regressionmodel to quantify the proportionof ju- plumagecharacteristics. Birds were consideredto be veniles in the population and in the Merlin's kills immatureif they had buffy edgeson their innermost (dependentvariables) as a functionof periodduring tertials or inner middle-wing coverts and adult if the winter season(independent variable). Because our these were white- or grey-tipped. Age can samplesof trappedbirds were largerin sizethan our also be determined by the shape of the outermost samplesof prey remains,we useda weightedleast January1984] DunlinMortality 71

1,00 0 POPULATION (N=218)

ß oMERLIN KILLS (N=66)

0.50

0.00 I I I I I I I I [ I I I I I I I 28 OCT- I7 NOV- 7 DEC- 27 DEC- 16 JAN- 5 FEB- 25 FEB- 17MAR- 6NOV 26NOV 16DEC 5JAN 25JAN 14FEB 6MAR 26MAR

PERIOD Fig. 2. Proportionof juvenile Dunlins in the populationand in the Merlin's diet at BolinasLagoon in winter, 1979-1980.

squaresanalysis (Dixon 1981)to comparethe slopes Figure 2 illustrates the proportion of imma- and Y-intercepts of the two lines. This technique ture Dunlins in the total population (Y = weightseach point in the regressionaccording to its 75.19- 3.26x) and in the Merlin's diet (Y = associatedsample size and thus correctsfor any in- 86.67 - 2.08x) at different periods throughout equalitiesin error variance that might exist between the winter. As has been previously reported by cases.All significancelevels presented are for two- tailed tests. Page (1974),juveniles arrive earlier than adults at Bolinas Lagoon, which accountsfor the ini- tial predominance of immature birds in the RESULTS population. The subsequent arrival of adult Dunlins causesthe proportion of young birds We caught a total of 218 Dunlins during sev- in the population to decrease.The age ratio sta- en sampling periods spanning the winter sea- bilizes by midwinter, although in Fig. 2 it ap- son. During these and four additional periods, pearsto continue to decline becauseof the lin- we collected 146 of the Merlin's prey remains, ear function used to describe the data. 73 of which were Dunlins. Of the Dunlin re- The relative decline in availability of imma- mains, 40% (n = 29) were collected after actual ture Dunlins during the winter is reflected in observationof the Merlin eating the bird. Of the Merlin's diet; yet, a comparisonof the two the 73 Dunlin kills, 9% (n = 7) were excluded regressionlines reveals that young birds con- from our analysisbecause not enough material sistently incurred a higher rate of predation was present to determine age. than did adults (comparison of Y-intercepts, We believe that our sample of prey remains P = 0.012; comparison of slopes, P = 0.44). In- was a good indicator of the Merlin's diet on spectionof the data for each sampling period the lagoon throughout the winter. For the win- in Fig. 2 shows that during the 28 October-6 ter of 1972-1973, Page and Whitacre (1975) es- November period, the Merlin's diet included a timated that the Merlin at Bolinas Lagoon ate substantially lower proportion of juvenile an average of 2.2 prey per day. Basedon this Dunlins relative to their occurrencein the pop- estimate, our sample of prey remains repre- ulation than during any other period of the sents 43% of the Merlin's prey for the 155 days winter. We are unable to explain this in either she was known to be on the lagoon. a methodologicalor biological context and on 72 Kus ETAL. [Auk, Vol. 101 these bases have no reason to exclude the ob- age-classeswith respectto these determinants servation from the data presentedhere. Statis- of risk could explain, in part, the high level of tically, however, it can be demonstratedthat juvenile mortality that we observed in the this particular point does not fall within the presentstudy (Kus unpubl. data).Additionally, confidenceinterval of the regressionline cal- juvenile vulnerability may be enhanced by a culated when the point is excluded (P = 0.02; general inexperience in avoiding attacks by Snedecorand Cochran 1967: 157). Comparison predators.Young birds may fail to respondap- of this second line (Y = 103.17 - 3.47x) with propriately to flock alarm calls or may be less that representing the population composition coordinatedin maintaining synchronywith the showsthat they differ significantly(P = 0.006) rest of the flock during an attack, thereby in- in their Y-intercepts. creasingtheir vulnerability even further.

DISCUSSION ACKNOWLEDGMENTS

The resultsof this studyindicate that Dunlin We are grateful to the many PRBOvolunteers who helped us capture, band, and count Dunlins on Bol- mortality due to predation is age related, ju- inas Lagoon. Michael Miller contributed invaluable veniles experiencinga higher rate of predation assistancewith statisticalanalyses. David Ainley, Joel than adults. Although we examinedthe diet of Berger, Robert Boekelheide, John Bulger, David only onepredator, we suspectthat theseresults DeSante, Jan Mendelson, Tom Miller, J.P. Myers, may apply to other predatorsas well. An anal- Matthew Rowe, Steve Smith, David Winklet, and an ysis of kills made by Short-earedOwls during anonymousreviewer provided helpful commentson the studyat BolinasLagoon also showed a pre- the manuscript. William J. Hamilton III lent support ponderanceof juvenilesamong the Dunlin re- and helpful commentsthroughout this study. Mar- mains (73% juveniles, n = 18). More complete garetGreene provided essential logistic support. This information concerning the hunting methods study was funded in part by grants to the senior au- thor from the U. C. Davis Jastro-Shields Research and diets of specificpredators will be required ScholarshipFund, the Frank M. Chapman Memorial before the generality of our resultscan be as- Fund, and Sigma Xi, the ScientificResearch Society. certained. This is contribution number 254 of Point ReyesBird The compositionof the Merlin's diet at var- Observatory. ious stagesof the winter appearsto be deter- mined, in part, by the relative availability of LITERATURE CITED adult and juvenile Dunlins on the lagoon, as evidencedby the nearly parallel decline in the BOYD,H. 1962. Mortality and fertility of European two regression lines in Fig. 2. Dunlins are Charadrii. Ibis 104: 368-387. clearlynot selectedat randomfrom the popu- DIXON, W. J. 1981. BMDP statistical software. Los lation, however. Angeles, Univ. California Press. Several aspectsof the distribution and be- GOss-CUSTARD,J.D. 1980. Competitionfor food and havior of Dunlins on the lagoon may contrib- interference among waders. Ardea 68: 31-52. --, S. E. A. LE V. DIT DURELL, H. SITTERS,& R. ute to this age difference in vulnerability (Kus SWINFEN.1981. Mist-nets catch more juvenile 1980,in prep.). Dunlins associatein highly co- Oystercatchersthan adults.Wader Study Group hesive flocks, both on the ground and in the Bull. 32: 13. air. Flocking in Dunlins appearsto facilitate HOLMES,R. T. 1966. Breeding ecology and annual early detectionof approachingpredators and is cycle adaptationsof the Red-backedSandpiper particularly important in thwarting attacksby (Calidris alpina) in Northern Alaska. Condor 68: the Merlin, whose successin hunting is deter- 3-46. mined, in large part, by its ability to surprise Kus, B. E. 1980. The adaptive significanceof flock- flockson the ground (Pageand Whitacre 1975, ing among wintering shorebirds.Unpublished M.S. thesis. Davis, California, Univ. California. Kus 1980). Additionally, the synchronousma- MYERS,J.P. 1980. Sanderlings(Calidris alba) at Bo- neuvers of airborne flocks appear to hinder the dega Bay: facts,inferences, and shamelessspec- Merlin's ability to single out a particularindi- ulations. Wader Study Group Bull. 30: 26-32. vidual for attack. Moreover, individual vulner- PAGE,G. 1974. Age, sex,molt and migration of Dun~ ability is inverselyrelated to flock size and is lins at Bollhas Lagoon. Western Birds 5: 1-12. increasedwith greater proximity to the flock , •: D. F. WHITACRE. 1975. Raptor predation periphery (Kus in prep.). Differencesbetween on wintering shorebirds.Condor 77: 73-83. January1984] DunlinMortality 73

, L. E. $TENZEL,& C. M. WOLFE. 1979. Aspects PIENKOWSKI,M. W., & W. J. A. DICK. 1976. Some of the occurrence of shorebirds on a central Cal- biases in cannon- and mist-netted samples of ifornia estuary. Studies Avian Biol. 2: 15-32. wader populations.Ringing & Migr. 1: 105-107. --, D. W. WINKLER, •t C. W. SWARTH. $NEDECOR,G. W., •t W. G. COCHRAN. 1967. Statistical 19•3.Spacing outat Mono Lake: breeding suc- methods. Ames, Iowa, Iowa State Univ. Press. cess,nest density, and predation in the Snowy Plover. Auk 100: 13-24.

The Frank M. Chapman Memorial Fund gives grants in aid of ornithologicalresearch and also post- doctoralfellowships. While there is no restrictionon who may apply, the Committee particularly welcomes and favorsapplications from graduatestudents; projects in game managementand the medicalsciences are seldomfunded. Applicationsare reviewed once a year and should be submitted no later than 15 January. Applicationforms may be obtainedfrom the Frank M. ChapmanMemorial Fund Committee,The American Museum of Natural History, Central Park West at 79th St., New York, New York 10024. Dr. Robert E. Bleiweisswas appointed Chapman Fellow for the period September 1983 through August 1984.He will studythe systematicsand speciationin Andean hummingbirdgenera. Dr. RobertM. Zink, also appointed a Chapman Fellow for September 1983 through August 1984, will study the systematicand bio- geographicrelationships of thrashers(Toxostoma) and the Brown Towhee complex (Pipilo)in the aridlands of North America. Chapmangrants during 1983,totalling $34,623with a meanof $533,were awardedto: Marianne G. Ainley, Americanwomen ornithologists:their contributionsto science1900-1950; Peter Arcese,correlates and con- sequencesof dominancebehavior in the Song Sparrow (Melospizarnelodia) on Mandarte Is., B.C.;Christopher Paul Lyman Barkan,risk sensitiveforaging behavior of Black-cappedChickadees; James C. Bednarz,ecolog- ical study of the cooperativelybreeding Harris' Hawk; Paul S. Bencuya,parental role partitioning in the Merlin and its adaptivesignificance; Craig W. Benkman,food availability, foragingefficiency and the regu- lation of Crossbills(Loxia) in easternNorth America;David E. Blockstein,reproductive behavior and parental investmentin the Mourning Dove (Zenaidarnacroura); Jeffrey D. Brawn, effectsof density on the breeding biology of WesternBluebirds; Mark Andrew Brazil,winter distributionand comparativeecology of Steller's and White-tailed SeaEagles in north-eastHokkaido, Japan;Charles R. Brown, costsand benefitsof coloniality in Cliff Swallowsin Nebraska;Angelo Paul Capparella,effects of riverine barrierson gene flow in Amazonian forest undergrowthbirds; Peng Chai, factorsaffecting prey selectionof variousinsect taxa by the Rufous- tailed Jacamar(Galbula ruficauda); Jeffrey D. Cherry, interaction of magnetic, stellar, and sunsetcues in the orientation of nocturnal restlessnessof migratory sparrows;Bruce A. Colvin, Barn nesting, population dynamics, and dispersion in southwest New Jersey;Nonie Coulthard, White-throated Bee-eater (Merops albicollis):cooperative breeding in relation to ecologicalfactors; Janice R. Crook, helping and infanticide in the Barn Swallow; Thomas A. Davis, water balance and incubation physiology of avian in wet nests; William JamesDavis, acousticcommunication in the Belted Kingfisher(Megaceryle alcyon); John Merrit Emlen and R. Cary Tuckfield, ecologicaland evolutionary determinantsof song variation in Black-throatedSpar- rows: R. Todd Engstrom,geographic variation in clutch size in birds; SusanEvarts, test of the incidence of multiple paternity in Mallard (Anasplatyrhynchos) clutches; Anthony David Fox, summer feeding ecologyof the GreenlandWhite-fronted Goose, with particularemphasis on pre-nestingbehaviour; Thomas L. George, density compensationon BajaCalifornia islands:the role of predation;J. ChristopherHaney, selectionpres- suresand the evolution of socialityin the White-throated (Cyanolycamirabills) and Silver-throated(C. argen- tigula)Jays; Lise A. Hanners, influencesof parent/offspringbehavior on Laughing Gull (Larusatricilla) re- productive success;Sara M. Hiebert, control of torpor in ;A.M. Hindmarsh, vocal mimicry of starlings on Fair Isle; Peter W. Houde, Lithornithidae of North America; David Hutchinson, Discovery Park Anna's projectand the range expansionof Anna's Hummingbirdsin the Pacificnorth- west; Victoria Ingalls, avian predator strategiesto insect startle defences:the habituation of blue jays to successivepresentations of startling stimuli; Paul A. Jones,breeding biology of a founding colony of Cali- fornia Gulls (Laruscalifornicus), Alviso, California; Jan Kalina, socioecology of Black-and-WhiteCasqued Horn- bills (Bycanistessubcylindricus) and their role as seed dispersersfor tropical rainforest trees; Chris Kellner, predation ecologyof American Kestrelswintering in central Kentucky; DouglasJ. Levey, seedsize and the

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