Postbreeding elevational movements of western songbirds in Northern California and Southern Oregon

Abstract

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.

Phenology and productivity in a montane bird assemblage: Trends and responses to elevation and climate variation

“…On average, our index of productivity was higher at high elevations, likely reflecting augmentation of local productivity with upslope movements of young birds to some extent. Although we are unable to distinguish the relative spatial scales of sampling at our monitoring stations, higher elevation sites may provide high-quality late-season habitat for both postfledging young birds and adults when habitats at lower elevations are relatively hot and dry (Gaines, 1988;Wiegardt, Wolfe, Ralph, Stephens, & Alexander, 2017).…”

Altitudinal migration: ecological drivers, knowledge gaps, and conservation implications

“…The loss of critical high‐elevation habitats will have profound effects on altitudinal migrants depending on them for part of their annual cycle. For example, some Neotropical migratory birds perform a post‐breeding upslope migration to moult, a time when they experience increased energetic costs and predation risk (Wiegardt et al ., , b ).…”

Long‐distance movements and evidence of post‐breeding elevational movements by Cassin’s Vireos

“…I maintain that the most likely explanation for my data is that Cassin’s Vireos in the Sierra Nevada Mountains, like the population studied by Rohwer et al () in Washington State, move to higher elevations in late summer. If so, this suggests a movement strategy similar to that of several other species that breed in western North America and show upslope movements in late summer, including Orange‐crowned Warblers ( Vermivora celata , Steele and McCormick ), Wilson’s Warblers ( Cardellina pusilla , Wiegardt et al ), and MacGillivray’s Warblers ( Geothlypis tolmiei ) and Swainson’s Thrushes ( Catharus ustulatus , Wiegardt et al ). Another common post‐breeding strategy for approximately half of the species of songbirds in western North America is to migrate to the monsoon region of northwestern Mexico during the post‐breeding period (Rohwer et al , , Pyle et al , Pillar et al ).…”

“…In western North America, complex post‐breeding movements are thought to be a common strategy for coping with the spatial, elevational, and seasonal variation in food resources characteristic of mountainous regions (Leu and Thompson ). Several species appear to move along elevational gradients in late summer, likely tracking the availability of food resources (Wiegardt et al ). Many other species leave breeding areas in late summer to molt in the monsoon region of Arizona and northwestern Mexico (Pyle et al , Pillar et al , Bridge et al ).…”