SONG-BASED SPECIES DISCRIMINATION AND BEHAVIOR ASSESSMENT BY FEMALE BLACKBIRDS, TURDUS-MERULA

The ability of female blackbirds to integrate information about a singer's species and possible ensuing behaviour from different parameters of the introductory part (i.e. the motif part) of the blackbird's song was tested. Females were made receptive to song stimulation by oestradiol injections and tested with tapes with modified motif parts in an anechoic chamber using a broad spectrum of female reproductive behaviour as response measures. Females, like males, attended to the natural upper and/or lower limits of variation in seven parameters in the process of species discrimination: (1) the frequency level and (2) the duration of sound figures (i.e. sounds separated by pauses greater than 50 ms); variation across the motif part in the sound figures' (3) frequency level, (4) duration and (5) amplitude; (6) the ratio of the sound pressure of the fundamentals to that of the first and second overtones; and (7) the relative proportions of the three types of sound figures (i.e. (a) whistles; (b) trills, gurgles and squeaks; and (c) compounds of a and b). The results suggest that females, unlike males, disregard both the upper limit of variation in the frequency level in the process of species discrimination, and the degree of modulation of the sound figures' amplitude functions (i.e. envelopes) and, perhaps, the pauses between the sound figures in the process of assessing the ensuing behaviour of the singer. The validity of such sexual differences in song decoding and possible factors leading to their evolution are discussed. The present differences all concern parameters that degrade more when passing through dense vegetation to females than when passing through the more open communication channels to males. This is consistent with the 'song-degradation' hypothesis that one of the sexes may pay less attention than the other to certain song features if these features are degraded to a different extent on their way to the two sexes. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.