Progress toward understanding factors that limit abundances of migratory birds, including climate change, has been difficult 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 fluctuations in the El Niño Southern Oscillation. Adult survival and fecundity were lower in El Niño years and higher in La Niña years. Fecundity, in turn, was positively correlated with subsequent recruitment of new individuals into winter and breeding populations. These findings 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 species, especially migratory songbirds (1). Quantifying the effect and timing of limiting factors for migratory species, however, is difficult because the birds spend different parts of their annual cycle in different locations. Furthermore, events during one stage of the annual cycle are likely to influence populations in subsequent stages (2). Here we show through longterm demographic studies of a migratory songbird that the El Niño Southern Oscillation (ENSO) impacts demographic rates in both the breeding and nonbreeding seasons. Our findings also reveal links in the population dynamics of this species between stages of its annual cycle.We measured the effect of ENSO on survival, fecundity, and recruitment of the blackthroated 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 demography from 1986 to 1998 at two locations during the annual cycle: the overwinter period at Copse Mountain, near Bethel Town in northwestern Jamaica, West Indies, and the breeding season at Hubbard Brook Experimental Forest, West Thornton, New Hampshire, USA. The species' habitat at both sites was mature mesic forest, relatively undisturbed by human activity. Demographic data were collected annually from the overwintering population in late October and from the breeding population in midMay through August. For all analyses, we used annual mean monthly values of the standardized Southern Oscillation Index (SOI) to represent ENSO conditions for each calendar year (4). High, positive values of SOI indicate La Niña conditions and low, negative values indicate El Niño conditions (5).Annual survival (6) of black-throated blue warblers in Jamaica (Fig. 1, A to B) was strongly associated with SOI: survival was low in El Niño years and high in La Niña years (Fig. 2). This result is best explained by the impact of ENSO on local climate and a con...

The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969-2002, strong density dependence in time-series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997-1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high-quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site-dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site-dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site-dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population.

Because populations of territorial birds are relatively stable compared to those of other animal taxa, they are often considered to be tightly regulated. However, the mechanisms that produce density-dependent feedbacks on demographic rates and thus regulate these populations are poorly understood, particularly for migratory species. We conducted a three-year density-reduction experiment to investigate the behavioral mechanisms that regulate the abundance of a Nearctic-Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding season. We found that the number of young fledged per territory, territory size, and the proportion of time males spent foraging were significantly greater on territories around which neighbor density was experimentally reduced compared to control territories. Territory quality, proportion of nests depredated per territory, and male countersinging rates were not statistically different between treatments. These results indicate that individuals with more neighbors (i.e., in neighborhoods with greater conspecific density) have reduced breeding productivity. The results also suggest that a crowding mechanism that mediates interactions among territory-holders could generate the density dependence needed to regulate local abundance, at least in areas of homogeneous, high-quality habitat. The effect of the neighbor-density reduction on warbler fecundity and behavior varied with annual fluctuations in weather and food availability, and was strongest in 1997, an El Niñ o year, when conditions for breeding were least favorable. This variation in our experimental results among years implies that density dependence due to crowding may have its strongest impact on local abundance when environmental conditions are relatively poor.

No abstract