Foraging ecology of yellow-rumped warblers in an Alaskan boreal forest following a spruce beetle outbreak

Abstract

I examined the foraging ecology of the Yellow-rumped Warbler (Dendroica coronata) several years after an outbreak of spruce beetles (Dendroctonus rufipennis) in the Copper River Basin, Alaska. With increased beetle-induced mortality of white spruce (Picea glauca), a preferred foraging substrate, we predicted warblers would respond through: (1) decreased overall use of white spruce, (2) increased selectivity of live white spruce that remained, and (3) reduced foraging efficiency, reflected by a greater proportion of time spent foraging and lower prey attack rates. We examined warbler foraging behavior and arthropod biomass on commonly used foraging substrates, and in stands with low-moderate (<40%) and heavy (>40%) spruce mortality. Live and dead white spruce, quaking aspen, and willow were the most commonly used foraging substrates, and selection of coniferous versus deciduous tree types varied by breeding stage. Yellow-rumped Warblers foraged extensively on dead spruce in stands with heavy spruce mortality, although they avoided it in stands with low-moderate spruce mortality. Dead spruce supported significantly lower arthropod biomass than any other tree type except black spruce, and warblers that foraged in dead spruce tended to have lower prey attack rates than when they foraged in live white spruce.