ABSTRACT. The Sierra Madre Occidental and neighboring Madrean Sky Islands span a large and biologically diverse region of northwest Mexico and portions of the southwestern United States. Little is known about the abundance and habitat use of breeding birds in this region of Mexico, but such information is important for guiding conservation and management. We assessed densities and habitat relationships of breeding birds across Sky Island mountain ranges in Mexico and adjacent portions of the Sierra Madre from 2009 to 2012. We estimated densities at multiple spatial scales, assessed variation in densities among all major montane vegetation communities, and identified and estimated the effects of important habitat attributes on local densities. Regional density estimates of 65% of 72 focal species varied significantly among eight montane vegetation communities that ranged from oak savannah and woodland at low elevations to pine and mixed-conifer forest at high elevations. Greater proportions of species occurred at peak densities or were relatively restricted to mixedconifer forest and montane riparian vegetation likely because of higher levels of structural or floristic diversity in those communities, but those species were typically rare or uncommon in the Sky Islands. Fewer species had peak densities in oak and pine-oak woodland, and species associated with those communities were often more abundant across the region. Habitat models often included the effects of broadleaf deciduous vegetation cover (30% of species), which, together with tree density and fire severity, had positive effects on densities and suggest ways for managers to augment and conserve populations. Such patterns combined with greater threats to highelevation conifer forest and riparian areas underscore their value for conservation. Significant populations of many breeding bird species, including some that are of concern or were not known to occur regionally or in mountain ranges we surveyed, highlight the importance of conservation efforts in this area of Mexico.RESUMEN. Abundancia y relaciones de h abitat de aves reproductivas en las Islas del Cielo y la contigua Sierra Madre Occidental del noroeste de M exico La Sierra Madre Occidental y las vecinas Islas del Cielo Madrenses abarcan una extensa y biol ogicamente diversa regi on del noroeste de M exico y porciones del sudoeste de Estados Unidos. Poco se sabe sobre la abundancia y el uso de h abitat de las aves que cr ıan en esta regi on de M exico, pero dicha informaci on es importante para guiar la conservaci on y el manejo de la zona. Evaluamos densidades y relaciones de h abitat de aves reproductoras a lo largo de las cadenas montañosas de las Islas del Cielo en M exico y puntos adyacentes de la Sierra Madre desde el 2009 al 2012. Estimamos densidad a m ultiples escalas, analizando la variaci on en la densidad entre las principales comunidades vegetales montanas, e identificamos y estimamos los efectos de importantes atributos de h abitat en las densidades locales. Estimaciones de densidad ...