TWEL Bret Graves Thesis

Grassland bird conservation on reclaimed surface mines: Evaluating the influence of vegetation structure on distribution, nest placement and nesting success
 
Bret M. Graves, MS
Advisor: Amanda Rodewald
 
 
Reclaimed surface mines represent a conservation paradox in the Midwest in that they are occupied by grassland birds, yet they are highly disturbed areas and often dominated by introduced grasses. I examined 1) associations among woody vegetation and relative abundance of the grasshopper sparrow (Ammodramus savannarum), Henslow’s sparrow (Ammodramus henslowii), eastern meadowlark (Sturnella magna), Savannah sparrow (Passerculus sandwichensis), bobolink (Dolichonyx oryzivorus) and dickcissel (Spiza americana), and 2) the influence of vegetation composition and structure on nest placement and nest survival of the grasshopper sparrow, Henslow’s sparrow and eastern meadowlark . From May – August 2005 and 2006, I surveyed grassland birds, monitored nesting success, and quantified vegetation structure and composition within reclaimed surface mines in eastern Ohio. Abundance estimates were derived from 101 point-counts along randomly located transects, and nest monitoring focused on eight study plots. I applied a principle components analysis to 3 woody vegetation metrics (i.e., percent cover of woody vegetation and number of woody patches within 100-m, and distance to woodland edge) to create a single index of woody vegetation that was used in subsequent analyses. Relationships between woody vegetation and relative bird abundance were analyzed using generalized linear models, whereas differences in vegetation structure and composition between nest and random locations were examined using a discriminant function analysis for each species. I used an information-theoretic approach, incorporating a set of a priori models into a logistic-exposure method to model daily nest survival. Grasshopper sparrows and Henslow’s sparrows were the most abundant grassland species recorded within both management areas. Although numbers of grasshopper sparrows, Henslow’s sparrows, Savannah sparrows and bobolinks were negatively related to woody vegetation cover within 100-m of survey locations, only the grasshopper sparrow showed strong evidence of responding to woody vegetation within 100-m of nest locations. Grasshopper sparrow nests were located in areas with less visual obstruction of the surrounding vegetation and more bare ground, whereas Henslow’s sparrow and eastern meadowlark nests were associated with deeper and greater coverage of litter within 100-m of the nest than randomly located plots. Daily nest survival was negatively related to the amount of woody vegetation in proximity to nests of grasshopper sparrow (n = 45) and Henslow’s sparrow (n = 18) and marginally related to eastern meadowlark nests (n = 18). Although grassland birds seem to select nest sites based on a variety of habitat features surrounding nests, the amount of woody vegetation may be one of the factors that most strongly influences nest survival. Thus, managers should consider controlling woody encroachment on reclaimed surface mines if the goal is to provide quality habitat for a diverse community of grassland-breeding birds.