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Wilson Warbler molting and plumage observations


Gilbert, William (2022), Wilson Warbler molting and plumage observations, Dryad, Dataset,


I made observations of a central California population of Wilson’s Warbler, Cardellina pusilla, on or after 1 July over ten breeding seasons. I sighted males in definitive prebasic molt as early as 4 July (in 2007) and as late as 1 September (in 1999). I determined that most territorial males at my study site molted on their breeding territories, and individual molt lasted a maximum of 46 days. Following prebasic molt, territorial males engaged in subdued “post-molt singing,” which appeared to last no more than seven days in most males, and which I first heard on 13 August (in 1004), and last heard on 6 September (in 1999). I sighted no female in definitive prebasic molt, nor fresh basic plumage, during the study. Of 13 females sighted on or after 21 July, I confirmed that 11 were in late breeding season uniparental brood care, and I could not rule out late brood care for the other two. Most, and possibly all, females not engaged in late season uniparental brood care appear to have vacated their breeding territories before 21 July, as I saw just two possible after that date. This apparent early departure of the majority of resident females from their breeding grounds was much earlier than the departures of resident males, the last of which I sighted on 10 September (in 1999). Early-departing females presumably underwent prebasic molt after 21 July, and at locations not known. Remaining late-nesting females must have molted much later than did resident males, and also likely later than early-departing resident females, and at unknown locations. I last sighted two uniparental brood-tending females, still in worn plumage, on 26 and 29 August respectively, whereas some males had completed prebasic molt as early as 13 August. Two unique findings of this study are a male/female difference in location of prebasic molt, and a likely dichotomy of prebasic molt timing between females which leave their breeding territories relatively early, and those which remain on their territories in uniparental brood care. Another finding, that most, and perhaps all, territorial male Wilson’s Warblers engaged in brief post-molt singing following prebasic molt, is a largely unrecognized behavior, but one previously reported in several other passerine species. Post-molt singing may be a reliable indicator that prebasic molt is complete.


Data collected through careful filed observation after 1 July over ten study seasons. Sightings of molt and plumage from the ten study seasons was divided into five day increments to obtain a record of the type of plumage (worn or basic) and the presence or absence of prebasic moltin in male and female Wilson's Warblers respecitively.

Usage Notes

No missing values. Data used to develop a graph of the frequency of worn plumage, prebasic molt, and basic plumage sighted in male and female Wilson Warblers, and viewer could dublicate the progression of these stages after 1 July from the data.