EXPERIMENTAL REMOVAL OF WOODY VEGETATION DOES NOT INCREASE NESTING SUCCESS OR FLEDGLING PRODUCTION IN TWO GRASSLAND SPARROWS (AMMODRAMUS) IN PENNSYLVANIA

The influence of vegetation structure on the probability of daily nest survival (DNS) for grassland passerines has received considerable attention. Some correlative studies suggest that the presence of woody vegetation lowers DNS. Over 3 years (2009-2011), we monitored 215 nests of the Grasshopper Sparrow (Ammodramus savannarum) and Henslow's Sparrow (A. henslowii) on 162 ha of reclaimed surface-mine grasslands in Pennsylvania. We removed shrubs from treatment plots with <= 36% areal coverage of woody vegetation in a before-after-control-impact-pairs (BACIP) design framework. Daily nest survival (95% CI: 0.94-0.96) was as high as previous studies have reported but was not associated with woody vegetative cover, proximity to woody vegetation, or woody stem density. Variation in DNS was best explained by increasing nonwoody-vegetation height. Grasshopper Sparrow fledgling production on treatment plots in 2010 (95% CI: 3.3-4.7) and 2011 (95% CI: 3.8-5.0) was similar to baseline conditions of treatment plots (95% CI: 3.4-4.9) and control plots (95% CI: 3.2-4.5) in 2009. Fledgling production was associated with thatch depth ((beta) over cap +/- SE = 0.13 +/- 0.09) and bare ground ((beta) over cap +/- SE = 2.62 +/- 1.29) adjacent to the nest and plot woody vegetative cover ((beta) over cap +/- SE = -3.09 +/- 1.02). Our experimental research suggests that overall reproductive success of Grasshopper and Henslow's sparrows on reclaimed surfacemine grasslands is driven by a suite of largely nonwoody-vegetation components, and both species can successfully nest and produce young in habitats with greater amounts of scattered woody vegetation than has generally been considered.