Riparian Influence on Eastern Forest Songbirds at Their Western Range Limit

Abstract

The Cross Timbers is a transitional landscape between the eastern temperate deciduous forests and the grasslands of the Central Great Plains. Here many eastern forest birds reach the western edge of their breeding ranges. In a water-limited environment, water resources are likely important in determining species distributions. Differences between riparian and upland forests may influence bird community composition and species distributions in this sub-humid to semi-arid climate. We conducted paired upland-riparian surveys for a total of 178 point counts across central Oklahoma. Ordination techniques were used to investigate how riparian and upland forest type influence eastern songbird distribution and how bird species distributions were organized by forest cover and surface water patterns along a precipitation gradient. Most eastern species, including the Kentucky Warbler and Red-eyed Vireo, were more common in riparian than upland forests. Riparian forests were more similar to eastern oak-hickory forests in structure and composition and had higher food availability for foliage gleaning insectivorous species. We also found that most eastern forest species were limited to regions of higher precipitation or to areas of higher flow accumulation, whereas generalist species showed no strong response to precipitation or surface water flow gradients. Flow accumulation was a better predictor of species occurrences in xeric Cross Timbers forests than in mesic oak-hickory stands for several eastern forest species typically found in wetter climates. With projected increases in temperature and declines in precipitation, it is important to identify species sensitive to water resource gradients and predict how changes will affect species distributions.