Availability and Use of Arthropod Food Resources by Wilson's Warblers and Lincoln's Sparrows in Southeastern Wyoming

Abstract

We examined availability and use of arthropods by Wilson's Warblers (Wilsonia pusilla) and Lincoln's Sparrows (Melospiza lincolnii) to determine whether all arthropods selected by these birds were equally preferred. Preferences were identified by comparing proportional availability of arthropods in willows to their proportional use by birds foraging in willows. Actively foraging birds were collected during the breeding seasons of 1984 and 1985 in montane riparian zones located on the Medicine Bow National Forest in southeastern Wyoming. Arthropods were sampled at bird foraging sites by applying a pyrethrin-based fogacide to the willows. We identified 115 families of arthropods in our spray samples from willows over the two breeding seasons. Of these families, 53 and 54 were identified in warbler and sparrow diets, respectively. Warblers were selective in the sizes of prey they consumed, with smaller prey (1 to 3 mm) being underrepresented in their diets for five of the eight arthropod groups considered. We employed a ranking procedure (PREFER) to investigate bird species' preferences among 10 arthropod food groups. The group ranked highest for both warblers and sparrows was Coleoptera, of which beetles with soft elytra were a major component. Of the remaining groups, arthropods that ranked high in preference for warblers were noncryptic and patchy in distribution, while those that ranked high for sparrows tended to be cryptic and more uniformly distributed. We suggest that the foraging strategies of Wilson's Warblers and Lincoln's Sparrows, and arthropod behavior, morphology, and distribution explain the differences we detected in proportional availability and use of arthropods by these birds.