Symposium: Brood parasitism

Abstract

Brown-headed Cowbirds Molothrus ater are very social birds. Males have a rich vocal repertoire, and much of their social behavior is mediated by vocalizations. For example, subtle differences in song reflect dominance status and are used by females during mate choice. Differences in song result from feedback from social interactions, and, in the absence of normal social feedback, subordinate males may produce inappropriate dominant-like songs (1) and above-normal testosterone levels (2). Another vocalization, the flight whistle, is equally interesting. It occurs in dialects, and can be used in long distance communication. In some populations parts of the flight whistle are produced antiphonally, probably to designate specific recipients. While the importance of intraspecific vocalizations to cowbird biology is well-established, surprisingly little attention has been given to the possibility that cowbirds also may attend to host vocalizations, which could act as one cue in locating nests. Given that the most frequently parasitized cowbird hosts are highly vocal oscines, it is reasonable that cowbirds could use host vocalizations to discover nests (cf. 3). Preliminary data suggest that, in one host species, median vocal rates are higher in parasitized than in unparasitized pairs (UYEHaRA, unpubl.). Further, some birds produce vocalizations that are associated with nest-building, and cowbirds may use these to detect nests. For example, female Hooded Warblers Wilsonia citrina produce a particular vocalization during nest construction. Hooded Warbler nests were heavily (20/25 nests) and selectively parasitized in one songbird community, probably due to the parasite's use of the host's nest-building vocalization (STuTCHBUt~Y, unpubl.). Other females have nest departure calls (4) that could also be used to discover nests. Another host vocalization that could be used to locate nests are nestling distress calls. These are produced when nestlings are in grave and immediate danger, such as when in the grasp of a predator. Female M. ater quickly approach playback of tape-recordings of distress calls, and often are the only species to respond. Approaching cowbirds do not mob, nor do they vocalize. Instead, they peer quietly and intently in the direction of the playback. This may represent an attempt by female cowbirds to determine the general location of a nest that is about to be lost, and it may alert her to a territory where nest building activity (and a new opportunity for brood parasitism) may occur within a few days. Thus, it represents a new "function" for distress calls. Additional study of the cowbird use of host vocalizations is warranted.