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Phone: 812-273-0783 |
Cerulean Warbler Study / Henslow's Sparrow Survey / Checklist
BIRDS
| Big Oaks NWR participates in the National Audubon Society's annual Christmas Bird Count (CBC). In 2001, the first year of the count on the refuge, a total of 67 species were observed, including a Le Conte's sparrow. The 2002 count tallied 45 species and provide volunteers a delightful day of birdwatching on the refuge. For more information on the CBC or to see count results go here.
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| The May Day Count is a spring migratory bird count administered by the Indiana Audubon Society. The count is held on International Migratory Bird Day, which is the second Saturday of May. Every bird seen or heard in the county is recorded. Since the refuge encompasses three different Indiana counties, the data are compiled to get a list of species and approximate numbers of birds using the refuge, however the data are submitted to the Audubon Society by individual county. Refuge wide counts have been conducted since 2001 with a record 97 species and 1627 individuals to date. Many more species and individuals use the refuge than what are recorded due to the limited number of volunteers available to assist refuge staff. If you would like to volunteer contact the refuge.
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A breeding bird survey is conducted on Big Oaks NWR every June to sample overall bird diversity. A total of 107 point counts were proportionately distributed among major habitat types along routes of travel on the refuge. The road-side survey was chosen because of the prevalence of closed areas (no access areas) throughout the refuge. These routes are the basis for long-term monitoring of bird populations on Big Oaks NWR. A total of 63 nongame species were recorded during the 2002 survey. Several (a total of 13) of the species observed are listed as a species of special management concern for the Fish and Wildlife Service. These included the red-headed woodpecker, northern flicker, acadian flycatcher, wood thrush, blue-winged warbler, prairie warbler, cerulean warbler, Kentucky warbler, field sparrow, grasshopper sparrow, Henslow's sparrow, eastern meadowlark, and orchard oriole. Additional information on the North American Breeding Bird Survey can be found here. |
Forest & Grassland Bird Productivity Study
A forest and grassland bird productivity study was conducted by Big Oaks NWR staff to better understand the appropriate mixtures of the two habitat extremes needed in the local landscape. The forest bird study was conducted during 1998-1999 and the grassland bird study during 1998-2001. Six (6) forested plots (~ 40-ha) were located in mature closed-canopy sites. Plots in forested habitat were selected to have replicates in the interior of large forest blocks, edges of large forest blocks, and isolated forest fragments. Five (5) grassland plots (~ 20-ha) were established in areas that were not closed due to munitions. With much of the grassland habitat being restricted to access, grassland sites that appeared to represent 'typical' refuge grassland habitat were selected. Nest searching and territory mapping were conducted on these 11 plots to determine breeding bird densities and nesting success. Study plots were marked with a grid to aid in relocating nests. Territory (spot) mapping followed standard procedures. Nest productivity was used to assess brown-headed cowbird parasitism, predation, and the general health of the forest bird population. Common forest nesting birds monitored included the red-eyed vireo, acadian flycatcher, ovenbird, woodthrush, and Kentucky warbler. Common grassland birds monitored included Henslow's sparrows, field sparrows, eastern meadowlarks, and common yellowthroats. Over 1140 nests of 41 different bird species were monitored during the entire study. And over 130 Henslow's sparrow nests were monitored during the 4-year grassland study, one of the largest samples for this species ever monitored. |
| Two graduate students (Master's students) from Ball State University and University of Tennessee and refuge staff completed a 4-year study (2002-2005) on habitat selection and reproductive success of cerulean warblers at Big Oaks National Wildlife Refuge. Cerulean warblers are a forest-nesting neotropical migrant that are undergoing a long-term population decline. The cerulean warbler is listed as a 'species of special concern' on the Region 3 Fish and Wildlife Resources Conservation Priorities List. Objectives of the studies included 1) determining the distribution and abundance of cerulean warblers in the study areas, including territory size and tract size, 2) determining habitat selection of cerulean warblers, 3) determining nest tree parameters selected by cerulean warblers, 4) determining the cerulean warbler productivity and the limits to productivity, and 5) the effects of periodic cicadas on nest productivity. Study plots were located in the northern third of the refuge in mature closed-canopy forest. Refuge staff and interns assisted with nest searching and vegetation measurements and territory mapping during the study. A total of 96 nests were located during the study, representing the highest number of nests found within a single study area in southern Indiana. A copy (pdf file, 2.2 Mb) of Dustin Varble's thesis can be downloaded here.
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Henslow's Sparrow Roadside Transect Surveys
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. |
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GRAPH 1 |
GRAPH 2 |
GRAPH 3 |
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 be decreasing due to shrubby intrusion in recent years. |
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