Geographical variation in bill colour in the Long-tailed Finch: evidence for a narrow zone of admixture between sub-species

The Long-tailed Finch (Poephila acuticauda) is endemic to Australia's northern tropics and comprises two sub-species differentiated by bill colour: yellow in the western sub-species acuticauda and red in the eastern sub-species hecki. We present an extensive survey of bill colour variation across the range of the Long-tailed Finch (more than 700 individuals collected across nearly 1500 km of range). Geographic clinal analysis using bill reflectance spectrophotometry data suggests that the greatest extent of bill colour admixture is only similar to 150 km wide and is centred approximately 50 km west of Katherine, Northern Territory (14.5 degrees S, 132.3 degrees E). Variation in bill colour is generally low outside of this zone, and across widely distributed allopatric populations of both sub-species. However, the best-fit clinal model supports a pattern of asymmetrical introgression of bill colour from the east into the west. The narrow geographic width of bill colour admixture between Long-tailed Finch sub-species and the estimated timing of secondary contact (14-21 kya) suggests a role for selection acting against hybrids and/or parentals in maintaining sub-species as independently evolving populations.