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Ecological factors on the breeding grounds are expected to have a relatively large effect on the timing of molt in migratory birds breeding at middle latitudes (e.g., southern North America), because constraints on the time available to molt are probably weak. We investigated the timing of prebasic molt in adults of two migratory passerines, the endangered Black-capped Vireo (Vireo atricapilla) and the common White-eyed Vireo (V. griseus), on their breeding grounds in Texas. We compared the onset and rate of molt between sites differing in habitat and between years differing greatly in precipitation due to the El Niño–Southern Oscillation (ENSO). Black-capped Vireos began molting 13 days later (8 July) and completed molt 12 days faster (50 days) in low quality, clumped habitat than in more typical, contiguous shrubby habitat (62-day molt starting 25 June). Long-term estimates of onset and duration of molt based on study skins were intermediate. White-eyed Vireos in the site with clumped habitat also started molting significantly later (13 July), but did not molt significantly faster (56 days), than those in the shrubby site (63-day molt starting 2 July). Both species started molting significantly later in the wet La Niña year than in the year with dry El Niño conditions. Body condition did not differ between sites or years. Our molt duration estimates of ~55 days for Black-capped Vireos and ~60 days for White-eyed Vireos suggest weak time constraints on molt in these populations. This study highlights the sensitivity of molt dynamics to events and conditions during the preceding breeding season.

PREBASIC MOLT OF BLACK-CAPPED AND WHITE-EYED VIREOS: EFFECTS OF BREEDING SITE AND THE EL NIÑO–SOUTHERN OSCILLATION