Nest Survival of Understory Birds in Longleaf Pine Forests Exposed to Fire and Fire-Surrogate Treatments

Abstract

Longleaf pine (Pinus palustris) forest ecosystems evolved with short-interval, low intensity fires. Fire suppression has reduced or eliminated fire and has caused extensive changes in plant community composition and structure. The National Fire and Fire-Surrogate study was implemented to evaluate effects of alternative habitat treatments, such as herbicide application and mechanical thinning, and to compare them with effects of prescribed fire. We evaluated how treatments influenced the nesting success of birds breeding in 10-ha plots of longleaf pine at Solon Dixon Experimental Forest, Alabama. We measured daily rates of nest mortality for 432 nests of 33 species in unmanipulated (controls), burned, herbicide-treated and burned, thinned, and thinned and burned plots during the three years of our study. Although we accumulated observations of 6,624 days that nests were exposed to possible predation, sample sizes were still too small to permit robust statistical analyses of treatment effects. Data from one (Eastern Towhee, Pipilo erythrophthalmus) of two species with the largest sample sizes suggested that nests in treatment plots experienced higher rates of mortality than did nests in control plots, but sample sizes were still small enough that high estimates of variance prevented strong statistical evaluation. Nest survival for the other common species, Northern Cardinal (Cardinalis cardinalis), showed no such suggestive differences. We present results from all nests of all species for which we found three or more nests so that future researchers can use the data during meta-analyses. We also encourage those designing future similar studies to consider expanding the size of study plots so that at least the most numerous open-cup nesting birds can be found in sufficient numbers for strong statistical analyses. In longleaf pine forests of southern Alabama, we estimate that minimum plot sizes would need to be 50 ha for the two most common species, Eastern Towhee and Northern Cardinal.

Short-Term Effects of Fire and Fire-Surrogate Treatments on Avian Nest Survival: A National-Scale Analysis

“…Available data suggests that the species experiences moderate to low nest survival in many locations studied to date, but is very tolerant of habitat disturbance [38]. In the Gulf Coastal Plain study site, which had the largest sample sizes of nests, cardinal nest survival was variable and not clearly associated with treatments [39].…”

The Challenges of Studying Vertebrates in Habitat Treatment Plots

“…This is particularly an issue when studying naturally rare species (often the species of most management interest) and studying ecosystems with relatively low productivity and therefore, lower abundances of most species. Robinson and Rompré [2], for example, estimated from studies of 10 to 12-ha plots in longleaf pine forest, a habitat supporting comparatively low numbers of birds overall [3], that a minimum plot size of 100 ha would have been required to conduct strong statistical comparisons of nest survival for just the most common species. The entire study of that longleaf community included 150 ha distributed across 15 plots, so enlarging plots by an order of magnitude would have also enlarged costs of setting up, manipulating, and maintaining the plots by at least that same amount.…”

“…Interestingly, even when nest survival was quite high, as in the South Carolina study site [9], sample sizes were low because few nesting attempts were made by the individuals breeding in those plots. In plots of longleaf pine in southern Alabama, the overall population densities of songbirds were so low that it was nearly impossible to find large samples of nests, even for the most common species and even when predation rates were high, leading to multiple re-nesting attempts [2]. The strategy of those studies was to present their findings, including the raw data on nest exposure days, so that data are available for future meta-analyses.…”