Distribution of Summer Birds Along a Forest Moisture Gradient in an Ozark Watershed

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Abstract
Vegetational and physical habitat factors were measured in 110.0.05—ha circular plots around singing ♂ ♂ of eight bird species in a small watershed in northwestern Arkansan during June and July 1973. Sixty birdless plots were similarly sampled in the driest and most moist areas of the watershed to determine a forest moisture gradient. Principal component analysis of the combined avian community habitat data determined that the forest moisture gradient is important in the distribution of the birds, although multiple analysis of variance showed that no one factor can account for the significant distribution. Principal component analysis of the individual species determined the unique characteristics of the realized niches of each species. An ordination of the species using linear discriminant function analysis of the 60 forest samples was similar to the ordination based on the bird distribution, but suggests that other factors are determining the precise distribution of the birds. No species shows a great affinity for the drier forest, although Tufted Titmice seem able to utilize the moist and dry forests equally. The Hooded Warbler, Ovenbird, and Acadian Flycatcher have shorter niche widths than early arriving migrants and permanent residents and can be considered obligatory moist forest species. The results show that the moist forests is of great importance to the avian community and reasons for the lack of birds in the dry forest are discussed.