Distribution, Abundance, Habitat, and Biogeography of Breeding Birds in the Sky Islands and adjacent Sierra Madre Occidental of northwest Mexico
Information on the status and habitat needs of wildlife is essential for conservation and management but often limited in remote regions with high conservation value. The Madrean Sky Islands region includes more than 40 mountain ranges located between the Sierra Madre Occidental in Mexico and the Mogollon Plateau in the U.S. These disjunct islands of montane vegetation and the broad valleys of desertscrub, thornscrub, and grassland that separate them, span the only gap in the vast highland cordillera that dominates western North America. Although the Sky Islands region is world renowned for its uniqueness, diversity, and conservation value, information on birds is very limited in Mexico and has not been synthesized since the 1950s. Between 2009 and 2012, I assessed the distribution, abundance, diversity, and habitat relationships of breeding birds in 26 Sky Island mountain ranges and six areas in the adjacent Sierra Madre Occidental in Sonora and Chihuahua, Mexico. I estimated the presence, density, and breeding status of birds during the breeding season at 1,562 points (n = 1,851 total point counts) along 210 transects (289 km in total length) that spanned all major montane vegetation communities in the region (1,150-2,750 m elevation), and described the composition and structure of vegetation, intensity of land use and disturbance, and other habitat features. Moreover, I evaluated biogeographical relationships among breeding bird communities in montane vegetation across virtually the entire Sky Islands region based on data gathered in Mexico and data from 22 additional mountain ranges in the U.S. My efforts represent the first assessment of biogeographical relationships based on data from across the Sky Islands region, and the first systematic study of bird communities and bird-habitat relationships in this region since Joe Marshall’s worked in nine Sky Islands and five areas in the adjacent Sierra Madre in Mexico in the 1950s.
I detected 199 species of birds including 165 species of landbirds that were at least presumably breeding and seven additional species that were possibly breeding. In the Sky Islands, I observed 152 species that were at least presumably breeding, eight additional species of possible breeders, and estimate 169 species breed in montane vegetation communities. Based on a comprehensive review of recent and historical observations, I found strong evidence of spatiotemporal changes in the status and distribution of numerous bird species and that more species seem to have expanded (vs. contracted) their distributions in recent decades. Although some of these patterns could be attributable to variation in survey effort, natural range expansion, changes in vegetation and land management that have affected habitat quality, and the effects of habitat area and isolation on extinction-colonization dynamics seem to have driven these patterns for many species. Many species I observed in the Sky Islands of Mexico or broader study area for the first time have strong Madrean affinities (e.g., Mountain Trogon, Brown-backed Solitaire, Crescent-chested Warbler, Slate-throated Redstart), which as a group have expanded their ranges northward in recent decades. Moreover, many other species I found distributed much more broadly than in the past are dependent on pines (e.g., Northern Goshawk, Hairy Woodpecker, Greater Pewee, Plumbeous Vireo, Grace’s Warbler, Olive Warbler) or are subject to hunting pressure by humans (e.g., Wild Turkey). These patterns were likely driven by the cessation of commercial logging and degradation of the extensive network of logging roads present in the past, and subsequent recovery of pines that have matured to varying extents and are now subject to virtually no active logging in the Sky Islands. Despite these auspicious trends, local extinctions of some populations due to natural disturbance, habitat degradation linked to past logging, the effects of habitat area and isolation on extinction risk, or climate change, have likely driven range contractions of other populations, especially those dependent on mature coniferous forest with large trees and cavities (e.g., Thick-billed Parrot, Flammulated Owl, Purple Martin, Pygmy Nuthatch).
Regional variation in the composition of breeding bird communities was largely spatially and biogeographically coherent but also produced some novel insights. In general, I found evidence of a discrete discontinuity in community composition between the Sierra Madre and Sky Islands but fairly continuous gradients in community composition across the Sky Islands. Nonetheless, I also found significant but variable levels of regionalization across the Sky Islands and evidence for three discrete groups of ranges; a small southwestern group with strong Sonoran and lowland Neotropical affinities, a larger southern group, virtually all of which were in Mexico, with stronger Madrean affinities, and an even larger northern group that had much higher levels of internal regionalization and strong Sonoran, Chihuahuan, Great Basin, or Petran affinities. These patterns reflect high levels of biogeographical complexity in the region that are driven by the convergence of two faunal realms and several major biogeographical provinces, steep but variable elevation profiles, and by Madrean and Petran influences at higher elevations and Sonoran, Chihuahuan, Great Basin, and lowland Neotropical influences at lower elevations.
I derived a large number of inferences on bird-habitat relationships that conformed to varying degrees with those from past studies in Madrean forest and woodland communities. Densities of most species (65% of 72) varied markedly among eight major montane vegetation communities, which ranged from oak savannah and oak woodland at the lowest elevations to mixed-conifer forest at the highest elevations. Much larger proportions of species occurred at peak densities in mixed-conifer forest and montane riparian vegetation, and more species were fairly restricted to these communities indicating their importance for conservation. In contrast, fewer species occurred at peak densities or were restricted to oak, oak-pine, or pine-oak woodland, or showed little evidence of variation in densities among communities, suggesting lower levels of habitat specialization. Habitat models that described variation in densities of 30 bird species most frequently included the effects of cover of conifers (10 species), a synthetic variable representing increasing tall-tree cover and decreasing shrub cover (9 species), and cover of broadleaf deciduous trees (7 species). Moreover, cover of broadleaf deciduous trees, fire severity, and tree density only had positive effects on bird densities among species. Collectively, these results and the remote and wild character of many Sky Islands in Mexico, suggest high conservation value and the importance of preserving this diverse and ecologically unique region for future generations.