An Unusual Sequence of Molts and Plumages in Cassin's and Bachman's Sparrows

An examination of feather replacement and plumage wear in study skins of Aimophila cassinii and A. aestivalis showed a sequence of molts and plumages for both as follows: Juvenal body plumage is replaced soon after fledging by a partial molt that produces plumage of adult-like body feathers combined with juvenal remiges, rectrices and greater primary and secondary coverts. This plumage has a spotted breast pattern intermediate between the streaked juvenal and spotless adult patterns, and is completely replaced in the bird's first autumn by a molt of all body, wing and tail feathers. A prolonged body molt of low intensity lasts through the spring-autumn breeding season, replacing feathers in all areas of the body. During autumn, adults have a complete molt coinciding with the second molt of the young birds. The replacement of all pennaceous body plumage twice within a bird's first six months of age, and a molt of body feathers in adults throughout the breeding season have not been reported before for a North American passerine, nor was this pattern described in an earlier study of the molt of these species. It is suggested that these partial molts renew plumage that otherwise would become severely worn in the birds' abrasive habitats.