Incomplete song divergence between recently diverged taxa: syllable sharing by Orchard and Fuertes’ orioles

ABSTRACT Orchard Orioles (Icterus spurius) and Fuertes’ Orioles (I. fuertesi) recently diverged from each other, making them an ideal system for investigating trait evolution and mechanisms of reproductive isolation during the early stages of speciation. These taxa differ in adult male plumage coloration and in their breeding and wintering ranges, but quantitative comparisons of their song characteristics have revealed no discernible differences. We assessed evolutionary song divergence in this group by investigating patterns of syllable‐type sharing within and between populations. Of 529 distinct syllable types, 142 (26.8%) were shared among individuals, and sharing appeared to decrease with geographic distance. The total number of syllables shared between Orchard and Fuertes’ orioles (26; 4.9% of the total) was similar to levels of sharing between populations of Orchard Orioles. Furthermore, hierarchical cluster analyses showed individuals of the two taxa intermixed. Syllables also used as calls were shared more frequently within and between taxa, suggesting that they have evolved more slowly than those used exclusively in songs. Our results show that at least some aspects of song have not yet diverged between these incipient species, either due to cultural exchange or because songs have evolved relatively slowly compared to plumage colors.