Ecological Consequences of Song Matching: Breeding Success and Intraspecific Song Mimicry in Indigo Buntings

Abstract

Mating and breeding success of males that matched the song of a neighbor were compared with success of nonmatching song individualists in Indigo Buntings (Passerina cyanea) in southern Michigan. Adult males had higher success than 1st—yr males that matched the song of an adult on a neighboring territory had significantly greater mating success, nesting success, and fledging success than did nonmatching 1st—yr males. First—year males with blue plumage did better than 1st—yr males with browner plumage. Song matching raised the success of the browner males by 50% to equal that of the nonmatching bluer males, and song matching in the bluer 1st—yr males increased their success to that of the adults. The advantage in birds that mimic was restricted to birds that copy an adult and remain on a territory near the song model, and was most pronounced in the mimics that resembled the song model in plumage as well as in song. This increase in success in the song—matching males cannot be explained by variation in body size, time of arrival, or habitat. No apparent disadvantage fell to the older male buntings that were mimicked. As the structure and function (territoriality and mate attraction) of song in the individualist 1st—yr males did not differ from that in the song mimics, the increased success of song mimics apparently resulted from matching the adult neighbor's song and not simply from a behavioral maturation of song learning. The mechanisms through which song matching enhances biological success appear to involve competitive mimicry and deceit of other males through mistaken identity based on the older territorial male model's song.