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Habitat Selection by Breeding Black-Throated Blue Warblers at Two Spatial Scales

Benjamin B. Steele
Ornis Scandinavica (Scandinavian Journal of Ornithology)
Vol. 23, No. 1 (Jan. - Mar., 1992), pp. 33-42
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3676425
Stable URL: http://www.jstor.org/stable/3676425
Page Count: 10
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Habitat Selection by Breeding Black-Throated Blue Warblers at Two Spatial Scales
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Abstract

I examined the effects of shrub density on the distribution and abundance of Black-throated Blue Warblers Dendroica caerulescens within a northern hardwood forest in New Hampshire, U.S.A. I found different responses to shrub density at two different spatial scales (plot and territory). At the scale of 15 ha study plots, the number of breeding birds on seven plots was positively related to shrub density, suggesting that areas having a high density of shrubs were selected by the birds. Selection of shrubs was further supported by an experiment in which the density of Black-throated Blue Warblers declined to zero in the three years after leaves were removed from shrubs on a 14 ha plot. In contrast to the plot scale results, the territory scale analysis demonstrated that shrub density was not greater inside than outside territories in any of 11 analyses (five plots, each studied for two or three years). Thus, within plots, territories were not located in areas where shrub density was high. No other vegetation variable consistently differed between inside and outside of territories within plots. The plot scale analyses provide an example of how the vertical structure of a habitat can affect the distribution of bird species. The conflicting results at the two scales point out the importance of considering several scales in studies of distribution and abundance, but they also raise the question of why a bird would preferentially select plots with dense shrubs and then not locate its territory so that shrub density is maximized. I argue that all territories on plots where average shrub density is above a threshold level will contain enough small dense patches of shrubs for the birds, especially if these patches are important for nesting rather than for foraging. If this argument is valid, then habitat selection at the scale of the territory may not be necessary for Black-throated Blue Warblers.

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