EFFECTS OF BROWN-HEADED COWBIRDS AND PREDATORS ON THE NESTING SUCCESS OF YELLOW WARBLERS IN SOUTHWEST COLORADO

We studied the effects of brood parasitism by Brown-headed Cowbirds (Molothrus ater) and predators on the reproductive biology of Yellow Warblers (Dendroica petechia) in La Plata County, Colorado, 1992–1996. Overall, 36.4% of Yellow Warbler nests (n = 66) were parasitized by Brown-headed Cowbirds. Although parasitized and nonparasitized nests were equally likely to fledge at least one warbler, in nonparasitized nests, warblers had larger clutches, hatched more young, and tended to fledge more young. Frequency of parasitism was not influenced by nest height and, on a weekly basis, was not correlated with number of warbler nests initiated. Overall, 39.1% of Yellow Warbler nests were lost to predators, but complete predation was not affected by nest height. On a weekly basis, percent complete predation of warbler nests increased with number of active nests. No significant relationship existed between frequency of complete predation and frequency of parasitism in Yellow Warbler nests.