Effects of Needle Architecture on Warbler Habitat Selection in a Coastal Spruce Forest

Habitat associations of mobile foragers are well documented, but causes behind such patterns are seldom as well defined. Black—throated Green Warblers (Dendroica virens) attain greater densities in red (Picea rubens) than in white spruce (P. glauca) forests in coastal Maine and use red spruce foliage significantly more than predicted on the basis of availability. These habitat associations may be explained best by small—scale differences in vegetation structure, which can influence foraging efficiency. Red spruce have shorter needles held at more acute angles 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 exaggerated in moist vegetation, since white spruce needle structure and arrangement may wet and reduce plumage insulation capabilities more quickly. Three aviary experiments were performed with D. virens to test the role of structural differences in vegetation on foraging—site and perching—site selection: wetting of vegetation, decreasing white spruce needle angle to resemble red spruce, and reduction of white spruce needle length to approximate red spruce. Moistening of red spruce branches did not affect the foraging or microhabitat use of experimental warblers. Needle angle reduction did not change perching—site preferences, but foraging significantly shifted in favor of white spruce. Needle length manipulations resulted in significantly greater use of white spruce than in controls, for both perching and foraging substrates. The relationships found in this study between individual habitat selection and fine—scale features of habitat structure are consistent with the larger—scale patterns of habitat occupancy seen in this system, and have important implications in understanding habitat requirements for declining forest passerine populations. _kw Dendroica virens