Ecology, 92(9), 2011, pp. 1789–1798 Ó 2011 by the Ecological Society of America Range-wide effects of breeding- and nonbreeding-season climate on the abundance of a Neotropical migrant songbird SCOTT WILSON, 1,4 SHANNON L. LADEAU, 2 ANDERS P. TøTTRUP, 3 AND PETER P. MARRA 1 1 Smithsonian Migratory Bird Center, National Zoological Park, Washington, D.C. 20008 USA 2 Cary Institute of Ecosystem Studies, Millbrook, New York 12545 USA 3 Center for Macroecology, Evolution and Climate, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark Abstract. Geographic variation in the population dynamics of a species can result from regional variability in climate and how it affects reproduction and survival. Identifying such effects for migratory birds requires the integration of population models with knowledge of migratory connectivity between breeding and nonbreeding areas. We used Bayesian hierarchical models with 26 years of Breeding Bird Survey data (1982–2007) to investigate the impacts of breeding- and nonbreeding-season climate on abundance of American Redstarts (Setophaga ruticilla) across the species range. We focused on 15 populations defined by Bird Conservation Regions, and we included variation across routes and observers as well as temporal trends and climate effects. American Redstart populations that breed in eastern North America showed increased abundance following winters with higher plant productivity in the Caribbean where they are expected to overwinter. In contrast, western breeding populations showed little response to conditions in their expected wintering areas in west Mexico, perhaps reflecting lower migratory connectivity or differential effects of winter rainfall on individuals across the species range. Unlike the case with winter climate, we found few effects of temperature prior to arrival in spring (March–April) or during the nesting period (May–June) on abundance the following year. Eight populations showed significant changes in abundance, with the steepest declines in the Atlantic Northern Forest ( 3.4%/yr) and the greatest increases in the Prairie Hardwood Transition (4%/yr). This study emphasizes how the effects of climate on populations of migratory birds are context dependent and can vary depending on geographic location and the period of the annual cycle. Such knowledge is essential for predicting regional variation in how populations of a species might vary in their response to climate change. Key words: American Redstart; Breeding Bird Survey; climate; hierarchical models; migratory connectivity; Neotropical migrant; population dynamics; Setophaga ruticilla. INTRODUCTION Identifying the causes of fluctuations in population size requires an understanding of limiting factors operating throughout the annual cycle. Much of our knowledge of population limitation in birds comes from species that are more or less sedentary year-round because individuals can be monitored among seasons and years (Newton 1998, Bo¨hning-Gaese et al. 2000). This task is far more challenging for long-distance migrants because of the vast distances they move among seasons. A typical individual will spend about three months on the breeding grounds, six to seven months on the wintering grounds, and the remainder on spring and fall migration. At each of these stages, they face a multitude of factors that can influence fitness (e.g., Sillett et al. 2000, Marra and Holmes 2001, Latta and Faaborg 2002, Hannon et al. 2009), and events during one stage of the annual cycle can carry over to affect performance during subsequent stages (Marra et al. 1998, Reudink et al. 2009). While detailed studies on the breeding or nonbreeding grounds have helped our understanding of the factors affecting reproduction and survival for specific populations, the difficulty of tracking individu- als among seasons means we still have a limited understanding of how different factors interact between seasons of the annual cycle and how they vary across a species range. Climate is one factor known to influence reproduction and survival of songbirds (reviewed in Newton 1998). For species breeding in north temperate regions, warmer temperatures typically result in an earlier onset of breeding (Both et al. 2004, Dunn 2004), which can enhance productivity because it increases the opportu- nities for multiple broods or re-nesting after failure (Wilson and Arcese 2003). Temperature and precipita- tion can also directly influence nestling and fledgling Manuscript received 8 September 2010; revised 18 February 2011; accepted 30 March 2011. Corresponding Editor: J. R. Sauer. 4 Present address: Canadian Wildlife Service, Environment Canada, 115 Perimeter Road, Saskatoon, Saskatchewan S7N 0X4 Canada. E-mail: scottd.wilson@ec.gc.ca 1789