Canopy cover as the primary factor affecting habitat use by grassland-shrubland bird species in central Texas, USA

Abstract

Woody vegetation continues to encroach into grasslands in many regions of the world. It often leads to habitat loss for grassland birds. Habitat loss is one reason for the ongoing declines in abundance of many bird species that are grassland obligates. However, species that can tolerate and perhaps even prefer some amount of shrub and tree cover presumably would not be as negatively affected by woody encroachment. Using bird point-count data from the Oaks and Prairies Joint Venture, we examined the habitat associations of eight grassland-shrubland bird species in two ecoregions of central Texas USA. In particular, we focused on canopy cover, canopy height, and shrub species composition given that woody encroachment entails increases or other changes in these habitat characteristics. Further, as with many bird species, structural components of the vegetation often characterize habitat. We treated these habitat characteristics as predictor variables in logistic multiple regression models wherein the response variable was probability of species occurrence at 478 point-count locations. Overall, canopy cover was a better predictor of probability of species occurrence than was canopy height or canopy species composition. Thus, canopy cover was an important habitat characteristic for most of the species. Bewick’s Wren ( Thryomanes bewickii ) exhibited its greatest probability of occurrence of approximately 0.63 at 40–60% canopy cover, Painted Bunting ( Passerina ciris ) approximately 0.74 at 15–45% canopy cover, and Rufous-crowned Sparrow ( Aimophila ruficeps ) approximately 0.09 at 50% canopy cover. The probability of occurrence of Field Sparrow ( Spizella pusilla ) consistently increased with increasing percent canopy cover, reaching a maximum value of 0.58 at 74% canopy cover. These results suggest that a limited amount of woody encroachment may actually benefit some grassland-shrubland bird species.

Keywords

• edwards plateau
• gis
• habitat association
• juniper
• mesquite
• northern bobwhite