POPULATION CHARACTERISTICS OF BLACK-THROATED BLUE WARBLERS WINTERING IN 3 SITES ON PUERTO-RICO

I studied ecology and behavior of wintering Black-throated Blue Warble rs (Dendroica caerulescens) at three sites on Puerto Rico over four ye ars. The site with tall mature forest and relatively few fruiting unde rstory plants had warbler populations characterized by: a predominance of males; relatively early fall arrival of returning adults; high sit e fidelity; large home ranges; low density of sedentary birds; few wan dering individuals; high over-winter persistence of sedentary birds; a nd an invertebrate-rich diet. In contrast, the shrubby second-growth s ite with an abundance of fruiting plants had warbler populations chara cterized by: a predominance of females; relatively late arrival of ret urning adults compared to juveniles; low site fidelity; small home ran ges; high density of sedentary and wandering birds; low overwinter per sistence of sedentary birds; and a nectar- and fruit-rich diet. At the third site, the population trails fell within these extremes. Some si te variation is attributable to sex differences in site persistence, w andering, and home-range size, which appear as population differences only because sex ratios vary among sites. However, differences in abun dance and seasonality of fruit and nectar may further contribute to si te differences in overwinter persistence of sedentary birds, wandering , home-range size, and density.