Written by Bryan Watts
March 3, 2010
The Bachman’s sparrow is endemic to southeastern North America and is the only member of the Aimophila genus found within this region. Following a dramatic northerly range expansion in the late 1800s and early 1900s, this species has been contracting back to the core of its historic range in the extreme southeast. Population declines along the northern fringe of the species range have likely been due to secondary succession within abandoned farmland and the increased use of sod-forming grasses. Declines within the core of the breeding range have been due to degradation and loss of the southeastern pine ecosystem and effective fire suppression programs.
Bachman’s sparrow is a disturbance-prone species that occupies a narrow disturbance/successional niche. The species requires pine or open savannas with a high density of grasses and forbs in the first meter layer above the ground and low densities of vegetation in the second to fourth meter layer above the ground. For most locations, habitat maintenance depends on a 3-5 year disturbance interval. In the absence of such disturbance, most habitat patches will become unusable within a short period of time as succession proceeds beyond where it is suitable for the species.
During the 1996 breeding season, the Center for Conservation Biology conducted a systematic survey of habitat patches for Bachman’s sparrows in Virginia that covered a 1-degree block or a 5,200 square kilometer area. The study area encompassed all records of the species for the previous 20 years except for those on military lands. This effort included 520 point counts within 280 clear cuts and resulted in the detection of only 6 individuals in 4 patches. Since this effort, the species appears to have disappeared from the state with no recent records, placing the northern range limit in the Carolinas.
Management of the Bachman’s sparrow population in Virginia has been difficult because the species has primarily relied on clearcuts for breeding. Clearcuts are ephemeral habitats that within a landscape context are spatially and temporally dynamic. Simulations in the late 1980s of Bachman’s populations within managed landscapes demonstrated that sustainability is very dependent on the availability of habitats that represent stable sources such as old-growth pine stands that are maintained with regular burning. Unlike clearcuts that may only provide breeding opportunities for short time windows, old-growth stands have the potential to support breeding populations for centuries. These populations may then serve as sources for other maturing forests and clearcuts.