Post‐breeding population dynamics indicate upslope molt‐migration by Wilson's Warblers

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.