Temporal variation in resource use by Black-throated Gray Warblers

Abstract

We studied foraging behavior and habitat use of male and female Black-throated Gray Warblers (Dendroica nigrescens) to quantify the effects of temporal variation on interpretations of avian resource use. Overall, both sexes primarily foraged by gleaning in single-leaf pinyon pine (Pinus monophyla); however, within-season and between-year variation in behavior and habitat use were found for both sexes. Warblers increased use of shrubs from May to mid-June, and decreased use of shrubs and increased use of pinyon pine from mid-June through August, during each year. Use of sagebrush (Artemisia tridentata) and Utah juniper (Juniper osteosperma) varied between years. Sex differences were found in proportional use of foraging maneuvers. Within-season shifts in plant species and habitat use corresponded to changes in arthropod numbers on the plant species used by the warblers, but between-year shifts in behavior did not correspond as closely with changes in arthropod numbers. Temporal variation in microhabitat use resulted from shifts within seasons in the plant species used for foraging, which was associated with temporal changes in food abundance. Our results also demonstrate the importance of considering the effects of temporal scale in studies of bird-resource interactions.