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

Black-throated Green Warblers (Dendroica virens) use coniferous vegeta tion as foraging and perching substrates at coastal sites in Maine and predominantly deciduous vegetation at inland sites in interior New Ha mpshire, despite the availability of both substrates. I tested the hyp othesis that these habitat use patterns result from intrinsic preferen ces (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 wi th equal substrate and prey availability. I also determined the streng th of these preferences by skewing experimental prey distributions tow ard the less preferred vegetation type of birds from each region and m easuring resulting substrate use. Individuals from coastal sites showe d significantly greater preferences for coniferous vegetation as both perching and foraging substrates than did inland birds, which preferre d deciduous vegetation. In skewed prey distribution experiments, indiv iduals maintained their regional preferences for perching substrates, but shifted foraging substrates typically after first capturing all pr ey from the preferred vegetation type. Significant morphological varia tion (larger maxillar width, tibiotarsi, tarsometatarsi, humeri, and r adii in coastal birds) was found among Black-throated Green Warblers f rom the two regions that corresponded to morphology- habitat associati ons found in other mixed coniferous and deciduous bird assemblages. Th ese data suggest that intrinsic factors may be influential as a proxim ate microhabitat selection mechanism in the Black-throated Green Warbl er.