Interspecific mate choice by late-courting male western grebes

In mixed populations of western and Clark's grebes, advertising calls by males and females play a critical role in mate choice and reproductive isolation. We conducted field playback experiments that tested whether courting western grebe males became less choosy in their responses to female Clark's grebe calls as the mating season progressed and mating opportunities diminished. Late-courting western grebe males were much more likely to answer and approach advertising calls of Dark's grebe females than were males courting earlier in the season. This change in responsiveness occurred as the operational sex ratio index of the population approached 3:1 male calls per female call. These and field census data support the hypothesis that late-season hybridization between these two closely related species may be a result not of species misidentification, but of active and adaptive mate choice by individuals with limited alternatives.