In the paper published in Ecology & Evolution, we report on experiments in which we assessed mate choice by both females and males for members of the opposite sex with bills colours that were either the same as their own, or those of the alternate sub-species. These findings differ from those found in some other hybridising species, where colour signals make an important contribution to pre-copulatory species barriers. The findings help us to understand the evolutionary processes that maintain the divergence between the two forms in the wild across northern Australia. The full paper, led by Callum McDiarmid, and part of his PhD on speciation in this species, can be found here.
Despite the lack of clear assortative mating on the basis of bill colour, the displacement of the contact zone illustrated above in orange, with the area in which most of the genomic divergence occurs (shown with a dashed line), suggests the introgression of the bill colour genes from the west towards the east. This finding can be partly explained by the potential dominance of at least some of the genes for bill colour. In our study we report the findings from experimental crosses between the subspecies in captivity where the colour of the bill in female F1 hybrids is dependent on their father, supporting this idea.