Local Survival of Dunlin Wintering in California ’
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The Condor 99:90&9 I5 B The Cooper Ormthological Society 1997 LOCAL SURVIVAL OF DUNLIN WINTERING IN CALIFORNIA ’ NILS WARNOCK~ Wildlife, Fisheries and ConservationBiology, Universityof California, Davis, CA 95616 and Biology Department,San Diego State University, San Diego, CA 92182 GARY W. PAGE Point ReyesBird Observatory,4990 ShorelineHwy., StinsonBeach, CA 94970 BRETT K. SANDERCOCK Department of Biological Sciences,Simon Fraser University,Burnaby, BC V5A IS6, Canada Abstract. We estimatedlocal annual survival of 1,051 individually color-bandedDunlin (Calidris alpina) at Bolinas Lagoon, California from 1979 to 1992. Resighting rates for birds banded as adults varied significantly among years, and resighting rates for first-year birds varied by sex and year. No significant differences in local survival rates were found between males and females in any age classes. First-year birds had lower local survival rates than adults. We suspectthat raptor predation accounted for much of this difference and other variation in survival rates. Adult Dunlin had lower local survival rates in the year of capture than in subsequentyears. Variation in resighting of some groups of individuals including transient Dunlin may accountfor some differences. However, capture and release of Dunlin may induce short-term behavioral changesthat increase the risk of depredation by avian predatorswithin the first few days after capture. Key words: D&in, Calidris alpina, local survival, shorebirds,predation, age and sex effects. INTRODUCTION Sandercock and Gratto-Trevor 1997). Away Demographic data are critical for assessingthe from the breeding grounds, where shorebirds status of avian populations (Lande 1988), but spend the majority of their life cycle, quantita- estimating these parameters and identifying fac- tive demographic data are lacking, despite the tors that affect populations usually requires contention that winter mortality limits many mi- long-term studies of marked individuals (Lebre- gratory bird populations (Evans and Pienkowski ton and North 1993). During the breeding sea- 1984, Conway et al. 1995). To understand life- son, a number of parametersincluding fecundity history cycles of birds or predict how variables and survival of adults influences the annual pro- such as habitat loss due to global warming or duction of young, whereas for the nonbreeding human expansion will affect wintering shorebird period, over-winter survival of first-year and populations (Goss-Custard et al. 1994), a better adult birds determines population status. De- understanding of population dynamics during mographic data such as mortality estimates are this winter period is required. the key to life-history theory. However, the rel- Dunlin (Culidris alpina) are one of the most ative importance of mortality during different widespread shorebird speciesin the world. They periods of a birds’ life-history cycle, and its in- have been studied extensively on their breeding fluence on population dynamics, is poorly un- (Holmes 1966, Soikkeli 1967, JGnsson 1991) derstood and difficult to study. and wintering grounds (Ruiz et al. 1989, War- Research on breeding shorebirds has pro- neck et al. 1995). Recent evidence suggeststhat duced a wealth of data on population dynamics Dunlin in Europe and North America are declin- (Ryan et al. 1993, Thompson and Hale 1993, ing (Tucker and Heath 1994, Wamock and Gill 1996), and even though an estimated 76% of ’ ’ Received19 September 1996. Accepted 27 June their total annual mortality is thought to occur 1997. during migration and on the wintering grounds 2 Current address:Forest and Rangeland Ecosystem (Evans 1991), there are no rigorous estimates of Science Center, U.S. Geological Survey, 3200 SW Jef- ferson Way, Corvallis, OR 97331, e-mail: survival for Dunlin based on nonbreeding stud- [email protected] ies. LOCAL SURVIVAL OF DUNLIN IN WINTER 907 Traditionally, return rates have been reported U.S. Fish and Wildlife Service band. Dunlin as minimum estimates of survival. Return rates were aged as either first-year birds (< 1 year old) are a composite of three rates: (1) the (true) rate or adults. First-year birds were identified by the of survival, (2) the rate of local site-fidelity, and presence of buffylchestnut-edges on the inner (3) the rate of resighting (or recapturing). New tertials or inner middle wing coverts. In adults, statistical techniques based on mark-recapture these feathershave white edges (Page 1974). We data now allow estimation of (3) the rate of re- measured the length of the exposed culmen to capture @) (Burnham et al. 1987, Lebreton et al. the nearest 0.1 mm, and body mass to the near- 1992). Local survival (4) is the product of rates est gram. Dunlin were sexed as male if their (1) and (2), and as such is an improvement over exposed culmen was I 37.7 mm, female if it return rates as it is a less biased measure of sur- was 2 39.8 mm, and as unknown if it was 37.8- vival (Sandercock and Gratto-Trevor 1997). We 39.7 mm (Page 1974). used these improved mark-recapture analyses to obtain estimates of local survival and resighting RESIGHTING CRITERIA AND EFFORT rates of a color-banded population of Dunlin Resighting data were used to determine whether based on a 1Cyear study along the Pacific Coast a specific bird was present at Bolinas Lagoon in of North America. a given year. We defined marking a bird as the This race, C. a. pacijica, winters along the event in which a Dunlin was initially caught, west coast of North America from southernCan- measured and color-banded. Subsequent recap- ada to Mexico (Warnock and Gill 1996) and ex- ture events of color-banded Dunlin were based hibits strong fidelity to winter sites, including on resightings where the bird was identified in our study site, Bolinas Lagoon, California (War- the field by its unique color-band combination neck et al. 1995). Previous work at this site with a field scope, without physical recapture. demonstrated that first-year Dunlin are more All sightings were recorded as either positive or likely to be depredated by raptors than adult probable. Sightings were recorded as probable birds (Kus et al. 1984). Weather also influences when the observer was not certain of one of the survival rates of shorebirds. In Lapwings (Vu- colors of the band combination. If the observer nellus vanellus) significant variation in adult sur- was unsure of more than one color, the bird was vival rates could be explained by mean winter not recorded. Generally, no more than two ob- soil temperatures and winter rainfall (Peach et servers resighted per year. From 1984 to 1992, al. 1994). Therefore, we tested whether sex, age, the majority of resighting was done by the senior or annual conditions affected the local survival author. Daily effort usually entailed l-5 hr scan- and recapture rates of wintering Dunlin. We also ning Dunlin flocks for color-banded Dunlin. investigated potential capture effects by testing Color bands on Dunlin were small, faded with the null hypothesis that newly banded Dunlin time, and bands were easily misread. In order to had the same local survival rates as returning, reduce resighting error, a bird was not recorded previously banded Dunlin. Testing these hypoth- as being present at Bolinas Lagoon in a given eses elucidated variables influencing local sur- year until it was either physically captured, pos- vival of Dunlin during winter months. The con- itively resighted at least twice, positively re- servation of shorebirds relies on identifying sighted once with at least two probable resight- these factors because the winter period is a par- ings, or recorded at least four times as a prob- ticularly sensitive and poorly understood seg- able resighting. ment of shorebirds’ life-history cycle (Myers et al. 1987). SURVIVAL ANALYSIS Local survival (4) and resighting (p) rates were METHODS estimated in two steps, following protocol out- lined by Lebreton et al. (1992) and Coach et al. STUDY SITE AND CAPTURE TECHNIQUES (1996). Program RELEASE (Burnham et al. Bolinas Lagoon is a 587-ha estuary on the 1987) was used to calculate the goodness-of-fit north-central coast of California. Birds were to a time-dependent model (&, pt or Cormack- captured in mist nets near nocturnal roosts.From Jolly-Seber model). The component statisticsof 1979-1992, 1,051 individuals were uniquely RELEASE are efficient for detecting variation in color-banded, and each bird received a metal capture probabilities of different individuals 908 NILS WARNOCK ET AL. (heterogeneity of capture), whether it is caused mediately following banding with subsequent by trap dependence (Test 2) or transients (Test years. A significant effect of age-class on local 3) (Coach et al. 1996). In calculating the overall survival may indicate age-specific mortality significance of the components of Test 2 (2.Ct rates, but it may also be due to handling effects, and 2.Cm, which deal generally with recapture transiency or heterogeneity of capture. Second, issues) and Test 3 (3SR and 3.Sm, which deal the difference between sexes (or groups) was generally with survival issues; see Coach et al. sometimes constrained to be a constant differ- 1996, Appendix, for fuller description and defi- ence in every year by using an additive model nitions of components), we included only those (sex+t). Comparing an additive and saturated tests where data were sufficient to calculate a x2- model (sex*t) is similar to testing whether the value. In the special case where 3 of the 4 com- interaction term is significant in a two-way ponents (3.Sm, 2.Ct, 2Cm) are nonsignificant, ANOVA. the remaining 3.SR can be used as a goodness- To examine potential effects of handling of-fit test to an age model (&2ac*trpt, Brownie and birds, we looked at the proportion of body mass Robson 1983). that captive Dunlin lost over time by weighing Next, we used program SURGE 4.1 (Pradel some birds immediately upon capture and then and Lebreton 1991) to model local survival and again at the time of release.