Effect of Hydrologic, Geomorphic, and Vegetative Conditions on Avian Communities in the Middle Rio Grande of New Mexico

Abstract

Anthropogenic regulation of the Middle Rio Grande (MRG) of New Mexico has altered the hydrogeomorphology of the system and led to a rapid decline in breeding riparian-obligate birds as native riparian forests are replaced by exotic tamarisk stands. In this study, I evaluated the relationships among hydrogeomorphology, vegetation structure and composition, and avian communities of the San Acacia Reach (SAR) of the MRG. I conducted avian point count surveys and collected habitat data to determine avian community structure and abundance, geomorphic feature, surface flooding, and vegetation structure and composition along 44 transects throughout the SAR. A total of 999 point count surveys and 1,801 vegetation plots were completed throughout the 2012 and 2013 breeding season. Sixty-nine land bird species were detected over the two breeding seasons. Avian guilds responded differently to various hydrogeomorphic conditions. Ground-nesting birds and low shrub-nesting birds were insensitive to hydrogeomorphic changes as they do not rely on native understory, but can use exotic understory or woody debris. In contrast, canopy-nesting birds required native overstory; therefore, they were sensitive to hydrogeomorphic changes as native overstory species, such as cottonwood (Populus deltoides) and Goodding’s willow (Salix gooddingii), require surface floods to germinate and establish. Similarly, water-obligate species were also sensitive to hydrogeomorphic changes as they require close proximity to surface water. I also evaluated hydrogeomorphic relationships with the habitat of the Western Yellow-billed Cuckoo (Coccyzus americanus occidentalis), a candidate species under the Endangered Species Act, using cuckoo presence/absence survey data, spatial statistics in ArcGIS 10, and vegetation and hydrology data. Results indicated that management of riparian forests that promotes overbank floods and regeneration and survival of cottonwood and Goodding’s willow overstory, with a mixed understory of Goodding’s willow, coyote willow (Salix exigua), and New Mexico olive (Forestiera pubescens) would provide long-term habitat for Western Yellow-billed Cuckoos. I also quantified the amount of variability within Hink and Ohmart vegetation structure classes, a vegetation classification scheme widely used throughout the MRG. Results indicated that including woody stem density classes in conjunction with the current Hink and Ohmart classification methodology will better inform songbird habitat management prescriptions.