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2015 SOUTHEASTERN NATURALIST 14(3):438–446
Area Sensitivity of Grassland Sparrows Overwintering in a
South Carolina Forested Landscape
Paul J. Champlin1,2, John C. Kilgo3,*, J. Drew Lanham1, and Frank J. Spilker4
Abstract - We assessed area sensitivity of overwintering Peucaea aestivalis (Bachman’s
Sparrow), Ammodramus savannarum (Grasshopper Sparrow), and A. henslowii (Henslow’s
Sparrow) within utility rights-of-way (ROWs) at the US Department of Energy’s Savannah
River Site (SRS) in South Carolina. We compared sparrow abundance among 4 ROW-width
classes (25–44.9 m, 45–64.9 m, 65–84.9 m, and ≥85 m) and used landform index (LFI; a
measure of topography and environmental exposure) as a covariate in our analyses to assess
potential effects of abiotic characteristics. Total number of sparrows flushed/ha, Grasshopper
Sparrows flushed/ha, and Henslow’s Sparrows flushed/ha were positively related to
ROW-width class. Total number of sparrows flushed/ha and Bachman’s Sparrows flushed/ha
were negatively related to LFI, indicating a positive relationship with site exposure. Utility
ROWs in the Southeast provide wintering habitat for grassland sparrows, especially on
exposed elevated plateaus within wide ROWs.
Introduction
Grassland-bird populations are declining more rapidly than any other North
American avian guild (Herkert 1995, Peterjohn and Sauer 1999, Sauer et al. 2008,
Vickery and Herkert 2001), and habitat fragmentation has been suggested as an
important factor in these declines (Askins 1993, Herkert et al. 2002, Winter and
Faaborg 1999). Smaller grassland-patch sizes can reduce density or productivity
of some grassland-bird species, although these effects vary regionally and among
species (Donovan et al. 1997, Ribic et al. 2009). Most efforts investigating area
sensitivity and edge effects have focused on breeding grassland birds and have included
the determination of minimum-area requirements for many species (Askins
1993, Davis 2004, Walk and Warner 1999). However, area effects on grassland
birds in their wintering sites have received little attention.
Differences in suitability of grassland patches of various sizes may be due in part
to variability in the abiotic environmental conditions that affect grassland habitats.
Thus, patch area actually may be a loose surrogate for various abiotic environmental
conditions, such as exposure (e.g., to solar radiation, wind, and precipitation).
Furthermore, grassland-bird density, diversity, and productivity are strongly linked
to vegetative composition (Carrie et al. 2002, Herkert 1994, Johnson and Igl 2001,
Winter et al. 2005), and vegetative composition and structure are dependent on
a host of abiotic environmental factors such as topography and environmental
1Clemson University, Department of Forest Resources, Clemson, SC 29634. 2Current address
- 1634 Q Drift Road, Westport, MA 02790. 3USDA Forest Service Southern Research Station,
PO Box 700, New Ellenton, SC 29809. 4Bowhead Science and Technology (BST), 3530 Manor
Drive, Suite 4, Vicksburg, MS 39180. *Corresponding author - jkilgo@fs.fed.us.
Manuscript Editor: Karl E. Miller
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2015 Vol. 14, No. 3
exposure (Chen et al. 1992, 1995; Knapp et al. 1993, Martin 2001, Yahner 1988).
Topography and exposure can be quantified by a single metric, the landform index
(LFI), which is the mean of a series of measurements of percent slope across the
horizon (McNab 1992). Because LFI is affected by both forest structure and slope,
it reflects the relative openness (i.e., exposure) of a site. Lower angle-measurements
indicate more exposed sites, and a negative response to LFI would represent a positive
response to exposure. We hypothesized that grassland birds may be associated
with LFI as well as an index of patch area.
The intensification of agriculture and land development in the southeastern US,
coupled with cessation of frequent natural disturbance regimes such as fire, have
greatly fragmented grassland and savanna habitats (Askins 1993, Hunter 1990).
Rights-of-way (ROWs) are common features of the landscape and are kept in an
early successional state over long time-periods through regular disturbances including
mowing, herbicide use, and burning. ROWs provide an opportunity to assess the
effects of abiotic traits such as topography and exposure in an unbiased fashion due
to the manner in which they bisect the landscape. Whereas the original positions of
fields may have been selected due to topographic considerations, ROWs allow for
essentially random transects through grassland habitat irrespective of topography.
ROWs and other such habitats present novel opportunities for surrogate grasslandhabitat
research and management because they support early successional avifauna
including overwintering grassland birds (Anderson et al. 1977, Champlin 2007).
The 3 grassland sparrow species of conservation concern most commonly encountered
at the Savannah River Site (SRS) in western South Carolina are Peucaea
aestivalis (Lichtenstein) (Bachman’s Sparrow), Ammodramus savannarum (Gmelin)
(Grasshopper Sparrow), and A. henslowii (Audubon) (Henslow’s Sparrow).
All 3 species winter in grasslands of Pinus (pine) savannas in the southeastern
US. Of these 3 species, only the Bachman’s Sparrow occurs regularly in SRS pine
forests (Champlin 2007), presumably due to the poorly developed grass-forb layer
of these forests (Imm and McLeod 2005). However, all 3 species occur in ROWs
at SRS (Champlin 2007). We took advantage of the properties of ROWs to investigate
whether grassland-sparrow density, as indexed by the number flushed/ha of
strip transect, may be a function of habitat area and the relative exposure of ROW
grasslands to abiotic influences, as reflected in the LFI, during winter on the predominantly
forested landscape of the SRS in western South Carolina.
Study Area
The SRS is an 80,267-ha US Department of Energy National Environmental
Research Park in the Upper Coastal Plain and Sandhills physiographic provinces
of western South Carolina (Fig. 1). At the time of our study, 85% of the SRS was
forested, with ~2700 ha of ROWs embedded in ~56,000 ha of pine-dominated forest
and ~18,000 ha of forested bottomland and wetland. Pine forests were primarily
Pinus taeda L. (Loblolly Pine) and Pinus palustris Miller (Longleaf Pine) managed
on 50- to 120-year rotations, depending on species and site-specific land-use objectives.
Wetlands ranged in size from a few to nearly 100 ha and included more than
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200 Carolina bays, many of the largest of which were grass-dominated (Imm and
McLeod 2005).
Rights-of-way at the SRS were dominated by xeric tall-grass prairie-like grasslands
on well-drained, sandy ultisol and quartzisamment soils with predominantly
moderate relief but also bisected topographically low, flat settings such as river bottoms
that supported wetland shrubs and hardwood species growing in entisol soils
Figure 1. Distribution of survey transects for wintering grassland sparrows on the Savannah
River Site, SC, 2003–2006.
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with high clay content (Imm and McLeod 2005). While most ROWs at the SRS were
predominantly grassland, gradients existed from dense sapling and shrub cover to
scattered sites of nearly bare sand. Soils at our research sites had not undergone
major human perturbation for ~30–50 years. Vegetation management in ROWs was
conducted in an ad-hoc manner and included a combination of intermittent mowing,
selective treatment of woody vegetation with herbicide, and prescribed burning.
Herbicide use occurred at ~3-year intervals, and burning occurred whenever adjacent
forest compartments were burned, typically at 3–5-year intervals, although prescribed
fires were infrequent during this study due to a severe drought.
Methods
We surveyed grassland sparrows on sixty 500-m long transects positioned at
least 500 m apart (Igl and Ballard 1999) within grassland-dominated portions of
utility ROWs (Fig. 1). We recorded all avian species, but we focus here only on
Bachman’s, Grasshopper, and Henslow’s sparrows. Single observers traversed
ROWs linearly while flushing birds from a 10-m-wide swath using two 5-m long,
telescopic, fiberglass fishing poles. After walking 500 m, the observer shifted laterally
90° from the direction of travel for a distance of no more than 25 m and walked
a parallel transect back to the initial starting point. Thus each transect surveyed 1 ha
(1000 m long x 10 m wide). Experienced observers trained in sparrow identification
walked transects at a pace of ~5 km/hr, only counting birds flushed from within the
reach of the poles and taking a conservative approach in the potential recounting of
previously flushed birds. Observers identified birds to species by their size, coloration,
flight characteristics, and tail length. Surveyors followed any unknown birds
until identification could be confirmed either visually or via capture for a concurrent
banding study.
We surveyed 30 transects once during winter (25 Dec–31 March of 2002–2003),
30 different transects once during the winter of 2003–2004, and all 60 of these transects
3 times each during the winters of 2004–2005 and 2005–2006. During winters
with multiple surveys per transect, we used data from the second survey round (the
round in the core of the winter season), unless a transect was mowed or burned
after the first survey of any year, in which case we used count data from the first
survey. We made no adjustments to survey results for detection probability due to
the narrow nature of flush transects (10 m) and the proximity of the 3 focal species
to observers when flushed (often less than 5 m); we assumed equal detection probability for
all individuals and transects.
We used ROW width as an index to habitat area. We defined ROW width as the
mean of 5 measurements taken perpendicular to the ROW centerline approximately
every 100 m along each surveyed ROW. We measured distance between the 2 tree
boles closest to the centerline of the ROW. ROWs ranged in width from 24 m to
102 m. We assigned ROW width to 4 classes: ≤44.9 m (n = 32), 45–64.9 m (n = 14),
65–84.9 m (n = 6), and ≥85 m (n = 8). Our focal species are known to be affected by
time since burn (Cox and Jones 2009, Korosy et al. 2013), but each of our ROWs
bisected or abutted multiple forest stands, each with a different burn history. Our
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transects therefore represented the full range of conditions available in SRS ROWs
with regard to vegetation management and burn histories, and these histories were
not confounded with ROW width.
We measured LFI at 5 points spaced approximately every 100 m along each surveyed
ROW and located random distances from the ROW centerline. We measured
percent slope to the lowest point of sky visible through the surrounding forest on
either side of each point in a direction perpendicular to the transect orientation. We
took the mean of these 10 measurements as the LFI for each transect. This procedure
is an alteration of the typical LFI measurement, which is made at a single point
and measured at 8 equal increments around the compass (McNab 1992).
We used a mixed model (Sokal and Rohlf 1995) analysis of covariance (SAS
2004) at α = 0.05 to compare the number of the 3 focal species flushed/ha, both individually
and combined, among width classes, and to evaluate the degree to which
LFI explained variation in abundance. We considered width class as a fixed effect,
LFI as a covariate, and year as a repeated measure on each transect nested within
each width class. Adjustments to estimates of the width class to account for effects
of LFI did not change results for any analysis, so we report un adjusted estimates.
Results
We detected 199 grassland sparrows, including 108 Bachman’s, 44 Grasshopper,
and 47 Henslow’s Sparrows during the first 2 years and during the second
round of surveys of the last 2 years of the study. No interaction between width class
and LFI was present. Total number of sparrows flushed/ha, number of Grasshopper
Sparrows flushed/ha, and number of Henslow’s Sparrow flushed/ha responded
positively to width class (Table 1, Fig. 2). Total number of sparrows flushed/ha and
the number of Bachman’s Sparrow flushed/ha were negatively correlated with LFI
(Table 1).
Discussion
The number of grassland sparrows flushed/ha was greater in wider and more
open ROWs than in narrower, less-exposed ROWs at the SRS. Although previous
Table 1. Effects of width class and landform index (LFI) on number of grassland sparrows flushed/
ha within utility rights-of-way at the Savannah River Site, SC, 2002–2006 (negative response to LFI
represents a positive response to exposure; P < 0.05).
Species/group Effect Estimate SE F df P
Total sparrows Width class 14.0 3, 49 less than 0.001
LFI -11.05 5.22 4.5 1, 124 0.036
Bachman’s Sparrow Width class 1.7 3, 49 0.185
LFI -8.51 3.70 5.3 1, 124 0.023
Grasshopper Sparrow Width class 4.9 3, 49 0.005
LFI -1.43 2.23 0.4 1, 124 0.521
Henslow’s Sparrow Width class 9.2 3, 49 less than 0.001
LFI -1.06 1.93 0.3 1, 124 0.586
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studies related habitat occupancy to habitat-patch area and juxtaposition to forest
or edge (Herkert 1994, Vickery et al. 1994, Winter and Faaborg 1999), the mechanisms
underlying these relationships are poorly understood. Positive relationships
between sparrows, ROW width, and exposure might indicate that larger habitatpatches
have an inherently greater likelihood of encompassing a wider array of
abiotic environmental attributes responsible for greater habita t complexity.
Even though our study was conducted within a relatively narrow range of size
classes compared to other studies, we were still able to detect trends in area sensitivity
both within and among species. The ratio of our smallest to largest ROW width
was 1:4 (24 m:102 m). Ratios of smallest to largest patch size studied reported in
other research on breeding birds include 1:34 (Winter and Faaborg 1999), 1:250
(Riffell et al. 2001), 1:644 (Davis et al. 2006), and 1:1347 (Vickery et al. 1994).
Thus, although ROW width is only an index to patch area, it appears that wintering
grassland birds may be sensitive to area (or abiotic factors related to area) within a
narrow range at the lower end of the area spectrum.
Hostility of environmental edge has been commonly suggested as an explanation
for edge avoidance and accompanying area sensitivity in breeding birds, with
the effect most acute within 50 m of the surrounding matrix (Burger et al. 1994,
Figure 2. Mean (± SE) number of Bachman’s Sparrows, Grasshopper Sparrows, and
Henslow’s Sparrows flushed/ha in 4 utility right-of-way width classes (1 = 25–44.9 m;
2 = 45–64.9 m; 3 = 65–84.9 m; and 4 = ≥85 m) at the Savannah River Site, SC during the
winters of 2003–2006.
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Johnson and Temple 1986, Winter et al. 2000). We observed no indication of actual
avoidance of edges; we regularly detected all 3 grassland sparrow species at the
forest edge (P.J. Champlin, pers. observ.). In addition, concurrent monitoring of
116 radio-tagged Henslow’s Sparrows on our research sites revealed that this focal
species frequently used portions of ROWs within 10 m of edges and that overall
mortality was low (~3%; Champlin 2007; P.J. Champlin, unpubl. data). Thus,
predator-driven edge avoidance (Lima and Valone 1991, Pulliam and Mills 1977)
seems an unlikely explanation for the apparent area sensitivity we found in ROWs
at SRS. We believe differences in vegetation composition and structure among size
classes were responsible for the presence of birds in relation to edge (Champlin
2007), but our sampling was not designed to address this effect directly. Whether
such differences exist and if so, whether they are attributable to fire effects or other
management activities, remains unclear.
Although it is important to assess the grassland-bird response to habitat-patch
size, it is perhaps of greater management utility to understand the habitat conditions
associated with patch size. Likewise it is important to understand whether
and how management may promote large-patch conditions within management
units of limited size. In addition, landscape-scale factors appear to play
an important role in determining grassland-sparrow distribution and abundance
(Dunning et al. 1995, Taillie et al. 2015). Future research should endeavor to
clarify the interactions between habitat-management activities in ROWs and the
effects of ROW-patch size and exposure on grassland vegetation and, in turn, on
grassland-sparrow habitat quality. Our ROW width classes were not correlated
with LFI, indicating that other factors may influence environmental exposure in
grasslands. In Louisiana, Carrie et al. (2002) found Henslow’s Sparrows most
frequently on dry, well-drained sandy soils similar to what occured on exposed
elevated sites at SRS. Large grassland-patches on elevated plateaus with soft forest
edges (limited height and density of adjacent canopy) may represent important
opportunities for the conservation of wintering grassland birds in the South
Atlantic Coastal Plain.
Acknowledgments
We thank E. Summers, D. Jones, and S. Junker for their efforts in the field, and J. Blake
and K. Wright for logistical assistance. We are indebted to M. Vukovich for assisting with
GIS and creating maps. The manuscript was improved by the comments of 3 anonymous
reviewers. Funding was provided by Clemson University, the US Forest Service Southern
Research Station, and the US Department of Energy-Savannah River Operations Office
through the US Forest Service Savannah River under Interagency Agreement DE-AI09-
00SR22188.
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