Sympatric avian brood parasites may compete for the same nests to parasitize. Host-resource partitioning, or “alloxenia,” is exhibited by several Old World cuckoos where they are sympatric in Africa, Japan, and Australia. I examined host use by sympatric Brown-headed Cowbirds (Molothrus ater) and Bronzed Cowbirds (M. aeneus) from 1997 to 1999 in pine-oak and montane riparian forests in southeastern Arizona. Bronzed and Brown-headed cowbirds partitioned hosts by host body size. Brown-headed Cowbirds did not parasitize larger hosts (i.e., Western Tanager, Piranga ludoviciana; and Hepatic Tanager, P. flava), while Bronzed Cowbirds did not parasitize smaller hosts (i.e., Painted Redstarts, Myioborus pictus; and Bell's Vireos, Vireo bellii). Although there was some host overlap (only 2/7 parasitized host species were parasitized by both cowbird species), only 3/48 nests (all Plumbeous Vireo, V. plumbeus) contained eggs of both parasite species. Parasitism by sympatric cowbirds in southeastern Arizona appears to fit the pattern of alloxenia.