What was done
The authors evaluated the relationship between bird abundance and a number of large-scale vegetation features, including the density and distribution of mesquite trees at the Buenos Aires National Wildlife Refuge in southeastern Arizona in an effort to understand changes that occur within bird communities as a result of changes in ecosystem composition, specifically, changes that have arisen due to a recent documented expansion of woody tree species into this region.

What was learned
Among the following variables - overall grass, herb and shrub cover, percent cover of native grasses, percent cover of an introduced grass (Lehmann lovegrass), average size of mesquite trees, and the density of mesquite trees - only the density and distribution of mesquite trees were found to influence bird populations.  Total bird abundance was found to increase with increasing mesquite density.  In addition, "greater bird species richness [was] found on plots with higher mesquite densities."

What it means
The results of this study suggest that both the total number and species richness of birds have been enhanced by the expansion of mesquite into this grassland area.  The expansion of woody species into grasslands is one of the many consequences of atmospheric CO2 enrichment.  Under higher CO2 concentrations, trees that were previously unable to grow in grassland regions due to certain temperature and/or moisture restrictions are now able to grow and mature there.  And it appears that their presence is having a pronounced positive effect on local bird biodiversity.