Brown-headed cowbirds are obligate brood parasites that lay their eggs in the nests of other species, thus enjoying the benefits of reproduction without the costs of parental care. Cowbirds parasitize a range of passerine species; one of their most common hosts is the red-winged blackbird. If brown-headed cowbirds parasitize red-winged blackbird nests so frequently, how do they find blackbird nests? Several hypotheses have been proposed to explain how brood parasites locate host nests. One hypothesis suggests that nests that are more exposed are easier to find, and thus are parasitized at higher rates than more cryptic nests. A second hypothesis predicts that nests that are close to convenient perching sites, perches from which brood parasites can scan for host nests, are parasitized at higher rates than nests away from perches. Two additional hypothesis propose that parasites cue in on host behavior to locate nests. One of these hypotheses predicts that parasites use the aggressiveness with which members of host species react to the presence of parasites as a gauge of whether or not the potential hosts are nesting, where higher levels of aggression are assumed to be a cue of nest presence. A related hypothesis predicts that parasites respond to nest-specific vocalizations in potential hosts, parasitizing nests at a higher rate when hosts are more vocal than when they are less vocal.

Ethan Clotfelter (1998. Anim. Behav. Vol. 55: 1181-1189) of the University of Wisconsin investigated the parasitism of red-winged blackbirds by brown-headed cowbirds in the prairie of southern Wisconsin to examine if any of the above factors facilitated the parasitism of blackbird nests by cowbirds. Clotfelter measured how close red-winged blackbirds= nests were to trees and perches, how high nests were, how much cover there was around nests, how much aggression red-winged blackbirds displayed at nests, and how vocal they were at nests. The results of Clotfelter's study provided clear support for the perch-proximity hypothesis; nests that were parasitized were significantly closer to trees than nests that were not parasitized in both years of the study. Clotfelter's results also supported the host vocalization hypothesis. Females at nests that were parasitized were observed to exhibit significantly more nest-associated "chit" (or "type I") vocalizations than females at nests that were not parasitized. No significant difference was observed in the degree of cover around parasitized versus unparisitized nests. In addition, no significant difference was observed in the amount of host aggression elicited by a mount of a cowbird between parasitized and unnparisitized nests. Thus, Clotfelter's results did not support either the nest-exposure or the host aggression hypotheses.

The findings of this study stimulate some interesting questions about the coevolution of brown-headed cowbirds and red-winged blackbirds. For example, given that cowbirds seem to use perches to scan for host nests, might there be some selective pressure for red-winged blackbirds to build nests further away from trees in prairie and marsh habitats? In addition, if cowbirds cue in on the nest-associated "chit" vocalizations produced by females, what selective benefits, if any, make the cost of producing these chit calls, in terms of the increased probability of parasitism, worth it? It has been suggested that the nest associated chit call may be used to communicate the need for nest vigilance in the male. Male vigilance may serve to deter nest predation as well as brood parasitism. There may be different selective pressures resulting from each type of defense and the sum of these pressures may result in the chit call in the female being favored (i.e., if such a call provides a large advantage against nest predation). Clotfelter (1998) states that "the importance of male vigilance in deterring brood parasitism is unknown." The study of topics such as the extent to which males of host species may deter parasitism, how this deterrence may vary with host species, and whether or not the cues attended to by parasites may change according to the different types of deterrence in different host species, offer fertile ground for future research.