Grassland Bird Productivity on Military Airfields in the Mid-Atlantic and Northeast Regions

摘要

Grasslands associated with airfields in the northeastern United States (both military and civilian) often support large numbers of regionally rare grassland birds. As grassland habitat area in the region continues to decline, the role that large airfields play in maintaining populations of these species is likely to increase. Despite this, relatively little is known regarding reproductive success in these habitats, and whether they act as population sources or sinks. This is a particular concern because vegetation management on airfields often involves regular mowing during the summer breeding season, a practice presumed to be harmful to nesting success. To obtain a general picture of grassland bird reproductive success on regional airfields, and to examine possible factors that may be affecting it (including mowing), we conducted a nest monitoring study in 2009 on three military airfields in the Mid-Atlantic and Northeast: Westover Air Reserve Base (Massachusetts), Lakehurst Naval Air Engineering Station (New Jersey), and Patuxent River Naval Air Station (Maryland). Nests of two target species (grasshopper sparrow (Ammodramus savannarum) and eastern meadowlark (Sturnella magna) and of other grassland-obligate species were located and monitored at regular intervals until success (fledging) or failure. We measured vegetation characteristics around each nest, and through direct observation and cooperation with mowing crews were able to determine: 1) if a nest was located in a regularly mowed area, 2) if a nest was directly mowed over while active, and 3) the condition of each nest immediately following a mow. We calculated daily nest survival rates (DSR), and examined the effects of various predictor variables using logistic modeling in program MARK. We also modeled the mean number of fledglings produced per nest using Generalized Linear Models (GLM).