Recent observations suggest that Gray Vireo is not ecologically separated from other species of vireos during the breeding season. In western Colorado, this species regularly occupies the same locations as Plumbeous Vireo, but details of habitat use and interactions were largely unknown. To illuminate this relationship, RMBO studied the breeding distribution and habitat use of Gray and Plumbeous at Colorado National Monument, Mesa Co., in western Colorado. We conducted ground searches and mapped the territories of singing males and pairs of each in the Monument. We found 160 Gray and 49 Plumbeous vireo territories in 1999 and 172 Gray and 44 Plumbeous vireo territories in 2000. Following breeding activities, we measured habitat variables within territories of both species and within randomly selected vegetation plots. Four variables - deciduous shrub density, juniper density, juniper height, and piņon height - discriminated among the three groups. Gray Vireo habitat contained the shortest trees and highest densities of junipers and deciduous shrubs; Plumbeous Vireo habitat contained the tallest trees and intermediate densities of junipers and deciduous shrubs; and random habitat contained intermediate height trees and the lowest densities of junipers and deciduous shrubs. Our findings were submitted to the National Park Service in December 2001.