LATE BREEDING-SEASON DEFINITIVE PRE-BASIC MOLT BY MALES, AND LATE BREEDING SEASON BROOD CARE BY FEMALES, IN CENTRAL CALIFORNIA WILSON'S WARBLERS

I made late breeding season (after 1 July) observations of a central California population of Wilson’s Warbler, Cardellina pusilla, over nine breeding seasons. I sighted males in definitive prebasic molt from 4 July in 2007 through 1 September in 1999. Possibly all territorial males molted on their breeding territories, and molt lasted a maximum of 46 days. Following prebasic molt, territorial males engaged in subdued “post molt singing,” which lasted 14 days in one male, but usually no more than seven days, and was last heard on 6 September in 1999. I sighted no female in definitive prebasic molt, nor in bright basic plumage, during the study. Of 12 color-banded females sighted after 22 July, I confirmed 11 had been in late breeding season uniparental brood care. Possibly all resident females not engaged in late uniparental brood care vacated their breeding territories earlier, and before resident males. They presumably underwent prebasic molt at later times and locations not known. Remaining late nesting females must have molted much later than resident males, and likely later than early departing resident females, and at unknown locations. I last sighted uniparental brood tending females still in dull alternate plumage on 26 and 29 August respectively, whereas some males had completed prebasic molt by 13 August. Three unique study findings are the occurrence of male post molt singing, a male/female difference in location of prebasic molt, and a likely dichotomy of prebasic molt timing between females leaving their breeding grounds early, and those remaining in uniparental brood care.