Variation in survivorship of a migratory songbird throughout its annual cycle

Abstract

1. Demographic data from both breeding and non-breeding periods are needed to manage populations of migratory birds, many of which are declining in abundance and are of conservation concern. Although habitat associations, and to a lesser extent, reproductive biology, are known for many migratory species, few studies have measured survival rates of these birds at different parts of their annual cycle. 2. Cormack-Jolly-Seber models and Akaike's information criterion model selection were used to investigate seasonal variation in survival of a Nearctic - Neotropical migrant songbird, the black-throated blue warbler, Dendroica caerulescens. Seasonal and annual survival were estimated from resightings of colour-ringed individuals on breeding grounds in New Hampshire, USA from 1986 to 2000 and on winter quarters in Jamaica, West Indies from 1986 to 1999. Warblers were studied each year during the May-August breeding period in New Hampshire and during the October-March overwinter period in Jamaica. 3. In New Hampshire, males had higher annual survival (0.51 ± 0.03) and recapture probabilities (0.93 ± 0.03) than did females (survival: 0.40 ± 0.04; recapture: 0.87 ± 0.06). In Jamaica, annual survival (0.43 ± 0.03) and recapture (0.95 ± 0.04) probabilities did not differ between sexes. Annual survival and recapture probabilities of young birds (i.e. yearlings in New Hampshire and hatch-year birds in Jamaica) did not differ from adults, indicating that from the time hatch-year individuals acquire territories on winter quarters in mid-October, they survive as well as adults within the same habitat. 4. Monthly survival probabilities during the summer (May-August) and winter (October-March) stationary periods were high: 1.0 for males in New Hampshire, and 0.99 ± 0.01 for males in Jamaica and for females in both locations. 5. These annual and seasonal survival estimates were used to calculate warbler survival for the migratory periods. Monthly survival probability during migration ranged from 0.77 to 0.81 ± 0.02. Thus, apparent mortality rates were at least 15 times higher during migration compared to that in the stationary periods, and more than 85% of apparent annual mortality of D. caerulescens occurred during migration. 6. Additional data from multiple species, especially measures of habitat-specific demography and dispersal, will improve our understanding of the relative impacts of the breeding, migratory, and winter periods on population dynamics of migratory birds and thus enhance future conservation efforts.