First prebasic molt in Herring, Thayer's, and Glaucous-winged Gulls

We made regular field observations over three winters of first-year Herring Gulls (Larus argentatus smithsonianus), Thayer's Gulls (L. thayeri), and G laucous-winged Gulls (L. glaucescens) in order to document the timing of fi rst prebasic molts. The first prebasic molt of North American Herring Gulls starts between September and February. Most resident birds on the Atlantic coast undergo extensive first prebasic molt by late November, while some m igrants wintering in California do not start molt until January or later. T he onset of first prebasic molt in 90% of Thayer's Gulls is not until Janua ry or later, and some birds migrating north in March appear to be still in juvenal plumage. Initiation of first prebasic molt in Glaucous-winged Gulls spans late October to March, such that some birds have largely finished mo lting before others start. Our findings differ from or clarify those of pre vious authors. We suggest that differences in molt timings within and among taxa of large gulls reflect the nature of their migrations. Largely reside nt or short-distance migrant populations can start molt up to several month s earlier than those populations that undertake longer migrations. Most mol t studies rely heavily upon specimens. However, while museum collections ma y have adequate series of passerines, this is rarely true of larger birds s uch as gulls. Critical field observations of species that lend themselves t o field study, in combination with documentary photographs and judicious co llection of voucher specimens, can more easily provide adequate samples to follow molt in a population.