Color aberration in the Yellow-legged Thrush (Turdus flavipes) in Brazil

Color anomalies in birds may be caused by several factors. Records of aberrant white feathers are relatively frequent, and have been reported in several Turdus species. In this study, we describe an aberration in an adult Yellow-legged Thrush and estimate the frequency of color aberrations in this species by analyzing published photographs. We assessed 1891 photographs of adults in the WikiAves (www.wikiaves.com.br) database, taken between Nov/1999 and May/2017 in 11 Brazilian states, in addition to 27 mist-netted individuals we banded in May-August/2016 in Rio de Janeiro state. The general frequency of aberrants was 1.66%, within the expected range (1-2%) for natural populations, and was similar between males (1.69%, n=1653) and females (1.51%, n=265). In 75% of the cases, the aberrant feathers were found exclusively or predominantly on the head. All aberrant individuals had varying quantities of totally white feathers, apparently due to progressive greying or partial leucism. The only aberrant individual we captured had brown plumage, typical of the female, but several (30-40%) unpigmented feathers, mainly on the head and scattered over the body, consistent with an advanced stage of progressive greying, a non-hereditary mutation restricted to older birds and usually affecting only the plumage. However, its bill was entirely yellow, which is an exclusive trait of the male. This individual may thus represent an unprecedented case of a leucistic female whose bill lacks the dark melanin pigmentation typical of the females of the species.