Alteration of hydrologic regime impacts breeding birds on the San Pedro River, Arizona

Alteration of hydrologic regime is a key threat to terrestrial vertebrates in the U.S. desert southwest.  Lowland river valleys cover <1% of land but support over 80% of the region’s vertebrate species including some of the highest avian densities in North America.  As one of the few undammed rivers in the southwest with one of largest remaining gallery forests of cottonwood-willow (Populus-Salix spp.) interspersed with monotypic stands of salt cedar (Tamarix chinensis), the San Pedro River provides an ideal study system to investigate the relative importance of hydrology versus vegetation for birds.  My first goal was to assess whether specific groups of bird species would be expected to decrease in abundance or go locally extinct based on altered vegetation and / or site hydrology.  My second goal was to assess whether species with equal or higher densities in the driest sites also maintained adequate nest productivity in those sites.  I found that canopy-nesting species were most impacted by changing vegetation from cottonwood-willow characteristic of perennial and intermittent flow sites to salt cedar associated with the driest sites. Within cottonwood-willow stands, wading and swimming birds were lost from sites that no longer maintained perennial surface water flow.  While maintaining similar densities across vegetation and hydrologic classes, the Arizona Bell’s Vireo (Vireo bellii arizonae) had substantially lower nest success in salt cedar than cottonwood-willow (9% vs. 33%) which was largely due to increased parasitism by Brown-headed Cowbirds (Molothrus ater) in the drier sites.  These results indicate that conservation and restoration of both in-stream flow and native gallery riparian forests are essential for maintaining the high avian diversity of the region.