Nestling growth and song repertoire sire in great reed warblers: evidence for song learning as an indicator mechanism in mate choice

Females of many songbird species show a preference for mating with males th at have larger song repertoires, but the advantages associated with this pr eference are uncertain. We tested the hypothesis that song complexity can s erve as an indicator of male quality because the development of the brain r egions underlying song learning and production occurs when young birds typi cally face nutritional and other stresses, so that song reflects how well a male fared during post-hatch development. A key prediction of this hypothe sis is that variation in nestling condition should correspond to variation in the adult song repertoires of individuals. We used data from a long-term study of the great reed warbler (Acrocephalus arundinaceus) to test this p rediction, correlating two measures of nestling development with subsequent repertoire size of males. We found that the length of the innermost primar y feather, a standard measure of development, significantly predicted first -year repertoire size. The relationship between repertoire size and body ma ss was nearly significant, in spite of the large variance inherent in this measure. These data support the idea that song may provide females with inf ormation about a male's response to developmental stress, which in turn is expected to correlate with indirect or direct benefits she might receive.