How do riparian songbirds (Passerines) coexist? Effects of vegetative structural complexity on habitat selection in willow (<i>Salix</i> spp.)

Abstract

The diversity of avian species and their coexistence in riparian habitat has become increasingly significant in the face of rapidly decreasing vegetation along hydric areas, primarily due to pollution, urbanization, overgrazing, and climate change. The complex and rare riparian habitat system is primarily supported by willow (Salix spp.) vegetation in the high-altitude areas along the East River Valley in the Rocky Mountains, which provide a multitude of niche types for many species of songbird (Passerines). The objective of this study was to evaluate songbird community response to differences in vegetative structure of willow, concentrating on the overall avian abundance, richness, and species diversity, as well as a number of individual focal species. We predicted that the warbler species would demonstrate a more specialized habitat selection to more complex willow vegetation, while the sparrow group would likely select for less complex vegetative structure. We sampled bird abundance and vegetative composition in riparian habitat along the East River near Gothic, CO for three weeks, performing both a series of point count observations and a suite of vertical and horizontal vegetation measurements at each site. Overall species richness and diversity was found to increase with higher vegetative complexity, while species abundance was shown to be significantly lower at sites with more complex vegetation. The focal species selected were the Wilson’s Warbler (Cardellina pusilla), Yellow Warbler (Setophaga petechia), MacGuillivray’s Warbler (Geothylpis tolmiei), White-Crowned Sparrow (Zonotrichia albicollis), Fox Sparrow (Passerella iliaca), Lincoln’s Sparrow (Melospiza lincolnii)), and the Willow Flycatcher (Empidonax traillii). The sparrows demonstrated selection of willow with less vertical and horizontal complexity, while the Yellow and MacGillivray’s Warbler, as well as the Willow Flycatcher, were present in greater abundance in more complex willow structure. The Wilson’s Warbler was found to demonstrate no significant relationship to any vegetative variable, suggesting the possible use of different strata of vegetation in a more complex life history. The results of this study indicate the importance of understanding the structural habitat requirements of the songbird species in riparian areas in order to effectively manage and improve threatened willow habitat.

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