Implicated in decline of several island bird populations, including yellow-shouldered blackbird in Puerto Rico. Other host species that should be monitored for possible cowbird-induced decline include Puerto Rican flycatcher, black-whiskered vireo, black-cowled oriole, and troupial (Cruz et al. 1989). Brood parasitism reduces nesting success and productivity of hosts (Wiley 1985).
Roosts communally in large numbers (1000s in Puerto Rico) with Agelaius xanthomus and Quiscalus niger (Post and Wiley 1977); otherwise occurs alone or more frequently in small loose groups. Females commuted daily about 4 kilometers between feeding and breeding areas (Woodworth 1993).