Understanding survival and abundance of overwintering Warblers: Does rainfall matter?

We investigated relationships between warbler abundance and survival rates measured on a Puerto Rican wintering site and rainfall patterns measured on the wintering site and in regions where these warblers breed, as estimated using stable-isotope analysis (deltaD) of feathers collected from wintering birds. We banded birds using constant-effort mist netting from January 1989-2003 in the Gunica Forest of southwestern Puerto Rico. Black-and-white Warblers (Mniotilta varia), American Redstarts (Setophaga ruticilla), and Ovenbirds (Seiurus aurocapilla) dominated the Neotropical migrant capture totals each winter, with resulting sample sizes large enough to estimate survival rates. Estimates of capture probability from survival modeling allowed us to estimate abundance from mist-netting capture totals for Black-and-white Warblers and Ovenbirds. Stable-hydrogen isotopes showed that the three focal species came mostly from the eastern United States. Black-and-white Warbler abundance was related to rainfall total deviations from normal in Guanica Forest, and Ovenbird abundance was related to total annual rainfall in the United States. Survival models with rainfall covariates were weakly supported overall, but apparent survival of Black-and-white Warblers and American Redstarts was negatively related to rain during the first 6 months of the year at Guanica, and Ovenbird survival was related to rainfall during the spring in the southeastern U.S. Abundance and apparent survival exhibited similar, species-specific patterns of association with rainfall for Black-and-white Warblers and Ovenbirds. Winter rainfall was important to demographic parameters of Black-and-white Warblers, and breeding-season rain was important to Ovenbirds.