Effects of burning on Henslow's Sparrow (Ammodramus henslowii) density and habitat quality in Louisiana

Abstract

The Henslow’s Sparrow (Ammodramus henslowii) is a grassland species of concern that winters in prairies and open pine savannas across the Gulf Coastal Plain. Previous studies have indicated that these birds occur at higher densities in recently burned habitats in winter, but this has never been examined over a large geographic area containing multiple habitat types. The objectives of the study were to identify areas of important winter habitat in Louisiana, to examine the relationship between time since burn, bird density, and bird condition, and to determine vegetation structure and composition used by birds. We found most of the new potential habitat and the highest numbers of birds in two specific ecoregions, the Lower West Gulf Coastal Plain and the East Gulf Coastal Plain (“east” and “west”) suggesting that these may be most important for wintering Henslow’s Sparrows. Time since burn explained little variation plot selection overall. Bird densities in the east peaked in the first winter after burn, while densities in the west were higher 1-2 years post burn, indicating that habitats in these regions may respond differently to fire. Vegetation measurements were not helpful in interpreting bird abundance patterns across plots, suggesting that characteristics of suitable habitat may vary widely across ecoregions. However, within a plot the probability of flushing a bird increased with increasing herbaceous density at ground level, and with increasing cover of the plant genera Andropogon and Rhynchospora, so Henslow’s Sparrows may be responding to structural components of the habitat on a small scale. Sex ratios and bird body condition, including mass, fat storage, and feather growth, did not vary with burn year. Therefore I found no evidence for any sex-related winter hierarchy. Results of this research will help land managers make decisions with appropriate consideration for the larger variation in bird abundance and vegetation associations found across a regional scale. We recommend a two year rotation of prescribed fire to maintain high numbers of Henslow’s Sparrows, and caution against more frequent fire rotations in the west.