Summer-Fall Survival of American Woodcock in Maine

Abstract

We estimated summer-fall (15 Jun-20 Oct) survival for 32 adult and 96 fledged young American woodcock (Scolopax minor) radiotagged in eastern Maine during 1982-84 using single-interval, multipleinterval, and nonparametric (product-limit) methods. Single-interval survival estimates were 0.90, 0.88, 0.57, and 0.73 for adult males, adult females, young males, and young females, respectively. Multiple-interval survival estimates (0.93, 0.88, 0.63, and 0.71) and product-limit estimates were similar (0.92, 0.90, 0.60, and 0.69). Within each age class, male and female period survival was not significantly different for any method. However, point estimates of survival for young females exceeded those of young males by 0.08-0.16. Regardless of sex, adults exhibited higher summer-fall survival (range = 0.89-0.92) than young woodcock (range = 0.64-0.68). Age-specific variation in survival was caused by different predation rates that may be related to age-specific differences in mobility. The survival rates sustained by young woodcock during our study were similar to 4-month rates derived from annual survival estimates that assume constant mortality throughout the year. The high summer-fall survival estimates exhibited by adult woodcock suggest that most of their annual mortality occurs during another season. J. WILDL. MANAGE. 54(1):97-106 Because of the long-term decline of American woodcock in the eastern United States (Tautin et al. 1983) the U.S. Fish and Wildlife Service (USFWS) implemented woodcock harvest restrictions in 1985. Changes in habitat conditions, through forest succession and urbanization, are believed largely responsible for the decline (Dobell 1977, Coulter and Baird 1982, Dwyer et al. 1983). However, the exact mechanisms that have resulted in lower recruitment or survival remain largely unexamined. To define the relationship between woodcock survival and habitat conditions, information is needed on timeand causespecific mortality factors. Estimates of annual survival for American woodcock relied on return rates of breeding birds (Sheldon 1956, 1967), life tables (Godfrey 1974), or band recoveries from hunters (Martin et al. 1969, Krohn et al. 1974, Dwyer and Nichols 1982). Analysis of band recoveries is least biased with the development of band-return and markrecapture models (Brownie et al. 1978). However, precise estimates of annual survival of woodcock are difficult to obtain because of the large sample sizes required by banding models (Dwyer and Nichols 1982). Fortunately, application of more rigorous statistical methods used in conjunction with radio-telemetry data permit relatively unbiased time-specific survival estimates from studies of free-ranging animals (Mayfield 1961, 1975; Trent and Rongstad 1974; Ri gleman and Longcore 1983; Cowardin et al. 1985; Heisey and Fuller 1985; Kirby and Cowardin 1986; Kurzejeski et al. 1987; Pollock et al. 1989). Our objectives were to estimate ageand sex-specific survival of American woodcock from June to October and compare survival estimates among 3 statistical methods with differing assumptions on survival constancy within the study period. We acknowledge C. M. Bunck, S. R. Winterstein, and G. L. Hensler for statistical consultation and thank G. L. Storm, T. J. Dwyer, M. A. Howe, C. M. Bunck, J. R. Sauer, R. H. Yahner, and W. M. Tzilkowski for thoughtful reviews of the manuscript. Support was providby the Patuxent Wildlife Research Center, the Moosehorn National Wildlife Refuge, the Pennsylvania Cooperative Wildlife Research Unit, the Ruffed Grouse Society, and the National Rifle Association.