Increasing nest success in the yellow-shouldered blackbird Agelaius xanthomus in southwest Puerto Rico

The yellow-shouldered blackbird, Agelaius xanthomus, is endemic to Puerto Rico and Mona Island, and endangered since 1976 mainly because of brood parasitism by the shiny cowbird, Molothrus bonariensis. In 1984 an artificial nest structures program was initiated, combined with the removal of M. bonariensis, to improve the reproductive success of A. xanthomus. The nesting success was high in artificial nest structures. From 1996 to 1999, 22 out of 804 nests in artificial structures and one of 203 nests in natural substrates were parasitized. The use of natural substrates increased from less than 1% (1996) to 35% (1999). Predation was the main cause of egg and chick loss and was high in natural substrates. The increase in the use of natural substrates for nesting is a trend favorable for the recovery of this species. Continued management is still needed to augment the breeding population and reproductive output.