THE EFFECTS OF MESQUITE INVASION ON A SOUTHEASTERN ARIZONA GRASSLAND BIRD COMMUNITY

We determined which vegetal features influenced the distribution and a bundance of grassland birds at the Buenos Aires National Wildlife Refu ge, Arizona. The density and distribution of mesquite (Prosopis veluti na) exerted the strongest influence on the grassland bird community. A bundances of Pyrrhuloxia (Cardinalis sinuatus; r(2) = 0.363, P = 0.025 ) and Lucy's Warbler (Vermivora luciae; r(2) = 0.348, P = 0.04), and t otal abundance of birds (r(2) = 0.358, P = 0.04) were positively corre lated with increasing density of mesquite (Prosopis velutina), whereas abundance of Cactus Wren (Campylorhynchus brunneicapillus; r(2) = 0.4 52, P = 0.02) was negatively correlated with increasing mesquite densi ty. Abundance of Loggerhead Shrike (Lanius ludovicianus; r(2) = 0.693, P < 0.001) was positively correlated with an increasing patchiness of mesquite. Shrub-dependent bird species dominated the community, accou nting for 12 of the 18 species and 557 of the 815 individuals detected . Species relying on extensive areas of open grassland were largely ab sent from the study area, perhaps a result of the recent invasion of m esquite into this semi-desert grassland.