Effects of Nest Predation and Brood Parasitism on Population Viability of Wilson’s Warblers in Coastal California

—We studied the consequences of nest predation and brood parasitism on a population of Wilson’s Warblers (Wilsonia pusilla) breeding in coastal riparian woodlands in northern California. We monitored 90 warbler nests from 1997 to 2000; only 16 of these produced Wilson’s Warbler young. Of 74 failed nests, 73% (54/74) failed due to nest predation. Overall, 33% (30/90) of the nests were parasitized by Brown-headed Cowbirds (Molothrus ater). Nest success, as calculated by the Mayfield method, was 0.085 and notably lower than values reported for other warbler species. We used a simple demographic population model—under scenarios of high, average, and low productivity and survival—to evaluate the viability of this population and found it to be at risk of local extirpation without immigration. This was due to the combined effects of high levels of nest predation and the impacts of brood parasitism. Received 16 June 2003, accepted 6 April 2004. Across their range, breeding populations of Wilson’s Warbler (Wilsonia pusilla) have been declining at both regional and local scales over the past few decades (Ammon and Gilbert 1999, Sauer et al. 2001). According to Breeding Bird Survey data, Wilson’s Warblers across the North American continent have been declining on average 2.0% per year during the period 1980 to 2000, and populations along the Pacific coast have been declining on average 1.8% per year over the same time period (Sauer et al. 2001). In contrast, data from a single site in coastal California indicate that the breeding population there is stable (Chase et al. 1997). Population declines in breeding songbirds have been attributed to a variety of factors, including, but not limited to, loss, degradation, and fragmentation of habitat and associated factors that affect reproductive success and survival. There is evidence that the Wilson’s Warbler population in coastal California is regulated primarily by breeding productivity (Chase et al. 1997); however, the factors that limit productivity are unknown. The leading causes of low reproductive success in songbirds are nest predation by vertebrate 1 Dept. of Biology, Sonoma State Univ., 1801 E. Cotati Ave., Rohnert Park, CA 94928, USA. 2 Point Reyes Bird Observatory, 4990 Shoreline Hwy., Stinson Beach, CA 94970, USA. 3 Current address: Prunuske Chatham, Inc., P.O. Box 828, Occidental, CA 95465, USA. 4 Corresponding author; e-mail: [email protected] predators and brood parasitism by Brownheaded Cowbirds (Molothrus ater; Brittingham and Temple 1983; Martin 1992a, 1992b). High levels of nest predation and brood parasitism have been implicated in the decline of many songbird populations by directly affecting productivity and, ultimately, population dynamics (e.g., Pease and Grzybowski 1995). While there have been few studies published documenting the breeding ecology and life history characteristics of western populations of Wilson’s Warbler (Stewart 1973, Stewart et al. 1977, Ammon and Gilbert 1999), little work has been done to explore causes of recent declines and, more specifically, factors limiting reproductive success. Population declines in the past have been attributed to loss and degradation of riparian breeding habitat (Ammon and Gilbert 1999). However, few estimates of reproductive success exist and, to our knowledge, no Mayfield (1975) estimates of nest success have been reported. There are even fewer accounts of cowbird parasitism and its effects on reproductive success of Wilson’s Warblers. In this study, we report on the breeding biology and population viability of a coastal population of Wilson’s Warblers breeding in Marin County, California. Our objectives were to (1) examine the effects of cowbird parasitism and nest predation on warbler reproductive success, and (2) develop a simple demographic population model to assess the viability of this local population. 42 THE WILSON BULLETIN • Vol. 116, No. 1, March 2004