Population Structure and Social Organization of Southwestern Riparian Birds

Breeding bird populations were studied on seven riparian woodland study plots in north-central Arizona. The densities of breeding birds varied from 193 to 847 pairs per 40 hectares. These differences were, in part, related to the vegetative structure of the habitat. Bird species diversity (BSD) was not correlated with foliage height diversity (FHD) in riparian habitats consisting of one major vegetational component (the cottonwood tree Populus fremontii) where these habitats are bordered by agricultural fields. These areas supported the largest breeding bird populations (847, 612, 512, and 425 pairs per 40 hectares) while areas consisting of a mixed deciduous tree and shrub component (Populus, Platanus, Salix, Acer, Fraxinus, Prosopis, etc.) had smaller populations of breeding birds (322, 312, and 193 pairs per 40 hectares). On the mixed deciduous woodland areas the correlation between BSD and FHD was similar to what others have found in Eastern deciduous forests. The average weights of individual birds were significantly greater on the homogeneous cottonwood areas (⁠X¯ = 46.4 g) than on the mixed deciduous areas (⁠X¯= 22.9 g). Data gathered indicate that two systems of social organization are being maintained. In areas of relatively low population density (mixed deciduous) 57% of the pairs maintained “class A” territories. On areas of relatively high population density (homogeneous cottonwood) only 22% of the pairs exhibited this form of spacing; here, most birds forage freely outside of the riparian habitat. We suggest that the differences in population density and average weights of birds found in the two habitat types are a result of the following: (i) relative productivity of areas adjacent to the riparian habitat, and (ii) degree of intra- and interspecific competition effected by avian populations found breeding in areas adjacent to the riparian habitat.