Preformative Molt in Brood Parasitic Screaming (molothrus Rufoaxillaris) and Shiny (m. Bonariensis) Cowbirds

– Molt and plumage cycles are poorly known in Neotropical birds in general and in South American brood parasitic cowbirds in particular. The Screaming Cowbird (Molothrus rufoaxillaris) is one of the more specialized brood parasites that use almost exclusively the Baywing (Agelaioides badius), while the Shiny Cowbird (M. bonariensis) is an extreme generalist brood parasite that uses more than 250 known hosts. Fledglings of Screaming Cowbird are mimetic in plumage coloration to fledglings of Baywings, while Shiny Cowbird fledglings have female-like plumage. We measured and analyzed plumage changes of Screaming and Shiny Cowbirds since the time of fledging (i.e. 12-14 days of age) until young completed the preformative molt. There were not sexual differences in molting pattern for both species. Screaming Cowbirds started to molt before Shiny Cowbirds (37 vs. 61 days of age). They first molted under-wing coverts, followed by chest, the rest of the body, and finally tail and external coverts. Shiny cowbird molt started at the crown, breast and back, followed by throat, under-wing coverts, and finally upper-wing covers and tail. In both species flight feathers began molting after body feathers and the pattern of molt was similar, starting with the primaries. For both species the preformative molt was completed at the same age (167-181 days). The beginning of the preformative molt in Screaming Cowbirds coincides with the age at which young become independent of their foster parents and join conspecific flocks. This shorter latency to start molting in Screaming Cowbirds could be related to the acceptance of juveniles in conspecifics flocks.