The Culpeper Basin IBA occupies the southern portion of a low-lying trough depression that extends from the northern Piedmont in eastern Loudoun County southwest through Prince William, Culpeper and Orange counties. The soils of the Basin originate from a combination of weathered volcanic rocks and sediments washed down from the Appalachians. The unique soils and flat, low topography have led to the presence of rare prairie-like vegetation and the predominance of agricultural land use in the area. Likewise, these characterisitcs attract an assemblage of rare or uncommon bird species in Virginia, primarily those associated with grass or shrubland habitats.
Several rare grass and shrubland bird species are found wihtin the Culpeper Basin IBA on a regular basis. In particular, this region is one of the Virginia strongholds for the Barn Owl, the population being partially supported by an active nest box program. At least 40 simultaneous nests have been active in boxes erected throughout the basin. Other common species that are likely to meet thresholds include Field Sparrows, Prairie Warblers, Grasshopper Sparrows, Eastern Meadowlarks, and Northern Bobwhites. Loggerhead Shrikes are uncommon but regularly reported in both the breeding season and in winter throughout the basin. The only known and consistent Upland Sandpiper breeding location occurs here near Remington Farms, where this species is also observed in groups of up to 10 birds during migration. Both Short-eared Owl and Henslow's Sparrow pairs have been recorded in the breeding season. Short-eared Owls are regular winter residents and are found locally in groups of 2-10 individuals. Very little systematic bird data are available for this region and much research remains to be done.
Primary threats to this area include development and the intensification of farming practices. Suburban development along the I-66 and I-95 corridors has been occuring at an explosive pace and has been enroaching upon historically rural areas of the IBA in recent years. This has led to an overall fragmentation and loss of habitat for grass and shrubland species as well as the loss of old farm structures that Barn Owls have historically used for nesting. Intensification of farming practices has reduced both the quantity and quality of large blocks of grassy fields that Barn Owls and Short-eared owls depend upon for foraging and has also reduced the availability of fallow, shrubby habitats used by successional species.
Very little of the IBA is held in protected lands. The largest conservation landholder is the National Park Service, who owns and manages Manassas National Battlefield Park in the northern portion of the IBA. Other landowners include the Virginia Department of Game and Inland Fisheries (Weston and C.F. Phelps Wildlife Management Areas), the Virginia Department of Forestry (Conway Robinson Memorial State Forest), the Association for the Preservation of Civil War Sites (Brandy Station), the U.S. Navy (Quantico Marine Corps Base), and the Prince William, Fairfax, and Fauquier County Park Authorities (several county parks). In addition, a number of private landowners have protected their property through conservation easements.
The current landscape within the IBA is highly altered by a history of human uses. It is now dominated largely by an agricultural mosaic of pasture and hay as well as crops such as soybeans and grains. This agricultural landscape is broken up by scattered woodlots of secondary forests comprised of hickories, oaks, white ash, and Virginia Pine. Tree and woody growth in this area is significantly less than other areas on the Piedmont. The soils are also generally more fertile in the Basin than elsewhere on the Piedmont due to the presence of rare, diabase soils leading to rare plant communities not found elsewhere on the Piedmont such as prairie grasses and forbs more characteristic of Midwestern grasslands. True 'historic' grassland communities, however, exist only at Manassas National Battlefield Park and in some powerline cuts that are mowed frequently. Pasturelands and fallow fields likely support a mixtre of native vegetation and introduced cool-season grasses.
Land use in the IBA is primarily agricultural with pasture and hay being the dominant form. Row crops such as soybeans and grains are also common. A limited portioin of the IBA is managed for conservation or wildlife.