Molt is an energetically costly process, and songbirds (Order Passeriformes) exhibit a diversity of strategies to maximize their survival and reproductive success while meeting the energetic demands of the annual prebasic molt. Nearctic‐Neotropic migrants in western North America commonly exhibit one of three strategies: (1) remain in breeding areas to molt, (2) migrate long distances to molt before continuing to wintering areas, or (3) migrate to wintering areas and then molt. Among species that molt in or near breeding areas, the nature of small‐scale movements to undergo molt remains largely unknown. We used banding data collected over a period of 27 yr and across an elevational gradient to examine the propensity of Wilson's Warblers (Cardellina pusilla) to molt and breed at the same or different locations in northern California and southern Oregon. We found that individual adult Wilson's Warblers were more likely to breed at lower elevations and molt at higher elevations, suggesting that some individuals move altitudinally after breeding to complete the definitive prebasic molt. Such altitudinal movements may be more common among Nearctic‐Neotropic migrants in western North America than previously thought.

Migratory species employ a variety of strategies to meet energetic demands of postbreeding molt. As such, at least a few species of western Neotropical migrants are known to undergo short‐distance upslope movements to locations where adults molt body and flight feathers (altitudinal molt migration). Given inherent difficulties in measuring subtle movements of birds occurring in western mountains, we believe that altitudinal molt migration may be a common yet poorly documented phenomenon. To examine prevalence of altitudinal molt migration, we used 29 years of bird capture data in a series of linear mixed‐effect models for nine commonly captured species that breed in northern California and southern Oregon. Candidate models were formulated a priori to examine whether elevation and distance from the coast can be used to predict abundance of breeding and molting birds. Our results suggest that long‐distance migrants such as Orange‐crowned Warbler (Oreothlypis celata) moved higher in elevation and Audubon's Warbler (Setophaga coronata) moved farther inland to molt after breeding. Conversely, for resident and short‐distance migrants, we found evidence that birds either remained on the breeding grounds until they finished molting, such as Song Sparrow (Melospiza melodia) or made small downslope movements, such as American Robin (Turdus migratorius). We conclude that altitudinal molt migration may be a common, variable, and complex behavior among western songbird communities and is related to other aspects of a species’ natural history, such as migratory strategy.