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

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Abstract
Foraging habits and relative abundances of 12 birds [Parus gambeli, P. rufescens, Sitta canadensis, Corthia americana, Regulus satrapa, Viceo solitarius, V. gilvus, Vermivora ruficapilla, Dendroica coronata, D. occidentalis, Piranga ludoviciana and Pheucticus melanocephalus] comprising the insect-gleaning guild in a Sierran mixed-conifer forest were studied during 2 breeding seasons to determine: foraging habitat preferences, the extent to which species differ in their use of various components of the foraging niche, patterns of relative abundance vs. niche breadth and 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 4 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. No correlation was found between niche breadth and species abundance for all guild members; however, a significant positive correlation existed for the 5 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. To provide for this guild, land managers should maintain natural levels of tree species diversity in the mixed-conifer forest type.