Population Characteristics of Black-Throated Blue Warblers Wintering in Three Sites on Puerto Rico

I studied ecology and behavior of wintering Black-throated Blue Warblers (Dendroica caerulescens) at three sites on Puerto Rico over four years. The site with tall mature forest and relatively few fruiting understory plants had warbler populations characterized by: a predominance of males; relatiyely early fall arrival of returning adults; high site fidelity; large home ranges; low density of sedentary birds; few wandering individuals; high over- winter persistence of sedentary birds; and an invertebrate-rich diet. In contrast, the shrubby second-growth site with an abundance of fruiting plants had warbler populations charac- terized by: a predominance of females; relatively late arrival of returning adults compared to juveniles; low site fidelity; small home ranges; high density of sedentary and wandering birds; low overwinter persistence of sedentary birds; and a nectar- and fruit-rich diet. At the third site, the population traits fell within these extremes. Some site variation is attributable to sex differences in site persistence, wandering, and home-range size, which appear as population differences only because sex ratios vary among sites. However, differences in abundance and seasonality of fruit and nectar may further contribute to site differences in overwinter persistence of sedentary birds, wandering, home-range size, and density. Received 26 April 1994, accepted 27 January 1995.