Canopy Characteristics Affecting Avian Reproductive Success: The Golden-cheeked Warbler

Habitat disturbances play a major role in wildlife distribution. Disturbances such as loss of breeding habitat and fragmentation are of particular concern for Neotropical migrant songbird populations. Additionally, different avian species respond differently to the surrounding environment at different spatial scales. Thus, multi-scale studies on bird abundance and reproductive success is necessary for evaluating the effects of habitat alterations. The golden-cheeked warbler (Dendroica chrysoparia) is a Neotropical migrant songbird that breeds exclusively in central Texas. In 1990, the U.S. Fish and Wildlife Service listed the golden-cheeked warbler as endangered, providing habitat loss among the list of justifications. Habitat requirements for this species are known to include mature juniper-oak (Juniperus-Quercus) woodlands; however, relationships between habitat characteristics and golden-cheeked warbler reproductive success remain unclear. Whereas the majority of golden-cheeked warbler research has focused on areas in the center of the breeding range, little is known about interactions between warblers and the environment at the edge of the range. Therefore, it is important to understand these relationships for successful golden-cheeked warbler management.

I investigated relationships between golden-cheeked warbler reproductive success and habitat characteristics, including canopy closure and tree species composition, at the study site and territory scale. My study took place within Kickapoo Cavern State Park and surrounding private properties in Kinney and Edwards counties in the southwest corner of the golden-cheeked warbler breeding range. I derived habitat characteristics from satellite imagery from the US Geological Survey National Land Cover Dataset (NLCD) and from field sampling. The NLCD provided data on canopy closure and tree species composition at a 30 m resolution. Additionally, I used spherical densitometers and transect evaluations to ground-truth data and take more detailed measurements. I determined reproductive success by nest monitoring and the Vickery index when nests could not be found. I monitored 80 territories across six study sites in 2009 and 2010. Reproductive success was 39.5 percent in 2009 and 59.4 percent in 2010. I found statistically significant results at the study site scale, whereas golden-cheeked warbler abundance increased as the portion of woodland increased. Similarly, I found that golden-cheeked warbler reproductive success increased at the study site scale as canopy closure increased. I did not find correlations between reproductive success and canopy closure or tree species composition at the territory scale. Results suggest that golden-cheeked warblers utilize a wider variety of habitat composition than previously thought, and habitat composition as a whole may not be the driving factors influencing warbler reproductive success in this region.