Long-Term Demographic Trends, Limiting Factors, and the Strength of Density Dependence in a Breeding Population of a Migratory Songbird

EFFECTIVE CONSERVATION AND MANAGEMENT of migratory bird species requires an understanding of when and how their populations are limited and regulated. From 1986 to 1999, we studied the processes that determine the abundance of a temperate-tropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding portion of its annual cycle. We found that young fledged per territory, fledgling mass, and yearling recruitment were negatively correlated with adult density, whereas the proportion of nests depredated per year was not correlated with density. Annual fecundity was also negatively correlated with density-independent nest predation and fluctuated with variation in weather and food supply associated with the El Niño Southern Oscillation. Using matrix population models, we discovered that local abundance of Black-throated Blue Warblers could be regulated by the negative feedback on annual fecundity generated by the density of breeding adults. Taken together, these results indicate that warbler abundance is determined in part by a complex interaction between biotic and abiotic factors during the breeding season and provide the first assessment of the strength of density dependence in a temperate-tropical migrant passerine.