Impacts of a Global Climate Cycle on Population Dynamics of a Migratory Songbird

Abstract

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...

Short- and long-term population dynamical consequences of asymmetric climate change in black grouse

“…A large variety of response types have been proposed, some of which are very complex (Stenseth et al 2002). In birds, climate change may influence demographic rates directly (Saether et al 2000;Sillett et al 2000), but more commonly it is linked to range shifts (Parmesan & Yohe 2003) or changes in breeding phenology. For instance, variation in climate change between wintering and nesting grounds may adversely affect arrival times in migratory birds (Forchhammer et al 1998;Brown et al 1999;Crick & Sparks 1999;Dunn & Winkler 1999), or the response of a non-migratory bird population may not match that of its prey (Visser et al 1998).…”

Predicting the consequences of carry-over effects for migratory populations

“…Populations can be influenced by events that occur during the over-wintering, migration and breeding periods, and regulation can occur by a combination of mechanisms operating in one or more of these seasons (Sillett et al 2000). In a general two-season model, a migratory population will be at equilibrium when the per capita net breeding output (reproductionKmortality) during the breeding season equals the per capita mortality during the wintering season (Fretwell 1972;Sutherland 1996).…”

Climate change suggests a shift of H5N1 risk in migratory birds

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