Climate models predict increasingly arid conditions in the southwestern United States. These conditions could facilitate replacement of native vegetation by exotic plants that are more tolerant of drought and wildfire. To evaluate effects of these changes on riparian-nesting birds, we examined their breeding biology along two New Mexico rivers that differ in abundance of exotic vegetation. Our sites along the Gila River were composed of a diverse, mostly native woody plant community and our Middle Rio Grande sites were to a larger extent composed of exotic species such as saltcedar (Tamarix spp.) and Russian olive (Elaeagnus angustifolia). At each river, we searched for nests of all landbirds encountered, compared nest plant use among nesting guilds, and calculated nest survival rates for black-chinned hummingbirds (Archilochus alexandri) and mourning doves (Zenaida macroura).
Results/Conclusions
Of the nests we observed, 49% along the Middle Rio Grande were constructed in exotic plants, compared to 4% along the Gila River. Canopy and cavity-nesting birds constructed less than 5% of their nests in exotic plants along either river. At the Middle Rio Grande, 92% of shrub nests and 65% of subcanopy nests were constructed in exotic plants. Nest survival rates of black-chinned hummingbirds were considerably higher along the Middle Rio Grande (0.55, SE: 0.03), where 68% of their nests were in exotic plants, than along the Gila River (0.23, SE: 0.04), where 98% of nests were in native species, primarily boxelder (Acer negundo). Mourning doves used similar proportions of native and exotic nest plants as black-chinned hummingbirds, but their nest survival rates were slightly higher along the Gila River (0.22, SE: 0.04) than the Middle Rio Grande (0.16, SE: 0.04). These results suggest that replacement of native trees such as cottonwoods (Populus spp.) by exotic plants would drastically reduce nest site availability for canopy and cavity nesting birds. Successful use of exotic plants by shrub and subcanopy-nesting birds, however, indicates that these guilds are less vulnerable to changes in forest composition.