Vegetation associations of riparian birds in successional woodlands along the regulated Missouri River

Abstract

River regulation by dams on the Missouri River has modified riparian forest successional patterns, with decreases in early and increases in later seral stages and higher occurrence of invasive tree species, including Russian olive (Elaeagnus angustifolia) and eastern red cedar (Juniperus virginiana). The effects of these altered successional trajectories on bird biodiversity are difficult to quantify because of limited data on bird-habitat associations. We surveyed riparian shrubland and forest bird species across a gradient of riparian forest ages along two segments of the regulated Missouri River in South Dakota and Nebraska, USA and explored relationships between bird abundance and patch- and landscape-scale vegetation characteristics for 46 bird species. Predicted abundances at sites assigned to five vegetation classes, estimated from Bayesian binomial N-mixture models, identified 11 early successional bird species and 19 forest bird species. Abundances of early successional bird species were similar at cottonwood-willow sites and Russian olive sites and were positively correlated with cottonwood (Populus deltoides) importance values for only one species, Willow Flycatcher (Empidonax traillii). Abundances of forest bird species were similar at sites in the three forest vegetation classes, although Ovenbird (Seiurus aurocapilla) and Baltimore Oriole (Icterus galbula) showed some affinity for mid- or late successional cottonwood sites over late-successional non-cottonwood sites. Abundances of three forest species, including Baltimore Oriole, were positively correlated with cottonwood or negatively correlated with eastern red cedar importance values. Fifteen species were positively correlated with shrubland land cover, whereas 21 species were positively correlated with forest land cover. For most bird species, correlations were strongest with land cover within a 200-m buffer compared to 400 or 1200 m. These data suggest that the trends in riparian forest change due to river regulation along the middle Missouri River may produce a mix of positive and negative effects on riparian bird species. While management plans to promote regeneration of early successional cottonwood-willow stands are likely to benefit conservation of early successional bird species, Russian olive may also provide suitable bird habitat for the majority those species.