A historical study of population dynamics and polymorphism of head mask colours in the Gouldian Finch (Chloebia gouldiae) from the Northern Territory and eastern Kimberley region

Gouldian Finches (Chloebia (Erythrura) gouldiae) occur across tropical Australia but numbers have varied considerably. Population sizes were estimated by the Capture–Mark–Release–Recapture method near Katherine and Pine Creek in the Northern Territory in 1966. Two to three thousand birds occurred at each locality, indicating they were abundant 60 years ago. Birds were relatively short lived. Gouldian Finches display a sex-linked genetic polymorphism in head mask colour with red mask dominant to black. Anecdotal evidence suggested that about 30% of birds in the wild have red masks. To help understand the forces that maintain this polymorphism, we recorded the number of red and black masked males and females separately. Overall 30.6% ± 3.8% of males and 22.7% ± 5.2% of females were red, with frequencies not differing significantly between Katherine and Pine Creek populations. Data collected subsequently by others from nearby and from the Kimberley area were consistent with this, suggesting a panmictic population occurs without geographic or temporal diversity. Extensive data from birds trapped in the eastern Kimberley region from 1950 to 1958 revealed divergence from Hardy-Weinberg expectations. Red females were less frequent than expected and red males more. These data included mostly juveniles in their first year. Samples limited to older birds consistently showed many fewer females overall than males. Selective loss of red females may contribute to this. If so, the polymorphism could be maintained by sexual antagonism, with selection against the red allele in adult females being balanced by its preferential transmission during breeding of red males.