Patterns of distribution and factors influencing riparian breeding birds along the Yellowstone River in Montana

Abstract

Riparian zones harbor some of the most diverse bird communities. However, most riparian zones have been significantly modified by human activities. It is important to identify relationships between riparian birds and characteristics of the environment to understand the potential influences of floodplain management on birds. I examined patterns of riparian bird species distribution, as well as the factors affecting occurrence and abundance of species along the Yellowstone River in Montana. Point counts were conducted and vegetation data collected at 234 locations along 725 kilometers of the river. Occupancy or linear regression models were used to estimate occurrence or abundance of 14 bird species. First, I examined patterns in the distribution of bird species and their habitats. A gradient exists along most rivers that represents downstream changes in environmental conditions; I expected the distribution of vegetation to reflect this gradient, and birds to respond to these changing habitat conditions. Mature cottonwood habitat types were identified using cluster analysis, and habitat was an important driver of bird occurrence and abundance. However, there was little evidence that habitats were organized along a downstream gradient. River location was very important to birds, even after accounting for the influence of habitat, suggesting that the distributions of bird species are related to gradients that are not always reflected by habitat. Second, I used model selection methods to investigate the influences of local habitat, forest cover, land use, and river location on the occurrence or abundance of bird species. There was strong evidence that these factors were important drivers of bird distribution, and the relative influences of each factor depended upon life history characteristics of each species. The effect of river location on the occurrence or abundance of species suggests that broad-scale influences are important predictors of bird distribution along rivers. River location may reflect unmeasured environmental resources (e.g. food) that are important to birds, or variation in a species' distribution within its range boundaries. The Yellowstone River may serve as a reference for understanding the factors influencing the distribution of birds along a river, and provide valuable information for the management of riparian species.