Among Yellow-rumped Warblers (Dendroica coronata)1 banded in Portland, Oregon, in the spring migration seasons of 2007 through 2010, prealternate molt was found in 40.0% of the Myrtle subspecies group (D. c. coronata group) but just 5.9% of the Audubon's subspecies group (D. c. auduboni group). The peak of molt frequency occurred prior to the peak of abundance for both subspecies groups, but that of the Myrtle Warbler substantially overlapped its peak in abundance and represented 82.4% of individuals captured. Twenty-two Yellow-rumped Warblers (1.4%) were recaptured. The mean stopover time for those recaptured within the same season was 8.27 d. Most of the same-season recaptures gained fat or became no leaner during their stay. Molting birds had lower fat scores than prior to and after completion of molt. Fat scores continued to increase following completion of molt. Multiple lines of inference point to the importance of Smith and Bybee Lakes Preserve and its vicinity as a migratory stopover site for the Yellow-rumped Warblers. Prealternate molt at a migratory stopover site to the extent shown by the Myrtle Warbler subspecies group has not previously been documented for any species.
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Vol. 92 • No. 2