Big Oaks National Wildlife Refuge is an approximate 55,000-acre property located in southeastern Indiana in Ripley, Jennings, and Jefferson counties, approximately 5 miles north of the town of Madison. Formerly known as the Jefferson Proving Ground, the site was used by the U. S. Army for ordnance testing until it was closed in 1995 under the Base Realignment and Closure Act. Big Oaks NWR was established on June 30, 2000, and is overlaid on approximately 50,000 acres of the former proving grounds. Most portions of the property are off-limits to public day-use due to danger from unexploded ordnance.
Big Oaks National Wildlife Refuge provides habitat for the fifth largest population of Henslow's Sparrow in the world; through roadside surveys, the refuge staff estimates the breeding population of HESPs on the property each summer. For the last survey (2004), the average density detected for HESP was 0.43+0.08 males/ha; this extrapolates to 629 singing male Henslow's sparrow on 1463 ha of the grassland habitat estimated on Big Oaks NWR/Jefferson Range. If an estimate of HESP detectability (0.716, from staff's territory mapping) is included, the average breeding density would be 0.60 singing males/ha. This extrapolates to 877 singing male Henslow's Sparrows for the summer of 2004.
In addition to Henslow's Sparrow studies, the Big Oaks staff completes four BBS-style survey routes within the refuge each June; a total of 107 point counts are proportionally distributed among major habitat types along routes of travel on the refuge. During the 2004 survey, a total of 65 nongame species were recorded. Species which were recorded during the survey which are listed as high-priority on the "Fish and Wildlife Resource Conservation Priorities (RCP) in Region 3" include Acadian Flycatcher, Blue-winged Warbler, Eastern Meadowlark, Field Sparrow, Henslow's Sparrow, Kentucky Warbler, Northern Flicker, Orchard Oriole, Prairie Warbler, Red-headed Woodpecker, Worm-eating Warbler, and Wood Thrush. In all, surveys of the refuge have documented 120 species of breeding birds.
Henslow's Sparrow densities have decreased over the years due to the loss of habitat from woody encroachment; all grassland units appear to be decreasing in size due to this type of succession. Many of the grassland fields which utilized by Henslow's Sparrows are old farm fields that were used by the JPG staff as range impact areas. During the proving ground's operation these fields were kept open with a variety of methods that included yearly prescribed burns, mowing, bulldozing, disking, and herbicide application. Many of these methods are unavailable to the NWR staff due to unexploded ordnance and lack of properly armored equipment. The refuge staff is currently considering alternative methods of maintaining grassland habitat in addition to prescribed burning.
Big Oaks National Wildlife Refuge encompasses 50,000 acres in 3 counties (Jefferson, Jenning, and Ripley) and overlays that portion of the former Jefferson Proving Ground (JPG) that lies north of the historic firing line. The Indiana Air National Guard operates an air-to-ground bombing range on the remaining 1,033 acres of the former proving ground north of the firing line, and this property is surrounded by by but not designated as part of the refuge. Beginning in 1996, the U.S. Fish and Wildlife Service managed the wildlife resources of JPG; Big Oaks NWR was established in June 2000 as an "overlay" national wildlife refuge through a 25-year real estate permit from the U.S. Army.