Survival, Productivity, and Abundance in a Wilson's Warbler Population

Abstract

-We analyzed patterns in the abundance, productivity, and survivorship of a coastal California population of Wilson's Warbler (Wilsonia pusilla) using capture-recapture data from 18 years of standardized mist-netting conducted during the breeding season. The numbers of adult and hatching-year birds captured each year showed no consistent trend through time. The total number of adults captured annually, an index of adult abundance, was positively related to productivity in the previous year as indexed by the number of hatching-year birds captured. In addition, the number of adults captured for the first time at the study site, an index of new adults entering the population, was positively related to the previous year's productivity. There was a positive correlation between the numbers of summer residents and transients captured in each year. Estimated annual adult survival of summer residents was 50.3%. The annual recapture probability for summer residents (68.8%) was much greater than for presumed transients (7.2%). When the difference in recapture probability between summer residents and transients was ignored, annual survival was underestimated (30.5 %). Year-to-year variation in survival showed no relationship to annual fluctuations in adult abundance. These patterns of survival and productivity parameters suggest that abundance in this population has been influenced primarily by circumstances on the breeding grounds. Received 17 May 1996, accepted 10 January 1997. EVIDENCE OF DECLINES in North American landbird populations has led to widespread efforts to monitor population trends and to identify the causes of population fluctuations (Robbins et al. 1989, Hagan and Johnston 1992, Finch and Stangel 1993). In theory, fluctuations in population size may be influenced by variation in reproductive success, survival and recruitment of young into the breeding population, adult survivorship, immigration, or emigration (Nur and Geupel 1993a). Variation in reproductive success has been suggested as a major cause of fluctuations in the abundance of Neotropical migrants, and several studies have found evidence that the recruitment of new breeders into populations of long-distance migrants is positively correlated with reproductive success in the previous summer (Nolan 1978, Sherry and Holmes 1991, Holmes et al.1992, Johnson and Geupel 1996). Evidence also suggests that variation in mortality during migration and over winter are important factors influencing the abundance of migratory birds (Holmes et al. 1989, Robbins et al. 1989, Peach et al. 1991, Rappole and McDonald Present address: Department of Biology, University of California, Riverside, California 92521, USA. E-mail: mkchase@citrus.ucr.edu 1994). Clearly, studies examining the demographic parameters of migratory bird populations are critical to identifying the proximate causes of population fluctuations (Temple and Wiens 1989; Sherry and Holmes 1991, 1992; Verner 1992). We studied the dynamics of a coastal California population of Wilson's Warbler (Wilsonia pusilla) using 18 years (1979 to 1996) of standardized mist-net data. We estimated three demographic parameters: abundance, productivity, and adult survival. Using the capture rate of adults as an index of abundance, we looked for evidence of long-term trends. To generate insight into the demographic mechanisms influencing population fluctuations, we investigated the relative influence of productivity and adult survival on adult abundance. To examine the influence of productivity on the variation we observed in abundance, we investigated the relationship between productivity in one year and the number of adults captured in the following year. We estimated annual recapture and survival probabilities of adult Wilson's Warblers by analyzing capture/recapture data (Lebreton et al. 1992). To examine the influence of survival on annual variations in abundance, we investigated the relationship between an-