EFFECTS OF NEEDLE ARCHITECTURE ON WARBLER HABITAT SELECTION IN A COASTAL SPRUCE FOREST

Habitat associations of mobile foragers are well documented, but cause s behind such patterns are seldom as well defined. Black-throated Gree n Warblers (Dendroica virens) attain greater densities in red (Picea r ubens) than in white spruce (P. glauca) forests in coastal Maine and u se red spruce foliage significantly more than predicted on the basis o f availability. These habitat associations may be explained best by sm all-scale differences in vegetation structure, which can influence for aging efficiency. Red spruce have shorter needles held at more acute a ngles to woody stems than do white spruce. I predicted that D. virens prefers red spruce because of ease of perching and foraging on needles with this structure. Moreover, red spruce preferences should be exagg erated in moist vegetation, since white spruce needle structure and ar rangement may wet and reduce plumage insulation capabilities more quic kly. Three aviary experiments were performed with D. virens to test th e role of structural differences in vegetation on foraging-site and pe rching-site selection: wetting of vegetation, decreasing white spruce needle angle to resemble red spruce, and reduction of white spruce nee dle length to approximate red spruce. Moistening of red spruce branche s did not affect the foraging or microhabitat use of experimental warb lers. Needle angle reduction did not change perching-site preferences, but foraging significantly shifted in favor of white spruce. Needle l ength manipulations resulted in significantly greater use of white spr uce than in controls, for both perching and foraging substrates. The r elationships found in this study between individual habitat selection and fine-scale features of habitat structure are consistent with the l arger-scale patterns of habitat occupancy seen in this system, and hav e important implications in understanding habitat requirements for dec lining forest passerine populations.