Use of discrete molting grounds by migrant passerines undergoing prebasic molt in southern Quebec

Abstract

We documented prebasic flight feather molt of passerines captured in fall 2013 and 2015 at McGill Bird Observatory (MBO) in Montreal, Quebec. We recorded active molt of flight feathers (remiges) in 11 species that do not breed on site. Flight feather molt was frequent among Swainson's Thrush (Catharus ustulatus; 64% of adults), Tennessee Warbler (Oreothlypis peregrina; 57%), Nashville Warbler (Leiothlypis ruficapilla; 67%), and Yellow-rumped Warbler (Setophaga coronata; 44%), and was observed less frequently in other species. The minimum stopover length of molting individuals was on average 8 times longer than that of non-molting individuals of the same species. Among Swainson's Thrushes and Yellow-rumped Warblers, far more females were undergoing molt than males, whereas for Tennessee Warblers molt was slightly more frequent among males. Frequency of molt was similar between years for most species but not Yellow-rumped Warbler, with 59% of adults captured in 2013 molting compared to none in 2015. We also observed molting site fidelity with multiyear returns of Tennessee and Nashville warblers. The use of separate breeding and molting sites is not well understood among eastern North American species, and with recent studies highlighting the importance of molt locations in western North America, we demonstrate the value in additional study of the use of discrete molt locations in the East.