Does bill size polymorphism affect courtship song characteristics in the African finch Pyrenestes ostrinus?

Vocal characteristics of bill morphs of the African finch Pyrenestes ostrin us were investigated to see whether there were differences between the morp hs that could potentially lead to assortative mating. Morphological differe nces between bill morphs are of a scale that could change resonance charact eristics, with as a physical consequence, a change in acoustic characterist ics. Song and variation in acoustic characteristics are very likely to be a n important factor in mate choice. We analysed recordings of large and smal l-billed birds, and measured II acoustic characteristics focusing on freque ncy use and possibly correlated temporal features. In addition, we investig ated in more detail the energy distribution within the frequency limits. We found no differences between bill morphs in acoustic characteristics of co urtship song. Our findings contrast with other empirical studies which show an impact of the suprasyringeal vocal tract on song output. One possible e xplanation could be that the morphological changes affect resonance charact eristics in multiple ways which do not operate in concert. Beside proximate aspects, we discuss the role of song and the bill polymorphism in the cont ext of sympatric speciation. (C) 2000 The Linnean Society of London.