REPLACEMENT OF PRIMARIES DURING PREALTERNATE MOLTS IN NORTH AMERICAN LARUS GULLS.

We document replacement of primaries during the prealternate molt in two and possibly three species of North American gulls of the genus Larus, including the first report of such replacement in an adult Yellow-footed Gull (L. livens), the first report in the Lesser Black-backed Gull (L. fuscus) in the Americas, and possibly the first report for the American Herring Gull (L. argentatus smithsonianus). The incidence and extent of replacement of primaries is greater during the second prealternate than during subsequent prealternate molts, which is likely related to second-cycle molts in Larus being earlier than the subsequent molts. The second prealternate molt of the Lesser Black-backed Gull includes up to all flight feathers (but not all wing coverts). The sequence of replacement of primaries during the prealternate molt matches that of the prebasic molt, starting at the innermost primary and proceeding distally; however, the sequence of replacement of secondaries can differ from that during the prebasic molt, perhaps because of a difference in the underlying mechanisms controlling these molts. Prealternate molt of inner primaries can begin before prebasic molt of outer primaries is completed, a pattern resembling Staffelmauser, but all evidence suggests that the ensuing prebasic molt of the primaries begins at p1, as in terns, rather than at the point where the inner molt wave is suspended, as during Staffelmauser in other large volant birds. We propose that the occurrence and extent of prealternate molt of the remiges in Larus is correlated with the latitude at which an individual winters and/or the timing of the prebasic molt the year before, as much as or more so than with phylogeny. The possible replacement of primaries during the second prealternate molt in North American but not European subspecies of the Herring Gull could relate to some individuals of the American subspecies wintering farther south. [ABSTRACT FROM AUTHOR]