Timing and Extent of the Molts of Adult Red-Eyed Vireos on Their Breeding and Wintering Grounds

Abstract

The molt of adult Red-eyed Vireos (Vireo olivaceus) on their breeding grounds was studied at three widely separated sites in northeastern North America: North Point, James Bay (NPJB), Ontario; Vermont Institute of Natural Science (VINS), Woodstock, Vermont; and Powdermill Nature Reserve (PNR), Rector, Pennsylvania. Unlike other North American vireos and most other North American passerines, adult Red-eyed Vireos undergo an incomplete molt on the breeding grounds. This molt typically replaces most or all of the body plumage, about half of the wing coverts, the tertials, and the central rectrices. Extent of molt did not differ among localities; however, timing of the molt was 1-2 weeks earlier and onset of molt occurred over a longer period at PNR than at VINS or NPJB. Based on examination of museum specimens, flight feathers not molted on the breeding grounds are replaced on the wintering grounds beginning in early January ; average duration of primary molt on the wintering grounds is 78 days. It is unclear whether or not feathers molted on the breeding grounds are molted again at this time. This issue needs to be resolved before Humphrey-Parkes terminology can be applied to breeding and wintering ground molt episodes in this species. Selective advantages associated with a protracted nesting season and with early onset of migration both may have had a role in shaping the molt strategy of adult Red-eyed Vireos. Rapid onset of fall migration following nesting in this seasonally frugivorous species may have been influenced by the phenology of fruit availability and abundance en route to and especially on its Amazonian wintering grounds.