Foraging and Habitat Relationships of Insect-Gleaning Birds in a Sierra Nevada Mixed-Conifer Forest

Abstract

-Foraging habits and relative abundances of 12 birds comprising the insect-gleaning guild in a Sierran mixed-conifer forest were studied during two breeding seasons to determine: (1) foraging habitat preferences, (2) the extent to which species differ in their use of various components of the foraging niche, (3) patterns of relative abundance vs. niche breadth, and (4) differences between resident and migrant species. Comparisons of proportional availability and bird use of foliage height classes and tree species showed that tree species and, to a lesser extent, heights were used selectively by the guild. Incense-cedar (Calocedrus decurrens) was consistently avoided by all species; other tree species were generally used in a complementary manner by different birds. Of four measured components of foraging niche, the use of foraging site (consisting of air or tree part) showed the greatest difference between species, followed by tree species, foraging techniques, and foraging height. We found no correlation between niche breadth and species abundance for all guild members; however, a significant positive correlation existed for the five resident species. Resident and migrant species groups showed few fundamental differences in foraging patterns, except that migrants tended to use a greater proportion of deciduous foliage than residents. Our results suggest that to provide for this guild, land managers should maintain natural levels of tree species diversity in the mixed-conifer forest type. Many studies have shown that syntopic insectivorous birds differ in their methods of foraging. In forested habitats, birds tend to use different foraging techniques, foraging sites, tree species, and heights. The relative importance of these foraging niche components in distinguishing species has received less attention. Differences in importance of niche components reported in previous studies may reflect either real differences that exist in different habitats and geographic areas (Balda 1969), or incomplete analysis of all potentially relevant factors (Holmes and Robinson 1981). In particular, the importance of differential plant species use has not been fully appreciated (Holmes and Robinson 1981). Additional information is needed before sound generalizations can be made regarding patterns of differentiation among insect-gleaning birds. In managed forests, plant species composition and vegetation structure may be altered by logging, other silvicultural activities, and disruption of natural fire regimes (Kilgore 1971, Franzreb and Ohmart 1978, Szaro and Balda 1979). Identification of habitat preferences of forest birds can suggest recommendations to mitigate impacts of manipulation. The mixed-conifer forest of the Sierra Nevada, California, supports a large number of arboreal insectivorous birds (Verner and Boss 1980). We studied foraging substrate preferences, foraging behaviors and relative abundances of the 12 most common members of an insect-gleaning guild. We sought to: (1) compare the guild's use of tree species and vertical foliage layers with the availabilities of these habitat components; (2) describe and compare foraging sites and techniques used by each species; (3) evaluate the relative importances of foraging heights, tree species use, foraging sites, and foraging techniques in differentiating species ecologically; (4) determine if resident and migrant species differed in their foraging and abundance patterns; and (5) suggest management recommendations to mitigate the effects of habitat manipulation on members of the insect-gleaning guild.