Aberrations of plumage coloration are caused by hereditary and environmental factors and are not rare in wild birds, but few studies have estimated their frequency in natural populations. Here, we provide the first report of a white plumage aberration in an insular population of the White-necked Thrush (Turdus albicollis) on Ilha Grande, in southern Rio de Janeiro state, Brazil. Four (two males and two females) of the 123 individuals examined in 2013–2015 presented unusual white feathers on some part of the body (especially the hindneck), which may represent progressive graying or the result of injury. The frequency of the white plumage aberration recorded in our study (3.25%) is approximately three times higher than that expected in natural bird populations, and three of the four aberrant individuals were captured in the most disturbed forest site sampled. Further studies will be necessary to identify the true causes of the high frequency of white plumage aberrations detected in the study population, which may include diet, longevity, behavior, parasitism or other environmental factors.