First molt of the Shiny Cowbird Molothrus bonaeriensis and the Screaming Cowbird M. rufaxilaris: description in captive individuals

FACCHINETTI, CAROLINA; URSINO, CYNTHIA; REBOREDA, JUAN CARLOS

Cuzco

Congreso; IX Congreso de Ornitología Neotropical y VIII Congreso Peruano de Ornitología; 2011

Differences in molting pattern between brood parasitic cowbirds with host-mimetic and host non-mimetic young. Facchinetti Carolina, Ursino Cynthia and Reboreda Juan Carlos Molt and plumage cycles are poorly known in Neotropical birds in general and in South American brood parasitic cowbirds in particular. The Shiny Cowbird (Molothrus bonaeriensis) is a generalist parasite that uses more than 250 hosts while the Screaming Cowbird (M. rufoaxillaris) is an extreme specialist that parasitizes almosts exclusively the Baywing (Agelaioides badius). Fledglings of Screaming Cowbirds are mimetic in plumage color to fledglings of Baywings while Shiny Cowbird fledglings have female like plumage. We measured plumage changes of Shiny and Screaming Cowbirds since the time of fledging (i.e. 10 days of age) until young completed their first molt and analyzed species and sexual differences in the molting pattern of body, tail and wings. Screaming Cowbirds started their molt before Shiny Cowbirds (37 vs. 61 days of age), but the length of the molt was similar for both species (screaming: 120 days, shiny: 124 days). Screaming Cowbirds first molted under wing covers and always started body molt before starting the molt of primaries. Shiny Cowbirds first molted crown feathers and started the molt of primaries either before or after the start of body molt. There were no sexual differences in molting pattern for both species. Our results show that Screaming Cowbirds start molting in the definitive plumage when they become independent from their foster parents and join conspecific flocks while Shiny cowbirds start molting one month after joining conspecific flocks. This shorter latency in the start of molt could favour the acceptance of Screaming Cowbird young in conspecific flocks.