Demographic and Population Responses of Cape May Warblers Wintering in Multiple Habitats

Abstract

Wintering Cape May Warblers (Dendroica tigrina) were studied in three habitats along an altitudinal gradient in the Dominican Republic. Results of this study are some of the first to link both population responses and individual condition of nonbreeding birds to prevailing ecological conditions across divergent habitats. Our data suggest that Cape May Warblers are habitat generalists and generally unspecialized resource opportunists, but that differences in food resources result in competitive interactions, sex and age class segregation, and differences in site fidelity and physiological condition among habitats. Diet of the Cape May Warbler varied between habitats: in desert, Cape Mays were primarily insectivores; in dry forest, they foraged primarily on homopteran honeydew; in pine forest, they were principally frugivores. Abundance of warblers as measured by mist-net captures was highest in pine forest sites. Males were more common than females in pine habitat, whereas there was a female-biased sex ratio in other sites. Adult birds were more common in the desert and the pine forest, but there was a bias toward immature birds in dry forest habitat. Overwinter site persistence was 76% in pine, but only 28% and 33% in dry forest and desert, respectively. Annual return rate averaged 57% and did not vary significantly among habitats. Adjusted body mass of site-persistent birds was highest in pine and dry forest and lowest in desert. Pectoral muscle mass scores increased in the pine forest throughout the sampling period, whereas muscle mass scores decreased in the desert. These data suggest that, among the habitats sampled, pine was preferred. It is likely that stability and predictability of resources, particularly fruiting Trema trees and nectar sources, attracted dominant Cape May Warblers to pine forests and kept them as site-persistent individuals in good body condition. In contrast, dry forest may have been suboptimal, unless an individual was able to hold and defend a tree with honeydew-producing homopteran scale insects. Desert thorn scrub was seemingly suboptimal because resources were consistently scarce, and conditions became increasingly difficult during the late-winter dry period.