Impacts of a global climate cycle on population dynamics of a migratory songbird

Progress toward understanding factors that limit abundances of migratory birds, including climate change, has been difÞcult because these species move between diverse locations, often on different continents. For black-throated blue warblers (Dendroica caerulescens), demographic rates in both tropical winter quarters and north temperate breeding grounds varied with ßuctuations in the El Nin÷o Southern Oscillation. Adult survival and fecundity were lower in El Nin÷o years and higher in La Nin÷a years. Fecundity, in turn, was positively correlated with subsequent recruitment of new individuals into winter and breeding populations. These Þndings demonstrate that migratory birds can be affected by shifts in global climate patterns and emphasize the need to know how events throughout the annual cycle interact to determine population size. The need to understand when and how bird populations are limited is made pressing by recent declines in the abundances of many spe-cies, especially migratory songbirds (1). Quan-tifying the effect and timing of limiting factors for migratory species, however, is difficult be-cause the birds spend different parts of their annual cycle in different locations. Further-more, events during one stage of the annual cycle are likely to influence populations in sub-sequent stages (2). Here we show through long-term demographic studies of a migratory song-bird that the El Niño Southern Oscillation (ENSO) impacts demographic rates in both the breeding and nonbreeding seasons. Our find-ings also reveal links in the population dynam-ics of this species between stages of its annual cycle. We measured the effect of ENSO on sur-vival, fecundity, and recruitment of the black-throated blue warbler, a migratory songbird that breeds in forested regions of eastern North America and overwinters primarily in the Greater Antilles. This species is territorial, largely insectivorous, and exhibits strong site fidelity in both its breeding and wintering grounds (3). We quantified warbler demogra-phy from 1986 to 1998 at two locations during the annual cycle: the overwinter period at Copse Mountain, near Bethel Town in north