Temporal variation in resource use by Black-throated Gray Warblers

We studied foraging behavior and habitat use of male and female Black-throa ted Gray Warblers (Dendroica nigrescens) to quantify the effects of tempora l variation on interpretations of avian resource use. Overall, both sexes p rimarily foraged by gleaning in single-leaf pinyon pine (Pinus monophyla); however, within-season and between-year variation in behavior and habitat u se were found for both sexes. Warblers increased use of shrubs from May to mid-dune, and decreased use of shrubs and increased use of pinyon pine from mid-June through August, during each year. Use of sagebrush (Artemisia tri dentata) and Utah juniper (Juniper osteosperma) varied between years. Sex d ifferences were found in proportional use of foraging maneuvers. Within-sea son shifts in plant species and habitat use corresponded to changes in arth ropod numbers on the plant species used by the warblers, but between-year s hifts in behavior did not correspond as closely with changes in arthropod n umbers. Temporal variation in microhabitat use resulted from shifts within seasons in the plant species used for foraging, which was associated with t emporal changes in food abundance. Our results also demonstrate the importa nce of considering the effects of temporal scale in studies of bird-resourc e interactions.