Response of nesting grassland birds to sand shinnery oak communities treated with tebuthiuron and grazing in eastern New Mexico
Smythe, Lindsay Allison
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Many grassland bird species have exhibited population declines during the past three decades. Major components of grassland bird habitat in eastern New Mexico include sand shinnery oak (Quercus havardii) communities, many of which have been subject to unmanaged livestock grazing and other impacts for decades. As a result, these sand shinnery oak communities tend to deviate from the historical grass/shrub vegetation mix in favor of shrubs. These communities are frequently managed with livestock grazing and herbicide application for shrub control. These techniques can be used for potential restoration of historical community structure. However, responses of spring migrating and nesting grassland birds to these management practices in shinnery oak communities are not well understood because of variation in species response. Additionally, most of the research in these communities has focused on game species. My study objectives, in the context of restoring shinnery oak communities, were to: 1) determine if tebuthiuron herbicide application or short-duration grazing had negative impacts on avian community structure (i.e., density, species richness, diversity, and evenness), 2) determine if the vegetation changes caused by tebuthiuron application and short-duration grazing negatively impacted avian reproduction (nest density and success), and 3) develop recommendations to benefit grassland birds when using herbicides and livestock grazing in sand shinnery oak communities. My study site in eastern New Mexico consisted of 1,040 ha divided into 16 plots of 65 ha each. The plots consisted of 2 treatments arranged in 4 combinations: tebuthiuron with grazing; tebuthiuron without grazing; no tebuthiuron with grazing; and a control of no tebuthiuron or current grazing. I performed biweekly point transects on these plots from February through July 2004 and 2005. Density of bird species was estimated using program DISTANCE. I also searched 4-ha subplots in each treatment plot for nests from April-June 2004 and 2005. I monitored nests to estimate daily survival rates and recorded vertical density and overhead cover measurements at each nest site to determine if vegetation structure affected nest daily survival or success. In 2005, I placed 64 artificial nests to supplement information from real nests. This study occurred over years of highly variable precipitation: 2003 represented the end of a 15-year drought with below-average precipitation, but during 2004, the area received 3 times the average amount of precipitation, the second highest amount ever recorded in the region. The above-average precipitation affected habitat conditions and likely impacted my results, but precipitation returned to near average in 2005, so these conditions may not persist. Density of all avian species did not differ between grazed and ungrazed plots. Tebuthiuron-treated plots had a higher density of all species than untreated plots. There was a higher density of all species during spring 2005 than 2004 but density was similar during the breeding season of both years. These trends were predominantly influenced by Cassin’s sparrows (Aimophila cassinii) and, in 2005, grasshopper sparrows (Ammodramus savannarum). Tebuthiuron-treated plots had a higher density of Cassin’s sparrows in both years. Grasshopper sparrows were not recorded in 2004 but were present in greater numbers on tebuthiuron-treated plots in 2005. The density of resident species (meadowlarks [Sturnella spp.] and loggerhead shrikes [Lanius ludovicianus]) exhibited little response to tebuthiuron or grazing treatments. Species richness was not affected by tebuthiuron or grazing, but was higher in the spring of 2005 than 2004 because of greater numbers of migratory sparrows. Decreases in both evenness and diversity in 2005 compared to 2004 were influenced by large flocks of chestnut-collared longspurs (Calcarius ornatus) and lark buntings (Calamospiza melanocorys). Diversity was also lower on ungrazed plots in February and March. Nest density of all species was similar among tebuthiuron and grazing treatments but was greater in 2005 than 2004. Nests of migratory species were more numerous in 2005 than 2004, whereas equal numbers of resident species nests were found in both years. Daily nest survival rates were similar between years and between incubation and the nestling periods, but varied among species and treatments. Daily nest survival rates during incubation were 6.3% higher in untreated plots than in tebuthiuron-treated plots, but during the nestling period were 17.3% higher in tebuthiuron-treated plots than in untreated plots. Depredation caused the majority of nest failures. Vertical density of vegetation differed among treatments and was greater in 2005 than 2004 but did not differ between nest sites and associated random points, nor between hatched and failed nests. Vegetation overhead cover did not vary among treatments but was greater in 2005 than 2004 and greater at nest sites than at associated random points. Overhead cover did not differ between nests that hatched and nests that failed. Unusually high rainfall in 2004 likely influenced results, but neither grazing nor tebuthiuron treatment had a substantial impact on resident birds. Migratory birds responded positively to tebuthiuron treatment, but overall density and nest success in this community (regardless of treatment) were extremely low.