EXPERIMENTAL-EVIDENCE FOR INTRINSIC MICROHABITAT PREFERENCES IN THE BLACK-THROATED GREEN WARBLER

Black-throated Green Warblers (Dendroica virens) use coniferous vegetation as foraging and perching substrates at coastal sites in Maine and predominantly deciduous vegetation at inland sites in interior New Hampshire, despite the availability of both substrates. I tested the hypothesis that these habitat use patterns result from intrinsic preferences (fixed behavioral choices independent of environmental influence) by quantifying coniferous and deciduous vegetation use by individuals from New Hampshire and Maine in ''commongarden'' aviary experiments with equal substrate and prey availability. I also determined the strength of these preferences by skewing experimental prey distributions toward the less preferred vegetation type of birds from each region and measuring resulting substrate use. Individuals from coastal sites showed significantly greater preferences for coniferous vegetation as both perching and foraging substrates than did inland birds, which preferred deciduous vegetation. In skewed prey distribution experiments, individuals maintained their regional preferences for perching substrates, but shifted foraging substrates typically after first capturing all prey from the preferred vegetation type. Significant morphological variation (larger maxillar width, tibiotarsi, tarsometatarsi, humeri, and radii in coastal birds) was found among Black-throated Green Warblers from the two regions that corresponded to morphology- habitat associations found in other mixed coniferous and deciduous bird assemblages. These data suggest that intrinsic factors may be influential as a proximate microhabitat selection mechanism in the Black-throated Green Warbler.