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K.A. Chadbourne and J.T. Anderson
2015 Vol. 14, Special Issue 7
344
Canaan Valley & Environs
2015 Southeastern Naturalist 14(Special Issue 7):344–356
Temporal Variation in Songbird Abundance on Grasslands
in Canaan Valley, West Virginia
Kelly A. Chadbourne1,2 and James T. Anderson1,*
Abstract - Grassland songbirds have been declining due to loss of habitat. In Canaan
Valley, WV, grassland habitats primarily consist of active and idle pastures and hayfields.
Our objectives were to document the species of breeding birds that occurred on grasslands
of the Canaan Valley National Wildlife Refuge and evaluate temporal patterns of
abundance among months and years. This study took place on 3 idle hayfields and 3 idle
pastures in the Refuge during the summers of 1999 and 2000. At the conclusion of the
first field season, half of each field was mowed. A total of 28 species—13 in 1999 and 27
in 2000—was documented. Dolichonyx oryzivorus (Bobolink), Sturnella magna (Eastern
Meadowlark), Passerculus sandwichensis (Savannah Sparrow), and Agelaius phoeniceus
(Red-winged Blackbird) were dominant. Additional species observed included Ammodramus
savannarum (Grasshopper Sparrow) and Circus cyaneus (Northern Harrier).
Species diversity, species richness, and total bird abundance varied among months (P <
0.05). Densities were higher in July than in May, June, and August; however, total number
of birds peaked later in 1999 than in 2000. Species richness and diversity were lower
in August than other months. Temporal variations in grassland bird diversity, richness,
and abundance were likely influenced by precipitation patterns and land-use practices on
and adjacent to the Refuge.
Introduction
Populations of North American grassland birds have undergone more consistent
and steeper population declines than the birds of other guilds (Jones and
Vickery 1997, Kantrud and Higgins 1992, Knopf 1994, Sauer et al. 2012). The
North American Breeding Bird Survey indicates that Eremophila alpestris (L.)
(Horned Lark), Passerculus sandwichensis (Savannah Sparrow), Ammodramus
savannarum (Grasshopper Sparrow), Ammodramus henslowii (Audubon)
(Henslow’s Sparrow), Pooecetes gramineus (Vesper Sparrow), Dolichonyx
oryzivorus (Bobolink), and Sturnella magna (Eastern Meadowlark) have all undergone
significant population declines in the last several decades (Sauer et al.
2012, Wells and Rosenberg 1999). Possible reasons for these reductions include
a change in agricultural technology, farmland abandonment, more frequent haycrop
rotation, and losses of habitat due to urbanization (Bollinger et al. 1990,
Herkert et al. 1996, Jones and Vickery 1997, Norment et al. 1999). In the northeastern
US, all of the common grassland bird species are showing long-term
decreasing population trends (Sauer et al. 2012; Table 1).
1Wildlife and Fisheries Resources Program, Division of Forestry and Natural Resources,
West Virginia University, PO Box 6125, Morgantown, WV 26506. 2Current address - US
Fish and Wildlife Service, Great Bay National Wildlife Refuge, 100 Merrimac Drive,
Newington, NH 03801. *Corresponding author - jim.anderson@mail.wvu.edu.
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2015 Vol. 14, Special Issue 7
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Many grassland bird species require large tracts of grassland and therefore are
vulnerable to habitat fragmentation (Norment et al. 1999). In the northeastern US,
grasslands have declined >60% since the 1930s (Norment et al. 1999, Vickery et
al. 1994). In part, because native grassland habitats have been severely altered
by agricultural practices, grasslands are among the continent’s most endangered
ecosystems (Vickery et al. 1999). Loss of grasslands has exceeded 80% in most
areas, and where land is well suited for agricultural crops, less than 0.1% of native
prairie remains (Vickery et al. 1999). However, in the Midwest and northeastern
US, agricultural lands have provided adequate breeding habitat for many species,
particularly the Bobolink and Eastern Meadowlark (Vickery et al. 1999).
Studies of grassland birds in West Virginia have focused on populations living
on reclaimed mine sites (Whitmore and Hall 1978); however, studies on
grassland birds in the state’s agricultural grasslands have not been conducted.
Moreover, survey design, population monitoring, and habitat-use studies need
to consider temporal trends in grassland bird populations (George et al. 1992,
Ralph and Scott 1981, Sauer and Droege 1990). The objectives of our study were
to document which species of breeding birds occurred on the grasslands of the
Canaan Valley National Wildlife Refuge (hereafter, the Refuge) and to evaluate
temporal patterns of their abundance among months and years.
Study Area
We conducted this study on the Refuge, which is located in Canaan Valley
(hereafter, the Valley), Tucker County, northeastern West Virginia. The Valley,
which is 1.9–3.7 miles (3–6 km) wide, 14.9 miles (24 km) long, and oriented
on a northeast–southwest axis, sits at an elevation of 3150–324 8 ft (960–990 m)
above sea level. Ridges surrounding the Valley rise 1001 feet (305 m) above the
Valley’s floor (USFWS 1979). The Valley’s floor hosts a large wetland complex
(~6025 ac [2438 ha]) that consists of meadows, bogs, hayfields, beaver ponds,
and boreal forests.
Table 1. Population trends of grassland birds (% change, 1966–2011) on the Canaan Valley National
Wildlife Refuge, Tucker County, WV (Sauer et al. 2012). Total trend = survey-wide grassland
bird species group population change; USFWS Region 5 NE) = grassland bird population change
data for USFWS refuges in Connecticut, Delaware, Maryland, Massachusetts, New Hampshire,
New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia, and West Virginia; WV =
population trend data for grassland birds found on the Canaan Valley National Wildlife Refuge for
1999 and 2000.
Species Total trend USFWS Region 5 (NE) WV
Savannah Sparrow -0.50 -2.28 -6.84
Bobolink -1.58 +0.15 +2.11
Eastern Meadowlark -2.74 -4.68 -3.40
Red-winged Blackbird -1.04 -2.62 -2.58
Vesper Sparrow -0.76 -5.29 -15.05
Grasshopper Sparrow -3.38 -4.28 -12.00
Northern Harrier -0.63 -3.11 -
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The Valley is characterized by cold winter temperatures and moderate summers.
Summer temperatures average 75–79 °F (24–26 °C) during the day and
50–55 °F (10–13 °C) at night (NOAA 1999). Because the climate and vegetation
are similar to more northerly regions, many boreal plants are at the southernmost
extent of their range in the Valley (Fortney 1993). An annual average of 92 frostfree
days extends from 31 May–1 September (Vogelmann 1978). Total average
rainfall is nearly 45 in (114 cm) (NOAA 1999). In 1999, the Valley’s total annual
rainfall was 37.6 in (95.6 cm), due in part to sub-average rainfall from June to
September (Fig. 1). In fact, West Virginia experienced a statewide drought during
the summer of 1999. In 2000, the total rainfall of 44.3 in (112.6 cm) was
near normal (Fig. 1; NOAA 2000). There were 8.6 in (21.9 cm) more rainfall for
May–August during 2000 than in 1999.
The history of human intervention in the Valley is discussed by Fortney (1993)
and Warren (2001). Although agriculture historically occurred here, it never
flourished. Nonetheless, some farms still exist. Many crops did not produce well
because of the short growing season, so after the area was logged, cattle became
the predominant agricultural product (USFWS 1979). Pastures and hayfields are
found in the slightly drier southern part of the Valley. The Refuge’s grasslands
consist of dry upland areas, Crataegus spp. (hawthorn) savannahs, saturated
wet meadows, and saturated scrub-shrub wetlands, which Vickery et al. (1999)
include in the broad definition of grassland systems.
Methods
Experimental design
We conducted this study on 6 grassland fields—designated as the Beall, Cortland,
Freeland, Harper, Herz, and Thompson tracts—that total 730 ac (295 ha)
Figure 1. Monthly
precipitation totals
(cm) for 1999–2000,
and normal monthly
rainfall values
for Canaan Valley,
Tucker County, WV
1999–2000.
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2015 Vol. 14, Special Issue 7
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(mean = 121.6 ac/field, SE = 31.4 [49.2 ha/field, SE = 12.7]). We classified each
plot according to its prior land use as (a) idle hayfield (Beall [230 ac (93 ha)],
Harper [180 ac (73 ha)], and Thompson [59 ac (24 ha)]) or (b) idle pasture (Cortland
[40 ac (16 ha)], Freeland [69 ac (28 ha)], and Herz [151 ac (61 ha)]). At the
end of the 1999 breeding season in late August, 50% of the grassland portions of
Beall, Cortland, Freeland, Harper, and Thompson were randomly chosen and 1
large block was mowed to determine the effects of habitat manipulation on breeding
bird success (Warren 2001). The Herz idle pasture was not mowed because it
was too wet. We established transects along the length of each field for bird and
vegetation surveys. We located transects 164 ft (50 m) away from field edges to
facilitate bird counts.
Breeding bird counts
To estimate breeding bird densities, we conducted strip-transect surveys (Best
et al. 1997, Franzreb 1981). We conducted 13 weekly bird counts using one person
on each field from May to August 1999–2000. We conducted weekly surveys
within 5 hours of sunrise, recording all birds seen or heard within 164 ft (50 m) of
the transect center, which provided a transect width of 328 ft (100 m) (Best et al.
1997, Gates 1995). The edges of the strip tansects were flagged to facilitate complete
survey coverage. We did not conduct counts when it was raining or when
wind speeds exceeded 10 mi (16 km) per hour (Best et al. 1997). As part of another
study (Warren 2001), we recorded bird species, gender, and behavior (e.g.,
singing, calling) when possible. We tried to walk the transects at a pace that was
slow enough to count all individuals but fast enough to avoid double-counting
individuals (Best et al. 1997). As a further check against double-counting birds,
we excluded individuals that flew overhead and did not alight within our study
fields (Best et al. 1997).
Statistical analyses
We calculated the average density (number of birds/ac [number of birds/ha])
for each month at each field. We also analyzed total birds and 3 target species—
Bobolink, Savannah Sparrow, and Eastern Meadowlark—to determine overall
differences in months and years for total bird and target-species densities. We
chose these 3 species because they occurred on both habitat types (pastures and
hayfields) during both years and because their populations are declining. We calculated
species diversity, specifically the Shannon-Weiner diversity index, using
the maximum number of individuals of a species observed on a field during each
month (Shannon 1948, Zar 1999). We calculated species richness by determining
the number of species found on each field during the month and standardized
them by field size.
We checked data for normality using the Shapiro-Wilk statistic and for homogeneity
of variances by plotting residuals (Cody and Smith 1991). Because
the assumptions for parametric tests were not met (Conover and Iman 1981), we
rank-transformed density, richness, and diversity data. We used 3-way analysis
of variance (ANOVA) to compare density, richness, and diversity (dependent
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2015 Vol. 14, Special Issue 7
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variables) among months (May, June, July, and August), years (1999 and 2000),
habitat types (pastures and hayfields), the 3-way interaction, and all 2-way interactions
(independent variables). Results of habitat-type comparisons are not
presented here (Warren 2001, Warren and Anderson 2005) and are only included
to test the interaction of habitat with year and month. Additionally, data from the
mowed portions of each field in 2000 were only used if results between mowed
and unmowed portions were not significantly different (Warren 2001). We considered
all tests to be significant at P < 0.05. Following a significant ANOVA, we
used Tukey’s multiple comparison test to separate means.
Results
We identified 13 species of birds in 1999, 27 in 2000, and a total of 28 different
species for both years (Table 2). The 3 most numerous species were Bobolink,
Eastern Meadowlark, and Savannah Sparrow. Agelaius phoeniceus (Red-winged
Table 2. Maximum number of birds counted during weekly bird surveys on 6 grasslands of the
Canaan Valley National Wildlife Refuge, Tucker County, WV, May to August 1999–2000. Tracts:
B = Beal, Ha = Harper, T = Thompson, C = Cortland, F = Freeland, and He = Hertz.
Tract
Common name Scientific name B Ha T C F He
Bobolink Dolichonyx oryzivorus (L.) 4 8 70 3 145 0
Savannah Sparrow Passerculus sandwichensis (Gmelin) 11 2 6 3 4 8
Eastern Meadowlark Sturnella magna (L.) 21 2 3 0 5 0
Red-winged Blackbird Agelaius phoeniceus (L.) 2 7 11 1 5 3
Grasshopper Sparrow Ammodramus savannarum (Gmelin) 3 2 0 0 0 0
Vesper Sparrow Pooecetes gramineus (Gmelin) 0 0 0 0 0 1
Chipping Sparrow Spizella passerina ((Bechstein) 0 0 0 1 0 5
Field Sparrow Spizella pusilla (Wilson) 0 1 0 1 0 1
Eastern Towhee Piplio erythrophthalmus (L.) 0 0 0 0 0 1
Indigo Bunting Passerina cyanea (L.) 0 1 0 1 0 0
Song Sparrow Melospiza melodia (Wilson) 0 2 0 1 1 0
Canada Goose Branta canadensis (L.) 0 0 19 0 0 0
Mallard Anas platyrhynchos L. 0 0 1 0 0 0
Wild Turkey Meleagris gallopavo L. 7 0 0 0 0 0
Northern Harrier Circus cyaneus (L.) 1 0 0 0 0 0
Ruby-throated Hummingbird Archilochus colubris (L.) 0 0 0 1 0 0
Northern Flicker Colaptes auratus (L.) 2 0 2 1 0 2
American Crow Corvus brachyrhynchos Brehm 0 0 0 1 0 0
Carolina Wren Thryothorus ludovicianus (Latham) 0 0 0 1 0 2
Gray Catbird Dumetella carolinensis (L.) 0 0 0 2 0 0
Eastern Bluebird Sialia sialis (L.) 0 0 0 1 0 0
American Robin Turdus migratorius L. 1 0 0 0 0 0
Cedar Waxwing Bombycilla cedrorum Vieillot 0 2 0 0 0 0
Red-eyed Vireo Vireo olivaceus (L.) 0 0 0 1 0 0
Common Yellowthroat Geothlypis trichas (L.) 0 0 3 1 1 4
American Goldfinch Carduelis tristis (L.) 0 0 0 1 0 1
Northern Cardinal Cardinalis cardinalis (L.) 0 0 0 1 0 0
Turkey Vulture Cathartes aura (L.) 0 0 0 0 0 4
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Blackbird) and Savannah Sparrows occurred most frequently and occupied all 6
fields. Bobolinks occurred on 5 of the 6 grasslands; we observed Eastern Meadowlark,
Colaptes auratus (Northern Flicker), and Geothlypis trichas (Common
Yellowthroat) on 4 fields; Spizella pusilla (Field Sparrow) and Melospiza melodia
(Song Sparrow) on 3 fields; and Grasshopper Sparrow and Spizella passerina
(Chipping Sparrow), Thryothorus ludovicianus (Carolina Wren), and Carduelis
tristis (American Goldfinch) on two. We recorded the other 16 species on only
one grassland (Table 2).
Bird numbers showed different temporal trends during May and June of the 2
years, but varied in later months (Fig. 2). During 1999, populations rose through
July and early August, then they declined (Fig. 2). In 2000, numbers peaked
earlier, in mid-July, but at a lower level before declining drastically (Fig. 2).
Observers frequently encountered many Bobolinks and Red-winged Blackbirds
on the Freeland and Thompson tracts during July and August 1999. At the end of
1999’s breeding season, observers found large flocks of Eastern Meadowlarks on
the Beall tract.
Total density (number/acre [number/ha]) was greater in 1999 (mean = 3.34/ac,
SE = 1.04 [8.25/ha, SE = 2.58]) than in 2000 (mean = 0.61/acre, SE = 0.13
[1.50/ha, SE = 0.32]) (F1, 32 = 11.53, P = 0.002). We also detected differences
among months (F3, 32 = 3.83, P = 0.009), with a higher mean density in July than
May, June, and August (Fig. 3). Densities were similar between years and among
Figure 2. Total
birds counted for
weekly bird surveys
on the Canaan
Valley National
Wildlife
Refuge, Tucker
County, WV,
May–August in
1999–2000. Week
1 is the 2nd week
in May, and Week
13 is the 2nd
week in August.
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2015 Vol. 14, Special Issue 7
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months for the Bobolink (years: F1, 32 = 0.06, P = 0.803; months: F3, 32 = 0.17, P =
0.915), Eastern Meadowlark (years: F1, 32 = 0.36, P = 0.551; months: F3, 32 = 0.69,
P = 0.563), and Savannah Sparrow (years: F1, 32 = 0.52, P = 0.477; months: F3, 32 =
3.04, P = 0.068) (Table 3).
Grassland bird diversity was similar between 1999 (mean = 0.12/acre, SE =
0.02 [0.29/ha, SE = 0.05]) and 2000 (mean = 0.16/acre, SE = 0.02 [0.39/ha,
SE = 0.04]) (F1, 32 = 3.31, P = 0.078). However, species diversity was lower in
August than other months (F3, 32 = 6.52, P = 0.001; Fig. 4). Grassland-bird richness
was higher in 2000 (mean = 0.73, SE = 0.10 [1.81/ha, SE = 0.25]) than in
1999 (mean = 0.28, SE = 0.11 [0.69/ha, SE = 0.26]) (F1, 32 = 23.32, P < 0.0001).
Figure 3. Breeding
grassland bird density
for all species
combined among
months on the Canaan
Valley NWR,
Tucker County,
WV, May–August
in 1999–2000.
The same letter
above bars indicates
no difference
(P > 0.05) among
months.
Figure 4. Breeding
grassland bird diversity
(Shannon-
Weiner diversity
indices; Zar 1999)
among months on
the Canaan Valley
NWR, Tucker
County, WV, May–
August in 1999–
2000. The same
letter above bars
indicates no difference
(P > 0.05)
among months.
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Table 3. Breeding grassland bird density (number/ha), diversity, and richness by year and month on the Canaan Valley National Wildlife Refuge, Tucker
County, West Virginia, May–August 1999 and 2000.
1999 2000
May June July August May June July August
Species Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE
Overall 2.13 0.49 5.44 1.13 14.64 7.07 10.8 7.27 1.65 0.56 1.65 0.56 2.52 0.98 0.95 0.45
Bobolink 0.31 0.21 0.3 0.23 2.45 1.91 5.18 3.57 0.27 0.21 0.71 0.56 1.45 1.15 0.25 0.16
Eastern Meadowlark 0.27 0.18 0.24 0.11 0.25 0.14 0.12 0.12 0.04 0.04 0.16 0.06 0.15 0.07 0.17 0.09
Savannah Sparrow 0.35 0.14 0.57 0.15 0.25 0.12 0.05 0.03 0.22 0.1 0.37 0.14 0.45 0.11 0.35 0.14
Diversity 0.35 0.09 0.42 0.11 0.35 0.06 0.03 0.02 0.48 0.06 0.47 0.07 0.52 0.06 0.22 0.06
Richness 0.72 0.12 0.98 0.23 1.02 0.3 0.23 0.1 1.09 0.14 1.11 0.19 1.32 0.16 0.54 0.09
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August data showed a lower mean richness than May, June, and July (F3, 32 = 4.38,
P = 0.011; Fig. 5).
Discussion
Overall, the diversity, richness, and often the abundance of the Valley’s grassland
bird species were low compared to other states such as Iowa (Bryan and
Best 1991); Kansas (Klute et al. 1997); and the Dakotas, Minnesota, and Montana
(Johnson and Schwartz 1993). Avian diversity and richness were also low
compared to other habitat types in West Virginia (Duguay et al. 2001, Forcey and
Anderson 2002, Sacilotto 2002). However, abundance estimates were similar to
those for grassland birds in Illinois (Kershner and Bollinger 1996). Our study was
conducted in interior grassland habitats, which precluded some edge-dependent
species from our surveys. Therefore, the Refuge grasslands’ low diversity is
probably due to the state’s few grassland species rather than a problem with the
Refuge’s habitat quality.
The Valley’s grasslands provide important habitat for grassland birds in West
Virginia (Warren and Anderson 2005). The three most abundant species—Savannah
Sparrow, Bobolink, and Eastern Meadowlark—are primarily ground nesters
that have limited numbers and distributions in the state due to a paucity of habitat.
Indeed, the Bobolink is listed as a state rare species (WVDNR 2000), and the
Eastern Meadowlark and Savannah Sparrow are declining at all scales (Sauer et
al. 2012). Grassland habitat is relatively scarce in the state, and only hayfields
and pastures, such as those evaluated in this study, or abandoned mine lands,
provide suitable habitat for many grassland bird species (Forcey and Anderson
2002, Whitmore 1980, Whitmore and Hall 1978, Wray et al. 1982). Our study
fields also provided edge and early-successional habitats, the latter a declining
habitat type in the eastern US (Hunter et al. 2001).
Figure 5. Breeding
grassland bird
richness by month
on the Canaan
Valley National
Wildlife Refuge,
Tucker County,
WV, May–August
in 1999–2000.
The same letter
above bars indicates
no difference
(P > 0.05) among
months.
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2015 Vol. 14, Special Issue 7
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The overall density and richness of birds living in the Refuge’s grasslands
varied between the two years. Birds occurred at a greater mean density in 1999
mainly due to the many Bobolink flocks (>100 birds) and abundant Eastern
Meadowlarks and Red-winged Blackbirds present. These differences may be
partly attributed to inter-annual differences in weather. West Virginia experienced
a drought in 1999 and normal precipitation in 2000 (NOAA 1999, 2000).
In the Valley, the May–August total precipitation increased 8.6 inches (21.9 cm)
between 1999 and 2000 (Fig. 1). Precipitation may have indirectly influenced
grassland bird distributions and patterns in the Valley and surrounding areas during
both years.
Variations in grassland bird densities have been linked with drought even
though this effect seems to be short-lived; most species recover within one year
(Cody 1985, Dale 1984, George et al. 1992, O’Connor et al. 1999). Although
drought may affect seasonal variations in grassland bird densities, it may ultimately
be farmers’ responses to weather conditions that affect variations in the Valley’s
bird densities. Most farmlands were mowed early in 1999 (June) because dry
weather allowed farmers to access their fields. The mowing on non-Refuge land
reduced available grassland habitat outside of the Refuge, causing birds to concentrate
on the Refuge’s unmowed grasslands. In 2000, most farmers delayed mowing
until mid- to late July because of abundant rain and wet fields, providing additional
off-Refuge habitat for birds in 2000. While differences in average rainfall may have
influenced the farmers’ responses, we suggest that it is unlikely that inter-annual
weather differences directly altered the Refuge’s grassland bird populations.
Differences between the Valley’s public and private lands may have also contributed
to the variation in bird populations. Although increases in rainfall may
not alter grassland bird density, diversity, or species richness directly, increased
rainfall may affect nesting success and land management techniques. Flooding
during prolonged periods of above-average rainfall can lead to nest loss of some
grassland bird species (Wray et al. 1982), just as the lack of rain in 1999 probably
reduced the number of species present on the fields. Thus, the results from 2000
probably indicate normal conditions.
The diversity, richness, and density of grassland birds on The Refuge were
lower in August than in other months because by then most of them had formed
flocks or abandoned their territories. By August, the birds were using alternate
sites that provided optimal foraging conditions for fledglings and for fueling their
upcoming fall migration. Most species aggregated in different parts of the Valley
or adjacent to transects. Bobolinks, among the first to migrate, began flocking in
mid- to late July, prior to molting. Savannah Sparrows, which tend nests through
August, were among the last species to leave.
Management Implications
The habitat provided in the Refuge for grassland birds in West Virginia is important
because outside the Refuge this type of environment is limited to farmlands
and reclaimed strip mines (Whitmore and Hall 1978). As small-scale farming
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2015 Vol. 14, Special Issue 7
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became less feasible, fields were abandoned and succession changed the vegetation’s
structural characteristics, greatly reducing grassland acreage (Whitmore and
Hall 1978). Moreover, prior to widespread strip-mine reclamation, the Grasshopper
and Vesper Sparrows were declining in their ranges (Whitmore and Hall 1978).
Idle and active agricultural fields and reclaimed mine sites, if managed correctly,
may provide essential habitats for these and other declining grassland species.
With active management, the Refuge’s grasslands will provide crucial habitat for
West Virginia’s grassland birds. Refuge staff must decide if they want to actively
manage these fields as grassland habitats for state-listed rare species or allow them
to revert to scrub-shrub and eventually to forest.
Acknowledgments
We thank the US Fish and Wildlife Service (Canaan Valley National Wildlife Refuge);
West Virginia Division of Natural Resources (Wildlife Diversity Program); West Virginia
University Division of Forestry and Natural Resources; the Davis College of Agriculture,
Natural Resources, and Design at West Virginia University (Brown Faculty Development
Fund); and the West Virginia Agricultural and Forestry Experiment Station (McIntire-
Stennis) for providing funds. We also thank J.D. Osbourne and two anonymous referees
for reviewing this manuscript. C.A. Rhoads and S.K. Reilly assisted with data collection.
R.C. Whitmore and L. Butler reviewed an earlier version of this manuscript. This
is manuscript number 3196 of the West Virginia University Agricultural and Forestry
Experiment Station.
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