Population Structure, Territoriality and Overwinter Survival of Two Migrant Warbler Species in Jamaica

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Abstract
We studied the ecology and behavior of American Redstarts (Setophaga ruticilla) and Black-throated Blue Warblers (Dendroica caerulescens) on their wintering grounds in Jamaica during 3 years, 1986-1989. Winter densities were comparable to (American Redstart) or slightly greater than (Black-throated Blue Warbler) those recorded for these species in breeding areas. Males, females, first-year and older individuals of each species occurred within the same habitats and study areas, with no evidence of intraspecific habitat segregation. Individuals of both sexes occupied territories, which were maintained by displays, vocalizations (chip notes), and stereotyped agonistic interactions. Territories were defended against conspecifics of the same sex and frequently of the opposite sex, although those of males and females sometimes overlapped, especially in the Black-throated Blue Warbler. Both sexes of each species responded aggressively to playbacks of breeding song/chip tapes and to decoys. Territories were occupied throughout the winter period, with most individuals of both species remaining through the winter within < 50 m of where they were captured, marked and observed in October-early November. Overwinter survival was high: 80% of 124 Redstarts and 66% of 53 Black-throated Blues color-marked in early winter (October-November) were still alive and present on territory in late winter (March-April). Individually marked birds disappearing during the winter were almost invariably replaced, usually by new (unmarked) individuals, indicating competition for territories and possibly the presence of floaters. This latter finding, coupled with those of strong overwinter site attachment and intense territorial behavior, suggests that events in winter are important in the population dynamics of these migrant species.