Document: JAM-3-53-5

Habitat and host selection by sympatric avian brood parasites in southeastern Arizona.

CHACE, J.F.*

University of Colorado, Boulder, CO 80309, USA ¹

Abstract:
Bronzed Cowbirds (Molothrus aeneus) and Brown-headed Cowbirds (M. ater) are obligate brood parasites that are sympatric during the breeding season from southern California east to Louisiana and south through central Mexico. In 1997-1999 I examined the degree of habitat and host partitioning in the San Pedro Valley of southeastern Arizona. Bronzed Cowbirds are less common and more habitat selective than the Brown-headed Cowbirds. While both cowbird species are most common within residential areas, Brown-headed Cowbirds are very common along the San Pedro River where Bronzed Cowbirds are mostly absent, and Bronzed Cowbirds are most common in the montane canyons of the Huachuca Mountains. These brood parasites also overlap in host resources, but partition by host size, with the smaller Brown-headed typically parasitizing smaller hosts than the larger Bronzed Cowbird. 47% of 161 nests of host species monitored were parasitized, 38% by Brown-headed Cowbirds and 11% by Bronzed Cowbirds. 3% of the host nests were parasitized by both cowbird species. Brown-headed Cowbird parasitism exceeded Bronzed Cowbird parasitism in all major habitat types studied. Overall, Brown-headed Cowbird parasitism was highest in the low elevation mesquite forests along the San Pedro River (64%, n=42), and Bronzed Cowbird parasitism highest in montane riparian canyons of the Huachuca Mountains (24%, n=43). The degree of habitat and host partitioning by these sympatric obligate brood parasites is low, resulting in competition for nests to parasitize.

Keywords: Brood Parasites, Molothrus, Niche Partioning, Competition

This abstract is being presented at: 10:15 AM in session:
Oral Session #3: Avian Ecology.