The
Henslow's sparrow population
has been surveyed annually at Big Oaks NWR since 1995. Much of the
grassland habitat present on the refuge was former pastures and
agricultural fields allowed to revert to natural vegetation under the
U.S. Army's prior management. Prescribed burning was a tool used
by JPG staff to reduce wildfires caused by ordnance testing and
encouraged this natural vegetation to flourish. The refuge staff
still use prescribed burning today to provide grassland habitat for not
only Henslow's sparrows but also other species such as bobwhite, deer
and other grassland and shrubland nesting birds. Since most of the
grassland habitat overlays former impact areas, the Henslow's sparrow
population is monitored via a roadside survey method. Singing
males are counted within grasslands adjacent to roads. From the
count data, an average density is calculated and applied to the total
available habitat to obtain an estimated population at Big Oaks NWR.
The counts are used as an index to gauge habitat conditions and monitor
general population trends. A total of 37 sites are surveyed twice
(June, July) by Service staff to determine the presence or absence of
Henslow's
sparrows.
Prescribe fire on Refuge photo credit - Laura Lake,
USFWS
Henslow's
sparrow densities have decreased over the years due to loss of small grasslands
fields that are being managed to create larger forest blocks for declining
forest interior bird species (Graph
1). Some grassland habitat is also being lost to woody encroachment.
Refuge staff are currently considering alternative methods of maintaining
grassland habitat in addition to prescribed burning (Graph
2). In 2002, the estimated count of singing male Henslow's sparrow was
556 ± 78 (95% CI, uncorrected for the detection
probability of territorial male Henslow's sparrows) on 1463 ha of grassland
habitat estimated on Big Oaks NWR/Jefferson Range (Graph
3). If we incorporate our estimate of Henslow's sparrow detectability
(0.716, derived from territory mapping), the estimated count would be 775 ±
143 (95% CI) singing male Henslow's sparrows. The Henslow's sparrow
population appears to have stabilized; the estimated number of Henslow's
sparrows was similar to the previous year (2001).
