Weak premating reproductive isolation despite divergence in secondary sexual traits in the variable seedeater

Abstract

The likelihood of hybridization after secondary contact is reduced when assortative mating is strong and fitness of hybrids is low; thus, maintaining reproductive isolation. To this end, signals that convey population identity may enhance recognition and reduce hybridization by promoting assortative mating. Here, we studied visual and acoustic signals, plumage colour and song, that commonly mediate species recognition in birds to evaluate their role in reproductive isolation in a recently diverged clade of Neotropical birds. Subspecies of the variable seedeater, Sporophila corvina, that differ dramatically in plumage colour have established secondary contact at two independent contact zones in Costa Rica and Panama. We first evaluated divergence in song in allopatric populations close to the most recent contact zone (Costa Rica), where the two subspecies do not hybridize, and found significant differences in song structure and composition of song elements between subspecies. Then, we used an experimental approach to measure a territorial male’s aggressive response to visual and acoustic signals, as a proxy of the role of divergent secondary sexual traits in reproductive isolation. These playback experiments did not show clear effects of divergent colour or song on aggressive response: we found no effect of plumage coloration or song in one subspecies and males of the other subspecies were more aggressive towards homotypic songs but, unexpectedly, also more aggressive towards to heterotypic plumages. Separately, we also characterized the song structure of hybrid individuals from the Panamanian contact zone to evaluate how songs covary given evidence for gene flow. These hybrid individuals sang songs of different element composition than allopatric subspecies but did not differ in most acoustic characteristics. Overall, our experiments suggest that reproductive isolation based on secondary sexual signals is weak in this group, and after secondary contact, the breeding phenology may synchronize and divergent acoustic signals may homogenize, facilitating further hybridization.