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The condor (Los Angeles, CA)


To understand why breeding Black-throated Blue Warblers (Dendroica caerulescens) select forests with dense shrubs, I assessed the value of this habitat in supplying opportunities for foraging and nesting. I predicted that these warblers would select shrub foliage for foraging if foraging substrate was important in their selection of habitat and that they would place their nests in areas of dense shrubs if nest-site availability affected habitat choice. To measure foraging and nest-site selection, I compared the proportion of foraging or nests in a particular habitat element to the availability of that element expressed as a proportion of all habitat elements. Foraging males under-utilized shrub foliage (below 3 m) in relation to its relative availability and over-utilized the sparse foliage between 3-9 m high. On a horizontal plane, males over-utilized areas of their territories with dense shrub foliage, but this could be due to the greater number of shrubs in these areas. Small samples of females and males feeding fledglings indicated that the lower foliage strata, but not necessarily dense shrub patches, might be important to these groups. These results demonstrate differences in foraging patterns between the sexes and between stages of the breeding cycle. More importantly, foraging Black-throated Blue Warblers showed no consistent selection of dense shrubs. As this species is more abundant in forests with dense shrubs, these analyses suggest that foraging may have a minor influence on habitat selection.

Black-throated Blue Warblers consistently selected areas of dense shrubs for nesting. Number of shrub stems and amount of foliage (0-1 m high) were significantly higher at nests than at random points. Furthermore, on an experimental plot with chemically defoliated shrubs, males continued to forage, but nesting was markedly reduced. These results suggest that, while shrubs are used for both foraging and nesting, nest-site requirements may be more important in determining what habitat is selected by Black-throated Blue Warblers. If nest-site requirements determine habitat choices in other forest birds, then the availability of suitable nest-sites should have an important effect on community structure.