Female Bobolink molts into male-like plumage and loses fertility

ABSTRACT.   Among wild birds, females expressing a male-like phenotype have rarely been documented. During a study of Bobolinks ( Dolichonyx oryzivorus ) in Vermont, I observed a female that changed secondary sexually selected plumage forms between breeding seasons. During the 2006 breeding season, I monitored an individually marked female Bobolink in typical alternate plumage, with a light brown back and yellow undersides. This female produced a clutch of four eggs, and two young eventually fledged. In 2007, this female returned to the same area with predominately male alternate plumage, including black wing and body feathers, buff nuchal collar, and white scapulars and rump. She laid and incubated five eggs for 21 d before abandoning the nest; none of the eggs were fertile. I banded 324 additional female Bobolinks from 2002 to 2007 in the same study area and none exhibited similar between-year changes in plumage. In addition, a review of the literature revealed no previous reports of such changes. The male-like plumage and loss of fertility by the female Bobolink may have been due to an infection that damaged the left ovary, increasing androgen production and reducing estrogen production in the right ovotestis.