Timing of molt is a critical life-history trait because molt is a nutritionally demanding process that must be completed before fall migration. We used data from 1992–2008 to assess hypotheses that initiation of the prebasic molt by Black Brant (Branta bernicla nigricans) was ultimately controlled by (1) the need to complete the molt before fall migration and (2) advantages of restoring depleted nutrient reserves before growing feathers. Specifically, we expected molt to occur sooner after hatching in years when nesting was delayed (prediction 1), but we expected individuals to delay molt in relation to hatch date when nesting was earlier (prediction 2). Our rationale for prediction 1 was that the need to complete molt before fall migration is exacerbated when nesting is delayed. Prediction 2 is suggested by the advantage of restoring nutrients depleted during nesting before initiating molt. We tested predictions by assessing patterns of ninth-primary length in relation to number of days posthatch, modal hatch date, and relative hatch date within years. We calculated molt initiation dates using parameter estimates from the best-supported model, using lengths and growth rates of ninth primaries. We estimated that Black Brant initiated molt an average of 14.5–19.5 days posthatch, depending on year. For modal hatch dates before 22 June (summer solstice), date of mean initiation of molt was negatively correlated with modal hatch date (β = -0.56), whereas for modal hatch dates after 22 June, mean initiation of molt began an average of 16.1 days after hatch. In years when modal hatch date was before 22 June, Black Brant waited longer after their clutches hatched before beginning molt, which suggests an advantage of greater stored nutrient levels before initiating molt. By contrast, in late-nesting years, Black Brant began molt 16 days after they hatched their clutches, which suggests that (1) a minimum average period posthatch was required before molt commenced and (2) there was a cost of further delay in molt in late-nesting years. Declining photoperiod following the summer solstice is a reasonable candidate for the signal that Black Brant use to initiate molt in late-nesting years.
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Vol. 129 • No. 2