Habitat Relations and Breeding Biology of Grassland Birds in Western New York: Management Implications

Abstract

In 1994 we began a study of the habitat relations and breeding biology of grassland birds in western New York. Most fields contained fewer than four grassland species, with Bobolink (Dolichonyx oryzivorous) and Savannah Sparrow (Passerculus sandwichensis) being the two most common species. Species of management concern in the Northeast, such as Henslow's Sparrow (Ammodramus henslowii) and Upland Sandpiper (Bartramia longicauda), were absent from the study area. Bird habitat models generated through Principal Components Analysis and stepwise multiple regression indicated that field area, or variables correlated with area, explained most of the variation in overall grassland bird species richness (partial r2 = 0.43) and abundance (partial r2 = 0.60) and in the abundance of Bobolinks and Savannah Sparrows. Grassland birds were generally absent from fields smaller than 5 hectares. Areas with few shrubs and low horizontal heterogeneity supported more grassland bird species than did fields with more shrubs and high horizontal heterogeneity, and fields with shorter, less dense vegetation had more individuals than did fields with taller, dense vegetation. Few grassland birds occurred in fields planted in switchgrass (Panicum virgatum) monocultures. More than 90 percent of all known nesting pairs fledged young by the end of the first week in July. Nest success was generally high; the proportion of nests fledging one or more young was 0.76 for Savannah Sparrows, 0.54 for Bobolinks, and 0.67 for Eastern Meadowlarks (Sturnella magna). Grassland bird populations in this study may benefit from management practices that increase field area, control shrub invasion, and encourage the growth of grasses other than switchgrass. The current low levels of grazing at Iroquois National Wildlife Refuge, with cattle allowed in pastures only after 15 July, do not appear to be harmful to grassland bird populations.