Wintering bird density and habitat use in chihuahuan desert grasslands

Many grassland bird species are of high conservation concern due to major population declines and continuing habitat loss and degradation over much of their range. More than 80% of grassland bird species breeding in western North America overwinter in the Chihuahuan Desert grasslands of the southwestern U.S. and northern Mexico. These grasslands are increasingly being lost and degraded through agricultural conversion, desertification and shrub encroachment, especially in Mexico. However, there is little information on wintering grassland bird distribution, abundance, habitat use and spatiotemporal patterns to guide strategic habitat conservation in the region. In January 2007, Rocky Mountain Bird Observatory (RMBO), together with Universidad Autónoma de Nuevo Leon, initiated a first-ever, region-wide pilot survey to inventory, research and monitor wintering birds in Chihuahuan Desert Grassland Priority Conservation Areas (GPCAs) in Mexico. This effort was refined and expanded in January and February of 2008 and 2009. In 2009 we conducted 682 one-kilometer line transects at randomly-selected grassland sites across 10 current or potential GPCAs. At each site we used distance sampling to quantify bird populations and visual habitat assessments to characterize vegetation types and conditions. These surveys generated abundance and distribution information on 34 grassland associated species, including 30 priority species of high regional or continental conservation interest. We obtained reasonably precise annual estimates of density for 33 species or species groupings, including 19 priority species, across GPCAs. We determined habitat relationships for 16 species using generalized linear models. Species density varied among the 10 GPCAs, but the degree to which densities differed varied among species. For example, Baird‟s Sparrow density in Cuchillas de la Zarca, a GPCA in northern Durango and southern Chihuahua, far exceeds that in any other GPCA. Other species like Sprague‟s Pipit were more evenly distributed, although some GPCAs still appeared to be more important than others. Some species showed broader regional patterns of abundance. For example, Chestnut-collared Longspur was most abundant in the northern Chihuahuan Desert GPCAs in most years. In 2008, densities dropped in the north and increased in the south, and most other species also showed evidence of inter-annual shifts in population density, both locally and across the region. Continued monitoring is needed to further evaluate these patterns. Grass cover, shrub cover, and/or other cover were important habitat variables that influenced the abundance of most grassland species. Several species of high concern, including Sprague‟s Pipit, Baird‟s Sparrow, Grasshopper Sparrow and Chestnut-collared Longspur, were negatively affected by shrub cover and positively affected by grass cover. Grasslands with low shrub cover and high grass cover were uncommon in many of the GPCAs. However, these conditions can be promoted through management and restoration. The identification of high-quality Chihuahuan Desert grasslands is impeded by a lack of resolution among grassland types in the available GIS. A refinement of the grassland classification, through an analysis of remote sensing data coupled with extensive ground- truthing, is needed in order to distinguish among shrub-infested grasslands, which have little to no present value for most grassland species of concern, and open grasslands that provides critical habitat for these species. Rapid agricultural expansion is threatening to severely reduce the availability of grasslands in the Chihuahuan Desert, particularly in flatland areas that appear to be important for many grassland birds of concern. Rates of grassland conversion to agriculture are not known, but they are increasing and appear to be unsustainable. Shrub encroachment is also reducing the availability of suitable habitat for open-grassland obligates. Given that migratory grassland birds from western North America concentrate in the limited grasslands of northern Mexico and southwestern U.S., continued loss of Chihuahuan Desert grasslands will create a wintering habitat bottleneck that will further limit population sizes of many grassland-obligate bird species. An immediate and broad array of conservation solutions are needed to slow and reverse current trends in Chihuahuan Desert grassland loss. Continued avian inventories and monitoring will allow us identify spatiotemporal patterns of abundance, species habitat requirements, important wintering areas and land use changes, while continuing to provide an avenue for outreach and education. Once we can determine key regions, habitat conditions, and other variables that affect distribution and abundance, efforts to protect, manage and restore habitat and build public support for conservation must be dramatically increased in these areas. RMBO would like to deploy stewardship biologists in the GPCAs to reach out to local communities and landowners, develop partnerships and deliver on the ground conservation. Together these strategies could help mitigate the loss of grasslands and improve the region‟s carrying capacity for grassland species