Over half of the breeding landbirds in North America make a yearly journey in the Fall to their wintering grounds in the Neotropics, only to return again to North America the following Spring (Rappole 1995).  Unfortunately, evidence of decline in many populations of Neotropical migratory birds has been documented in recent years (Sherry and Holmes 1993).  An understanding of where species are limited in their annual cycle and which ecological factors are involved in these declines is a topic of much debate and discussion (Latta and Baltz 1997).  Determining the influence of specific factors is a difficult task because of the complex life-history patterns of these migrants and the seasonal environments they inhabit (Moore et al. 1993, Sherry and Holmes 1993, Yong et al. 1998).  Populations of Neotropical migrants may be limited by events on their breeding grounds, wintering grounds and/or migratory stopover habitats (Sherry and Holmes 1993, Yong et al. 1993).  From a management perspective, an understanding of the events and circumstances at each stage during the annual cycle must be taken into account when developing conservation strategies (DeSante 1995).

On their breeding grounds, the decline of many songbird populations across the country has been attributed in part to the breeding behavior of the Brown-headed Cowbird (Molthrus ater) (Trail and Baptista 1993).  The cowbird is an obligate brood parasite, laying its eggs in the nest of other species, and leaving the responsibility of rearing and caring for cowbird young up to the host species.  The breeding strategies of the Brown-headed Cowbird have had a significant impact on many host species (Payne 1998).  These effects are due in part to the high fecundity of the parasite and the reproductive losses incurred by the host (Brittingham and Temple 1983, Clotfelter 1998).  As a consequence of brood parasitism, many hosts suffer reduced nesting success and/or decreased seasonal fecundity.  Thus, high rates of brood parasitism can have a substantial effect on host population trends (Pease and Grzybowski 1995).  An accurate assessment of the impacts of cowbird parasitism on individual host species is important for understanding the causes of host population declines and the potential impacts of this brood parasite.

In this proposed study, I will examine the dynamics of Brown-headed Cowbird parasitism in a migratory songbird, the Wilson's Warbler (Wilsonia pusilla chryseola) (in collabartion with the Point Reyes Bird Observatory in Marin County, California).  Specifically, I will attempt to address the following questions: 1) What is the frequency and intensity of brood parasitism in the Wilson's Warbler?; 2) Does nest-site selection influence parasitism rates?; 3) What impact does cowbird parasitism pose to warbler nesting success? and 4) What are the overall effects of parasitism on the long-term survival of this population of Wilson's Warbler?

This work is being completed with the collaboration of Tom Giraldi and Nadav Nur of the Point Reyes Bird Observatory.