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Song Repertoire Development in Male Cowbirds

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Journal of Comparative Psychology 19Xh. Vol. 100. No. 3.196-303 Copyright 1986 by the American Psychological Association, Inc. 0735-703h/8h/$OO.75 Song Repertoire Development in Male Cowbirds (Molothrus ater): Its Relation to Female Assessment of Song Potency Meredith 1. West University of North Carolina at Chapel Hill Andrew P. King Duke University Two studies were conducted to investigate the relation between the male cowbird's (MO/Olhrus aler aleI') development of a song repertoire and the female cowbird's assessment of song potency. Male development was assayed by vocal copying and female assessment by copulatory responsiveness to song playback. The results demonstrate that males do not copy most often the particular songs that females respond to most often. Whereas rank orderings of potency were highly correlated across two independent samples of playback females, male and female rank orderings were not significantly correlated. The data highlight the potential significance of social interactions between and across the sexes for repertoire development. Bird song is a social enterprise involving a singer and a recipient and, as such, necessitates the study of two organisms. Although the proposition that song is a social behavior is commonly accepted, most empirical work has been focused on how the behavior of the singer affects that of a listener; many fewer studies have addressed how the behavior of the listener affects that of the singer; and in even fewer have the mechanisms linking song production and song reception been investigated. New neurophysiological and behavioral evidence suggest the need for such a proximate analysis. At a neurophysiological level, the adult female canary's (Serinus canaria) brain appears to undergo neurogenesis. as does the male's, even though only the male sings (Nottebohm, 1984). Presumably, such neuronal development reflects the critical need for the female to evaluate and respond to new songs produced by males. At a behavioral level, the nonsinging female cowbird can socially influence a male to modify his song repertoire to suit her native preference (King & West, 1983b; West & King, 1985). In this species, the female appears to be a silent but influential partner in the epigenesis of song. Here we explored other potential linkages in song development in cowbirds. Specifically, we investigated the male cowbird's development of his song repertoire in relation to the female's copulatory responsiveness to song. We have defined song potency as a song's ability to elicit copulatory postures from captive female cowbirds deprived of male companions (King & West, 1977, 1983a; West, King, Eastzer, & Staddon, 1979). A song of high potency consistently elicits This work was supported by Grant BNS 84-0 IllS from the National Science Foundation and Grant I KO NS00676-4 from the National Institute of Neurological and Communicative Disorders and Stroke. The birds were collected under Federal Permit 2-4046. We thank Jill M. Trainer for her able assistance in preparing the tutor tapes, recording the males, and scoring tapes. We thank the three anonymous reviewers for incisive and productive reviews. Correspondence concerning this article should be addressed to Meredith J. West, Department of Psychology, 221 Davie Hall 013A. University of North Carolina. Chapel Hill, North Carolina 27514. 296 copulatory postures from females (on more than 50% of playback trials); a song of low potency infrequently does so (on less than 25% of playback trials). Using song potency as a measure, we have documented that female cowbirds can discriminate their species and subspecies by song alone (King & West, 1977; King, West, & Eastzer, 1980; West, King, & Harrocks, 1983). In addition, we have demonstrated that females do not respond equally to the songs of all males even when the "choice" is among familiar males from the same locality. Female cowbirds can discriminate differences in songs that indicate the "right" class of male, that is, the male's subspecific or geographic identity, but, more important, they can discriminate differences that reveal the most dominant or reproductively successful male (King & West, 1983c; West et aI., 1983). That the female's response to playback songs is biologically significant is indicated by data showing that playback potency predicts mate choice in captivity (Eastzer, King, & West, 1985; West, King, & Eastzer, 1981; West et aI., 1983). The sensitivity of the female cowbird to song content would perhaps be expected in a parasitic species in which information about a male's reproductive quality is more limited because nest building, territorial maintenance, defense, and parental activity do not occur. A female appears to require little to no postnatal experience with song in order to respond selectively, a capacity that may also reflect the ontogenetic implications of brood parasitism (King & West, 1983a; but see King, West, & Eastzer, in press, for populational differences in the role of postnatal experience). Here we asked about song potency from the male's perspective. Does a male develop his repertoire on the basis of song potency, an attribute operationally defined in terms of female responsiveness to song? As a first step, we asked whether males, without opportunities for interaction with females, would select songs for imitation that we knew females found to be attractive. Positively correlated selectivity between males and females might seem likely given the presumed evolutionary linkages between signal perception and signal production necessary for mate identification (Doherty & Gerhardt, 1983; Hoy, 1974; Marler, 1976).REPERTOIRE DEVELOPMENT 297 We chose as the independent variable songs already identified as potent or not potent by previous playback to females and as the dependent variable song copying by juvenile males. We knew naive males could produce a potent, albeit structurally atypical, song without


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