Spring migrant use of native and saltcedar-dominated riparian areas along the lower Colorado River in Arizona

Abstract

Riparian systems in the western United States provide essential stopover habitat to en-route migrant birds, and there is concern that the invasion and dominance of saltcedar (Tamarix) in many areas may inhibit use by some species. However, evidence from recent studies is challenging the widely held belief that invasive plants universally reduce habitat quality. Moreover, where many studies have compared avian use of riparian habitats dominated by saltcedar with those comprised primarily of native trees, few have investigated how birds use shrub communities, which are becoming more prevalent in western riparian zones because of widespread hydrologic modifications. We compared spring migrant use of 125-m sections of riparian habitat dominated by five different habitat types in southwestern Arizona in 2006 and 2007. We found that migrant abundance, species richness, and community assemblages were all influenced by the composition of riparian vegetation. Habitats completely dominated by saltcedar supported fewer migrants and migrant species than any other habitat type, but the presence of small amounts of native vegetation as a part of the overall riparian plant community greatly bolstered habitat use. Habitats dominated by native shrubs tended to support the greatest total migrant abundance, total species richness, and abundance of many individual species. Our findings suggest that riparian areas dominated by saltcedar are avoided by many western migrant species and have relatively low value as stopover habitat. In places where this species is a predominant component of the riparian plant community, restoration of at least a portion of native riparian vegetation may be effective for encouraging use by stopover migrants.