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Models for Guiding Management of Prairie Bird Habitat in Northwestern North Dakota

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  • Eagle Environmental

Abstract and Figures

With grassland bird populations in the Great Plains exhibiting steep declines, grassland managers require information on bird habitat needs to optimally manage lands dedicated to wildlife. During 1993–1994, we measured bird occurrence and corresponding vegetation attributes on mixed-grass prairie in northwestern North Dakota. Three hundred and ten point-count locations over a wide range of successional stages were sampled. Ten grassland passerine species occurred commonly (i.e., at >10% of point count locations), including two species endemic to the northern Great Plains [Baird's sparrow (Ammodramus bairdii) and Sprague's pipit (Anthus spragueii)], and several species of management concern [bobolink (Dolichonyx oryzivorus), grasshopper sparrow (Ammodramus savannarum), clay-colored sparrow (Spizella pallida)]. Some species were ubiquitous and had generalized habitat associations [e.g., savannah sparrow (Passerculus sandwichensis)]. Others exhibited more finely tuned, closely overlapping use of relatively short, sparse to moderately dense, grass- and forb-dominated habitat. We used logistic regression models to predict bird species' occurrence based on nine vegetation variables. Previously undefined limits of vegetation height and density were identified for Baird's sparrow and Sprague's pipit, and of shrub cover for Baird's sparrow. Our findings underscore the need for a mosaic of successional types to maximize diversity of prairie bird species. Managers may reduce confusion created by generic treatment prescriptions for grasslands by focusing on absolute rather than relative measures of vegetation, and by integrating standard data from multiple bird habitat studies across regions.
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Am. Midl. Nat. 144:377–392
Models for Guiding Management of Prairie Bird Habitat in
Northwestern North Dakota
ELIZABETH M. MADDEN
1
Biology Department, Montana State University, Bozeman 59717
ROBERT K. MURPHY
U.S. Fish and Wildlife Service, Des Lacs National Wildlife Refuge Complex, Kenmare, North Dakota 58746
ANDREW J. HANSEN
Biology Department, Montana State University, Bozeman 59717
LEIGH MURRAY
University Statistics Center, New Mexico State University, Las Cruces 88003
A
BSTRACT
.—With grassland bird populations in the Great Plains exhibiting steep declines,
grassland managers require information on bird habitat needs to optimally manage lands
dedicated to wildlife. During 1993–1994, we measured bird occurrence and corresponding
vegetation attributes on mixed-grass prairie in northwestern North Dakota. Three hundred
and ten point-count locations over a wide range of successional stages were sampled. Ten
grassland passerine species occurred commonly (
i.e.,
at .10% of point count locations),
including two species endemic to the northern Great Plains [Baird’s sparrow (
Ammodramus
bairdii
) and Sprague’s pipit (
Anthus spragueii
)], and several species of management concern
[bobolink (
Dolichonyx oryzivorus
), grasshopper sparrow (
Ammodramus savannarum
), clay-
colored sparrow (
Spizella pallida
)]. Some species were ubiquitous and had generalized hab-
itat associations [
e.g.,
savannah sparrow (
Passerculus sandwichensis
)]. Others exhibited more
finely tuned, closely overlapping use of relatively short, sparse to moderately dense, grass-
and forb-dominated habitat. We used logistic regression models to predict bird species’ oc-
currence based on nine vegetation variables. Previously undefined limits of vegetation height
and density were identified for Baird’s sparrow and Sprague’s pipit, and of shrub cover for
Baird’s sparrow. Our findings underscore the need for a mosaic of successional types to
maximize diversity of prairie bird species. Managers may reduce confusion created by generic
treatment prescriptions for grasslands by focusing on absolute rather than relative measures
of vegetation, and by integrating standard data from multiple bird habitat studies across
regions.
I
NTRODUCTION
With grassland bird populations in the Great Plains exhibiting steep declines (Droege
and Sauer, 1994; Knopf, 1994, 1996) grassland managers require information on bird hab-
itat needs to optimally manage lands dedicated to wildlife. Responses to activities such as
cropping, haying, grazing and prescribed burning have been documented for many grass-
land bird species including passerines (Owens and Myres, 1973; Kantrud, 1981; Pylypec,
1991; Johnson, 1996; Dale
et al.,
1997, Madden
et al.,
1999), but these responses can vary
greatly both spatially and temporally. Studies of effects of management activities on Great
Plains avifauna often cannot be extrapolated across the region due to differences in envi-
ronmental conditions (
e.g.
, moisture, soil types, and plant species composition). Thus, it
remains difficult to summarize effects of management on individual bird species. For ex-
1
Corresponding author: Medicine Lake National Wildlife Refuge, Medicine Lake, Montana 59247.
Telephone (406)789-2305; e-mail: elizabethpmadden@fws.gov
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ample, in North Dakota, Sprague’s pipit was most abundant in moderately- and heavily-
grazed areas (Kantrud, 1981), but in Alberta and Saskatchewan it was most abundant in
ungrazed or lightly-grazed prairie (Owens and Myres, 1973; Dale, 1983). Study results that
appear conflicting may not be when vegetation type and structure are considered.
An alternative to summaries of management impacts to birds is the use of habitat models
based on vegetation attributes associated with the species of interest (
see
Verner
et al.,
1986).
Vegetation attributes are important determinants of grassland bird abundance and distri-
bution (Cody, 1968; Wiens, 1969; Rotenberry and Wiens, 1980; Whitmore, 1981; Zimmer-
man, 1988). Vegetation structure and, to a lesser extent, composition are readily manipu-
lated by land managers. Presented with a profile of the vegetation used by a particular bird
species or group of species, a manager can use a variety of defoliation or other management
tools to obtain it, either through the incorporation of natural disturbance regimes, or
through activities that may mimic them (
e.g.
, mowing).
Despite the fact that studies of bird habitat associations based solely on occurrence or
abundance data ignore important facets of population ecology (
e.g.,
source/sink dynamics)
and therefore may not reveal highest quality habitat for a species (Van Horne, 1983), they
often are the only cost-effective option, especially when developing models for managing
multiple species (Hansen
et al.,
1993). We used such a habitat-based approach for grassland
passerines in northern, mixed-grass prairie in the Missouri Coteau physiographic subregion
of North Dakota. Our objective was to quantify relationships between passerine birds and
vegetation structure and composition. Our ultimate goal was to provide managers of north-
ern, mixed-grass prairie with models that predict occurrence of bird species based on hab-
itat conditions.
S
TUDY
A
REA
Lostwood National Wildlife Refuge (NWR) covers 109 km
2
of rolling to hilly, mixed-grass
prairie in Mountrail and Burke counties, northwestern North Dakota (488379N; 1028279W).
It lies within the Missouri Coteau, a dead-ice moraine characterized by knob-and-kettle
topography (685–747 m elevation). The large tract of grassland is interspersed with more
than 4100 wetland basins and 500 clumps of quaking aspen (
Populus tremuloides
). Major
vegetation is a needlegrass-wheatgrass (
Stipa
spp.-
Agropyron
spp.) association, and habitat
composition is 55% native prairie, 21% previously-cropped grassland revegetated with tame
and native prairie plants, 20% wetland, 2% trees, and 2% tall shrubs (Murphy, 1993). Cli-
mate is semiarid and mean annual precipitation (1936–1989) is 42 cm, with most (.75%)
falling as rain between April and September.
Before European settlement in the early 1900s, the landscape of Lostwood NWR was
treeless mixed-grass prairie, maintained in a shorter grass, or even barren state by frequent
fire and bison impacts (Murphy, 1993). The region likely supported a 5- to 10-y fire-return
interval (Wright and Bailey, 1982:81; Murphy, 1993; Bragg, 1995). Although some of pre-
sent-day Lostwood NWR was tilled and farmed during the early 1900s, most (70%) upland
areas remained unplowed and were either left idle or lightly grazed season-long by livestock
during the 1930s–1970s. With settlement came suppression of wildfires and a concomitant
loss of early successional, herbaceous vegetation. Western snowberry (
Symphoricarpos occi-
dentalis
), quaking aspen and exotic grasses [Kentucky bluegrass (
Poa pratensis
), smooth
brome (
Bromus inermis
), and quack grass (
Agropyron repens
)] proliferated and now domi-
nate much of the mixed-grass community of Lostwood NWR.
Since the late 1970s, the U.S. Fish and Wildlife Service (USFWS) has mostly used pre-
scribed fire to reduce woody vegetation, control exotic plants, enhance vigor of native plant
species, and restore a more natural diversity of successional stages to the landscape. Avail-
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ability of this wide range of vegetation successional stages made Lostwood NWR an appro-
priate place to study relationships between prairie birds and vegetation in various stages of
succession.
M
ETHODS
We measured bird use and vegetation characteristics on 160 (1993) and 150 (1994) sam-
ple points distributed over nine prescribed burn units that encompassed a wide range of
postfire successional stages, ranging from ,1yto.80 y. This provided a broad gradient
of vegetation, from short and sparse to tall and dense. We selected sampling points from a
grid of potential points that encompassed the study area. The sampling scheme was system-
atic in 1993 (i.e., each square mile of the study area was gridded into 268-m 3268-m blocks
and each grid intersection was a potential sample point), but was changed to randomized
systematic in 1994 to better meet sampling assumptions for statistical testing. We generated
the random, systematic grid for each square mile section by randomly picking 2 numbers
between 1–250 m to serve as the X and Y coordinates of a point in the southwest corner
of the section. From this random start point, we gridded points every 250 m north and
south across each section. Points were $250 m apart to provide statistical independence in
terms of birds and vegetation (Ralph
et al.,
1993), and were randomly selected from the
grid of potential points that met the following criteria: (1) located in ‘‘upland prairie’’ as
delineated by the National Wetland Inventory map of cover types of Lostwood NWR, (2)
$200 m from any aspen clump, (3) $50 m from roads or firebreaks and (4) currently
ungrazed by livestock. We selected 160 sample points in 1993, and a new set of 150 points
in 1994. In 1994, we reduced the buffer from aspen to 100 m based on 1993 sampling
observations. We also added a 50-m buffer to seasonally-flooded wetland zones because of
high water levels.
We estimated occurrence of individual bird species using 50-m radius (1993) and 75-m
radius (1994) point count surveys on fixed-radius plots (Hutto
et al.,
1986). [Based on an
analysis of bird species detectability (Rotella
et al.,
1999), the radius was increased to 75-m
in 1994 to increase the number of birds sampled per point]. The observer stood at a survey
point center for 10 min and recorded all birds seen or heard. We conducted counts from
0.5 h before sunrise until about 0900 h CDT, during the breeding season from late May
until early July, whenever weather conditions did not impede detection of birds (
i.e.
,no
rain, fog or wind .15 km/h). Each point was surveyed three times. Observers and the
order in which points were visited were rotated to minimize sampling bias.
Vegetation structure and general composition were measured at each bird sur vey point
during late June through early August in 1993 and late June through late July in 1994. Ten
subsample points were located at 5-m inter vals along each of two transects within each fixed-
radius bird plot, for a total of twenty subsamples. Both transects were positioned on the
same random compass bearing (
i.e.
, parallel), each a different random distance (between
5–30 m) from plot center.
At each subsample point, we estimated visual obstruction (dm) at a height of 1 m and a
distance of 4 m (Robel
et al.,
1970). Litter depth was measured directly (cm) by lowering
a 6-mm diameter rod vertically into the litter layer. Dead vegetation from previous years
that was standing but no longer vertical was considered litter where it formed a mat-like
layer, roughly continuous to the ground. Using the same rod, we counted the total number
of ‘‘hits’’ or contacts of vegetation on the rod in each dm height interval (Wiens, 1969).
Each hit was recorded as either live (current year’s growth) or dead (previous years’
growth). Total number of hits represented vertical density. We also calculated the percent-
age of total hits represented by live vegetation. Percentage areal cover of shrubs, forbs and
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grasses was visually estimated at each subsample (Daubenmire, 1959) within a 1-m diameter
circular quadrat. Finally, each circular quadrat was assigned to one of nine general plant
groups of management interest: native grass, Kentucky bluegrass, native grass/Kentucky
bluegrass, broad-leaved exotic grass (
i.e.,
smooth brome or quack grass), shrub (primarily
western snowberry), shrub/broad-leaved exotic grass, shrub/Kentucky bluegrass, shrub/
native grass and wetland.
For each vegetation structure variable, we calculated the mean and coefficient of variation
(CV) for the 20 subsamples near each point. CV indicates horizontal ‘‘patchiness’’ or het-
erogeneity of a particular vegetation attribute (Roth, 1976). Plant groups were expressed
as the mean frequency of a given group in the 20 subsamples. In all, 14 vegetation structure
variables (
i.e.,
mean and CV of: litter depth, visual obstruction, % live vegetation, vertical
vegetation density, shrub cover, grass cover and forb cover) and nine plant groups were
considered. Our collection of bird occurrence and vegetation data overlapped only slightly
in time (mostly June, vs. mostly July). We acknowledge this temporal disparity may have
slightly affected our assessment of visual obstruction and vertical density influences, but not
other vegetation structure and composition variables.
Because each point count location was independent in terms of birds and vegetation, we
used point-level analyses with a sample size of n 5160 and n 5150, in 1993 and 1994,
respectively, to explore relationships between bird species and vegetation characteristics.
Bird species’ occurrence at a point was defined as the presence of at least one singing
male of a given species during at least one of three survey visits. (Singing male observations
were used for all species except cowbirds, because female grassland birds are secretive and
are not reliably detected with these methods.) Only bird species detected at .10% of points
were used in statistical analyses. These included the same nine species each year, and a
tenth only in 1994.
Because many variables of vegetation structure were correlated, we used principal com-
ponents analysis (SAS Institute, 1989) of the 14 variables to identify major gradients in
vegetation structure. Using the resulting gradients, we plotted the subset of points at which
individual bird species were present on a plot of Principal Component 1 (PC 1) versus
Principal Component 2 (PC 2) to describe each species’ relative location on the vegetation
gradients in each year. We then plotted a 95% confidence ellipse based on the mean for
each species, and superimposed the ten species onto one graph for purposes of comparison.
Logistic regression (SAS Institute, 1989) was used to develop predictive models of species’
occurrence based on vegetation characteristics. We chose logistic regression over linear
regression because bird abundance data were heavily weighted with zeros and violated as-
sumptions of linear regression (
i.e.,
normality and constant variance). Densities of birds in
mixed-grass prairie are typically low such that point count abundances are essentially re-
cords of species’ occurrence.
The logistic model was:
P
(presence) 51/(1 1exp{ 2[
b
o
1
b
1
(
x
)]})
where
P
(presence) was the probability that a bird species was present,
b
o
and
b
1
were in-
tercept and slope coefficients and
x
was the predictor variable (
i.e.
, vegetation variable).
Two-thirds of the 1994 data (n 599) was used to generate the models, and the remaining
one-third (n 551) was used as a ‘‘hold-out’’ data set to retest the model’s predictive power
for correct classification. Only 1994 data were used here because the smaller (50-m) radius
point counts used to sample birds in 1993 yielded few observations of birds at sample points
(
i.e.,
birds were often using an area, but the small size of the plot precluded them from
being recorded).
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1.—Eigenvector loadings of principal components (PC 1 and PC 2) of vegetation structure
variables in 1993 and 1994
Variable
1993
PC 1 PC 2
1994
PC 1 PC 2
Shrub cover
Forb cover
Grass cover
CV shrub cover
CV forb cover
CV grass cover
Visual obstruction
CV visual obstruction
Litter depth
CV litter depth
Vertical density (total # hits)
CV vertical density
% live vegetation
CV % live vegetation
0.33
20.17
20.27
20.20
0.20
0.29
0.33
0.08
0.33
20.32
0.34
20.08
20.33
0.24
0.29
20.39
0.27
20.32
0.42
0.12
0.24
20.12
20.23
0.28
20.02
20.11
0.33
20.29
0.33
20.15
20.28
20.19
0.22
0.29
0.31
0.02
0.32
20.30
0.37
20.24
20.31
0.20
0.34
20.28
20.20
20.30
0.24
0.26
0.23
0.16
20.29
0.34
20.16
0.25
0.36
20.22
Eigenvalue
% variance explained
Total variance explained
5.51
39.32
39.32
2.46
17.6
56.93
5.33
38.10
38.10
2.82
20.11
58.22
To choose the best multivariable model for each bird species, a backward-elimination
routine was used on a set of nine variables (grass cover, forb cover, shrub cover, shrub
frequency, native grass frequency, exotic grass frequency, litter depth, visual obstruction and
vertical vegetation density). These nine variables were chosen based on their perceived
importance to birds, their relevance to managers and to minimize collinearity (r
s
,0.85)
within the set of predictor variables. A variable was eliminated from the model if its observed
significance level for the regression coefficient (based on the Wald chi-square) was P .
0.05. In one case (bobolink), a 4-variable model was selected by the backward-elimination
method, but scrutiny of possible 3-variable models indicated a more parsimonious fit, and
a 3-variable model was ultimately chosen.
Although multivariate models can provide excellent predictive power, their individual
regression coefficients can be difficult to understand and interpret, especially in 3- and 4-
variable models where interactions and collinearity cloud relationships among variables. To
facilitate interpretation of relationships between bird occurrence and individual vegetation
variables, we additionally scrutinized and graphed single-variable models (of management
interest). These models were then used to generate incidence functions that predict the
probability of a bird species’ occurrence given a certain value of a selected vegetation var-
iable. We limited incidence function graphs to the range of vegetation values observed in
this study, to ensure that interpretation would not extend beyond the observed range of
data.
R
ESULTS
Results of principal components analyses for both years of vegetation data were remark-
ably similar in terms of eigenvectors and gradients (Table 1). The first axis (PC1) accounted
for 39% and 38% of the variation in 1993 and 1994, respectively, and represented a gradient
from short, sparse, grass-dominated vegetation to tall, dense, shrub-dominated vegetation.
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. 1.—Composite 95% confidence ellipses based on the mean for plots at which individual bird
species occurred in 1993 and 1994, with reference to Principal Component 1 and 2. Number of plots
on which the species occurred indicated in parentheses
The second axis (PC2) accounted for 18% and 20% of the variation in 1993 and 1994,
respectively, and represented a gradient from deep litter and forb-dominated vegetation to
mostly live vegetation with few forbs and low litter.
On composite graphs for bird species’ occurrence across the vegetation gradients (Fig.
1), we interpreted nonoverlapping confidence ellipses to represent use of different vege-
tation by individual species, and ellipses that overlapped the plot origin (0,0) to indicate
use of average vegetation conditions available on the study area. For example, savannah
sparrow and clay-colored sparrow overlapped each other almost completely in 1994, and
partially in 1993, and both were located near or over the origin, indicating similar use of
vegetation near the average available. Confidence ellipses of brown-headed cowbirds (
Mol-
othrus ater
) were among the largest, and overlapped with all species except common yel-
lowthroat (
Geothlypis trichas
) [and western meadowlark (
Sturnella neglecta
) in 1994]. Con-
fidence ellipses for common yellowthroat were distinctly separate from those of other spe-
cies. This species alone was located wholly in the quadrant with high shrub cover.
The remaining six grassland species were clustered together in the short, sparse to mod-
erate, grass- and forb-dominated quadrant of vegetation space (Fig. 1). Baird’s sparrow,
grasshopper sparrow, and bobolink showed greatest overlap. Sprague’s pipit, western mead-
owlark and Le Conte’s sparrow (
Ammodramus leconteii
) overlapped these species, but had
larger confidence ellipses. Of this group, Sprague’s pipit used shortest and sparsest vege-
tation, and Le Conte’s sparrow (observed in 1994 only) used the tallest and densest. On
PC 2 most of these six species were near or below zero, indicating use of areas with moderate
amounts of litter and forbs.
Predictive models for 8 species included 1 to 4 significant predictor variables (Table 2).
A 4-variable model best classified Baird’s sparrow occurrence (Table 2). Grass cover was the
best single-predictor model for Baird’s sparrow (Wald chi-square significance ,0.001),
classifying presence/absence correctly 68% and 75% of the time in the original and hold-
out data sets, respectively. Incidence (
i.e.,
probability of occurrence) of Baird’s sparrow
increased with grass cover, and reached 50% at about 42% grass cover (Fig. 2). It also
increased with forb cover (Wald chi-square significance 50.040), reaching 50% incidence
at 35% forb cover (Fig. 2). Shrub cover and Robel visual obstruction were also good single
predictors (Wald chi-square significance ,0.001 for each). Incidence of Baird’s sparrow
decreased as these increased, and dropped below 50% at about 18% shrub cover and 1.5-
dm visual obstruction (Figs. 2, 3). Frequency of native grasses was also a significant single
predictor (Wald chi-square 50.014; 66% and 67% correct classification), predicting 50%
Baird’s sparrow occurrence at native grass frequency of 0.42 (Fig. 2).
Sprague’s pipit occurrence was best predicted by visual obstruction (Table 2). Incidence
of pipits declined quickly as visual obstruction increased, even more rapidly than Baird’s
sparrow and grasshopper sparrow incidence for the same variable (Fig. 3). Sprague’s pipit
incidence decreased to 50% at 0.8 dm and to less than 5% at 1.9 dm.
Bobolink occurrence was best predicted by increasing forb cover and grass cover, and
decreasing frequency of native grasses (Table 2). Frequency of broad-leaved, exotic grass
was also a good single predictor of bobolink occurrence (Wald chi-square significance 5
0.021) (Fig. 4).
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2.—Logistic regression models that best predicted occurrence of grassland birds in 1994. Variables were selected from a set of nine vegetation
variables using a backward-elimination routine
Species Fitted model for logit response
a
o.s.l.–
model
b
% correct
classification
Data
to fit
c
n599
Hold-out
data
d
n551
Baird’s sparrow 28.17 21.31(visual obstruction)
e
10.12(forb cover) 10.14
(grass cover) 10.09(shrub cover)
0.000 75.8 68.6
Brown-headed cowbird 0.04 10.06(forb) 10.04(grass) 20.15(vertical density) 0.000 71.7 68.6
Bobolink 24.03 10.05(forb) 10.06(grass) 20.22(native grass) 0.003 77.8 72.5
Clay-colored sparrow 0.42 10.13(shrub) 0.002 93.9 88.2
Common yellowthroat 2.89 20.08(forb) 20.07(grass) 0.000 75.8 92.2
Grasshopper sparrow 2.37 21.55(visual obstruction) 10.35(exotic grass) 0.000 68.7 62.7
Le Conte’s sparrow No significant model found
Savannah sparrow No significant model found
Sprague’s pipit 2.24 22.77(visual obstruction) 0.000 87.9 82.4
Western meadowlark 26.90 10.11(forb) 10.07(grass) 0.000 80.8 82.4
a
Logit 5ln[P(Presence)/P(Absence)] 5
b
o
1
b
1
(
x
1
)... 1
b
p
(
x
p
), P(Presence) 5l/[l1exp{2(
b
o
1
b
1
x
1
...1
b
p
x
p
)}] and P(Absence) 5l-P(Presence)
b
observed significance level (o.s.l.) for the overall model
c
correct classification rates for data used to fit the model
d
correct classification rates for ‘‘hold-out’’ data set used to re-test classification
e
Visual obstruction 5Robel visual obstruction, forb 5forb cover, grass 5grass cover, shrub 5shrub cover, vertical density 5total number of vegetation
contacts, native grass 5frequency of native grasses, exotic grass 5frequency of broad-leaved, exotic grasses
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. 2.—Incidence of Baird’s sparrows as predicted by logistic regression models for four vegetation
variables. Dashed lines indicate 95% confidence intervals for the predicted probabilities. Incidence
lines do not extend beyond observed ranges of data
Visual obstruction combined with frequency of broad-leaved, exotic grasses best predicted
grasshopper sparrow occurrence (Table 2). Grasshopper sparrow incidence increased with
decreasing visual obstruction (Fig. 3), and with increasing frequency of broad-leaved, exotic
grasses.
Clay-colored sparrow occurrence was best predicted by shrub cover (Table 2). However,
little shrub cover was necessary for high probability of clay-colored sparrow occurrence.
Probability of occurrence was 69% at only 3% shrub cover, and reached 95% at 20% shrub
cover (Fig. 5).
Forb cover and grass cover were best predictors of common yellowthroat and western
meadowlark occurrence, although relationships in the models were opposite. Yellowthroat
incidence increased with decreasing forb and grass cover (Table 2). Meadowlark incidence
increased with increasing forb cover and grass cover, and did not reach 50% probability
until 44% forb cover (Fig. 5).
Brown-headed cowbird incidence increased with forb cover and grass cover, and de-
creased with vertical vegetation density (Table 2). No significant predictive models were
found for savannah or Le Conte’s sparrows.
D
ISCUSSION
Bird species examined were well distributed over gradients of vegetation structure and
composition. Savannah sparrow and clay-colored sparrow, the most widespread and abun-
dant passerines at Lostwood NWR (Madden
et al.
, 1999), used mixed-grass prairie vegetation
near the average available. Clay-colored sparrow was distinctly associated with shrubs. Baird’s
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. 3.—Incidence of three grassland bird species as predicted by logistic regression models for Robel
visual obstruction (
i.e.,
height and density) of vegetation. Dashed lines indicate 95% confidence inter-
vals for the predicted probabilities. Incidence lines do not extend beyond observed ranges of data
F
IG
. 4.—Incidence of bobolinks as predicted by logistic regression models for frequency of native
and exotic grasses. Dashed lines indicate 95% confidence intervals for the predicted probabilities.
Incidence lines do not extend beyond observed ranges of data
sparrow, grasshopper sparrow, Sprague’s pipit, and western meadowlark used moderate
amounts of chiefly grass and forb cover, with Sprague’s pipit using the shortest cover. Bob-
olink and Le Conte’s sparrow used taller, denser, grassy vegetation (mostly represented at
Lostwood NWR by exotic grasses, such as smooth brome and quack grass). Common yel-
lowthroat used the densest, shrubbiest habitats. Of species examined here, it is the only
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. 5.—Incidence of clay-colored sparrows and western meadowlarks as predicted by logistic regres-
sion model for shrub cover and forb cover, respectively. Dashed lines indicate 95% confidence intervals.
Incidence lines do not extend beyond observed ranges of data
one not considered a grassland bird (Mengel, 1970; Johnsgard, 1978), but instead is a shrub-
or wetland-associated species. Vegetation use by brown-headed cowbirds overlapped with
nearly all other bird species, a good characteristic for a brood parasite.
The vegetation gradients described in our study (principal components analyses) resem-
ble Ryan’s (1990) prairie continuum model, which portrayed grassland habitats ranging
from short/sparse, grass- and forb-dominated to tall/dense, shrub- and tree-dominated veg-
etation over gradients of soil moisture and fire and grazing frequency and intensity. Ryan
classified general (qualitative) habitat affinities of Sprague’s pipit as short-sparse grass;
Baird’s sparrow, grasshopper sparrow, savannah sparrow, and western meadowlark as mid-
grass; clay-colored sparrow as mid-grass/shrub; Le Conte’s sparrow and bobolink as tall-
dense grass and common yellowthroat as tall-dense grass/shrub. Our results agree with these
descriptions.
General habitat affinities are well known for grassland birds, but few published studies
provide quantitative summaries of species’ preferences for vegetation structure. Especially
lacking are data for endemic grassland species (Mengel, 1970; Johnsgard, 1978) such as
Baird’s sparrows and Sprague’s pipits, which are of increasing concern due to recent steep
population declines, habitat loss and a public policy shift toward ecosystem and native spe-
cies management (Knopf, 1994, 1996; USFWS, 1996). Most public grasslands devoted to
wildlife traditionally have been managed to promote tall, dense nesting cover for game
birds, especially waterfowl. Our results reveal that Baird’s sparrows and Sprague’s pipits use
relatively short, sparse grass structure, and are most associated with native bunch grasses,
rather than with the broad-leaved, exotic grasses often planted for nesting cover. To maxi-
mize native species diversity, managers charged with maintaining grassland communities
should consider such needs.
At Lostwood NWR, presence of Baird’s sparrow was best predicted by relatively high
percentages of grass cover and forb cover, low shrub cover and low visual obstruction. Areas
occupied by Baird’s sparrow in other studies (Dale, 1983; Renken, 1983; Winter, 1994) had
fairly similar values for these attributes. Occupied areas in central North Dakota had a mean
visual obstruction of 1.3 dm and litter depth of 3.6 cm (Renken, 1983), similar to values of
1.6 dm and 3.7 cm at Lostwood NWR (Madden, 1996). Winter (1994) also studied Baird’s
sparrow at Lostwood NWR, and similarly summarized preferred habitat as having 0.3–3 cm
litter depth, visual obstruction of about 1.2 dm, and 1.5–3.5% shrub cover. Dale’s (1983)
values for south-central Saskatchewan were not as directly comparable, but height and den-
sity of vegetation and presence of some forbs or shrubs were important, and litter depths
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of occupied areas were about 0.8–1.2 cm during breeding season. Mean shrub cover of
areas occupied by Baird’s sparrow ranged from less than 1% (Renken, 1983) to 3–8% (Dale,
1983) to 20% in this study (Madden, 1996).
Our logistic regression models for visual obstruction and shrub cover define previously
unidentified upper limits of suitability for Baird’s sparrow. A Habitat Suitability Index (HSI)
based on limited data suggested 1.5–4 dm visual obstruction as optimum for Baird’s sparrow
(Sousa and McDonal, 1983). Lacking evidence that visual obstruction over 4 dm might be
detrimental for Baird’s sparrows, the model assumed no upper limit. Our model captured
distinct limitations in Baird’s sparrow occurrence at higher visual obstruction levels. Prob-
ability of Baird’s sparrow occurrence was highest at about 1 dm, fell below 25% between
2.0–2.5 dm, and below 1% at about 4.5 dm of visual obstruction. Baird’s sparrows were
completely absent from areas with greater visual obstruction (5–7 dm) (Madden, 1996).
Lower limits of visual obstruction cannot be assessed from our model because the lowest
mean readings on sample plots were 0.7 dm. Lower thresholds have been identified else-
where from studies in drier, grazed areas where heavy grazing reduced vegetation to un-
suitable height and density levels for Baird’s sparrow (Owens and Myre, 1973; Dale, 1983).
The HSI also suggested that Baird’s sparrow is inhibited by over 25% shrub cover, but lacked
supportive data. Probability of occurrence of Baird’s sparrow in our models dropped below
50% at about 18% shrub cover and below 10% at 54% shrub cover. Although sparse shrub
cover appears acceptable or even preferred by Baird’s sparrow (
see
Dale, 1983), the species
has been notably absent on idle (
i.e.,
no fire or grazing) prairie invaded by shrubs (Arnold
and Higgins, 1986; Renken, 1983; Madden
et al.
, 1999).
Occurrence of Sprague’s pipit at Lostwood NWR was best predicted by low visual obstruc-
tion by vegetation. In addition, this species was consistently associated with native grasses
and low amounts of shrub cover (Madden, 1996). In Saskatchewan, sites used by Sprague’s
pipits also had lower shrub cover, but had higher vegetation density than unused sites (Dale,
1983). Absolute values of vegetation densities used by Sprague’s pipit in the two studies,
however, were similar (
i.e.,
vegetation that is ‘‘less dense’’ at Lostwood NWR is ‘‘more
dense’’ in Saskatchewan). Throughout their range, Sprague’s pipits are less abundant (or
absent) in areas of introduced grasses compared with native prairie (Kantrud, 1981; Wilson
and Belcher, 1989; Johnson and Schwartz, 1993; Dale
et al.,
1997).
Vegetation attributes preferred by these endemic species are characteristic of grasslands
receiving periodic defoliations such as those produced by fire or grazing. Prescribed fire,
for example, promotes prairie dominated by herbaceous cover (Wright and Bailey, 1982).
Both Baird’s sparrow and Sprague’s pipit were most abundant at Lostwood NWR on prairie
burned 2–4 times in the previous 15 y (Madden
et al.
, 1999). Depending on location within
the species’ ranges, higher or lower intensities of habitat disturbance will achieve the req-
uisite vegetation structure. For example, periodic fire or moderate grazing provide pre-
ferred Baird’s sparrow and pipit habitat in North Dakota (Kantrud, 1981; Renken, 1983;
Messmer, 1990; Madden
et al.
, 1999), whereas long-idle or lightly-grazed native prairie sup-
ports preferred vegetation structure in drier portions of their ranges (Dale, 1983; Wershler
et al.,
1991; Sutter
et al.,
1995).
Focus on absolute, rather than relative, measures of vegetation may reduce conflicts as-
sociated with attempts to standardize habitat treatment prescriptions on the Great Plains,
as well as problems inherent in defining, for example, ‘‘heavy’’ or ‘‘light’’ grazing. Integra-
tion of many small-scale, bird habitat studies into broader guidelines incorporating the full
range of grassland habitats would likely resolve apparent contradictions in bird-habitat as-
sociations, but lack of standardized methodology makes this challenging.
Several other bird species examined here are of management interest. The clay-colored
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sparrow, which breeds mainly in the northern Great Plains, is considered an endemic grass-
land species by some ( Johnsgard, 1978), and its population is declining (Knopf, 1996; Sauer
et al.,
1997). The species’ well-documented association with shrubby prairie (Owens and
Myres, 1973; Kantrud, 1981; Dale, 1983; Renken, 1983; Knapton, 1994) was corroborated
by our results. Proliferation of western snowberry at Lostwood NWR has benefited clay-
colored sparrow, as it was the most abundant bird we observed. Although associated with
shrubs, clay-colored sparrows at Lostwood NWR occupied areas with as little as 2–3% shrub
cover (Madden, 1996), indicating that prairie with small amounts of shrub may suffice.
Although bobolinks are much more widespread than these endemic species, their pop-
ulations are also declining (Sauer
et al.
, 1997). Bobolink habitat use in our study was similar
to that found in many other studies, namely hayfields and exotic, tall grasses (Kantrud,
1981; Renken, 1983; Bollinger and Gavin, 1992). These nonnative habitats seem to effec-
tively simulate the now rare tallgrass prairie types that bobolinks preferred historically. In
1873, bobolinks were noted breeding in large numbers in north-central North Dakota in
‘‘meadowy’’ areas on ‘‘open prairie adjoining the (Souris) river.’’ (Coues, 1878:587). Ap-
parent preferences of bobolinks and other grassland birds
(e.g.
, Le Conte’s and savannah
sparrow) for exotic grasses may more accurately reflect affinities for mesic prairie sites.
Whereas the native plant composition of xeric prairie at Lostwood NWR is relatively intact,
most tallgrass, mesic sites have been invaded by exotic grasses. The tall, rhizomatous (usually
broad-leaved) exotic grasses (
i.e.
, smooth brome and quack grass) are structurally similar
to the native grass species they have replaced, and associated bird species have adopted this
introduced vegetation as breeding habitat.
Whereas bobolinks showed a distinct preference for exotic grasses, grasshopper sparrow
use of grass habitats is less easily summarized. Whitmore (1981) indicated that grasshopper
sparrows normally inhabit open grasslands of bunch grasses rather than rhizomatous grasses
in West Virginia and Wisconsin. This is apparently contradicted by mixed-grass-prairie stud-
ies in which grasshopper sparrows were more abundant in exotic rhizomatous grass than
native bunch grass habitats (Wilson and Belcher, 1989), and were one of the most common
species in Conservation Reser ve Program (CRP) lands planted mostly with exotic grasses
( Johnson and Schwartz, 1993). Our study concurred with the mixed-prairie studies; grass-
hopper sparrows used areas with high frequencies of exotic grasses. We conclude that both
habitats are likely acceptable to grasshopper sparrows, and suitability may depend on mois-
ture conditions. Their preference for moderate amounts of vegetation may involve either
use of sparser bunch grasses in dense, relatively tall grass habitats, or use of denser, rhizo-
matous exotic grasses in shorter-grass prairies.
Savannah sparrow was so ubiquitous (93% frequency) at Lostwood NWR that modeling
any habitat preferences there would have been difficult. Other work at Lostwood NWR
revealed little else about savannah sparrow habitat use, except for a positive relationship
between savannah sparrow abundance and broad-leaved, exotic grasses (Madden, 1996). Le
Conte’s sparrow appeared to use a limited array of vegetation types, but occurred so infre-
quently (12% frequency) that predictive modeling was again made difficult. Other analyses,
however, revealed patterns in habitat use. Points occupied by Le Conte’s sparrows had lower
shrub cover than unoccupied points and a greater frequency of broad-leaved, exotic grasses
(Madden, 1996).
This study represents one of the initial efforts in modeling habitat use of grassland pas-
serines in the northern Great Plains. It is limited, however, by lack of information on avian
reproductive success and habitat area needs, issues clearly central to the long-term viability
of these bird populations. Regardless, our results underscore the need for a mosaic of
available vegetation types to maximize avian biodiversity (Ryan, 1990). Where traditional
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wildlife management emphasized late successional stages because these areas usually had
the greatest number of species, managers now recognize the importance of beta diversity,
i.e.
, diversity across the gradient of prairie habitats, as well as the importance of emphasizing
habitat for endemic species (Samson and Knopf, 1982; Knopf, 1994; Johnson, 1996).
Predictive models developed here quantify structure and composition of vegetation used
by individual bird species. Managers of northern mixed-grass prairie can maximize proba-
bility of occurrence for species of interest by using these models to select appropriate ranges
of vegetation parameters, and then manipulating vegetation accordingly. When manage-
ment treatments are periodic (
e.g.
, about every 5 y in mixed-grass prairie in this study),
vegetation mosaics created are likely to be both spatial and temporal. An array of prairie
birds should be favored in a given area over decades of management, and quantitative
models for multiple species could be integrated to forecast such communities. For example,
six of nine grassland bird species we studied had relatively specific but closely overlapping
habitat needs that also encompassed the broad range of conditions supporting two more
generalized species. Visual obstruction and/or presence of grass, forb or shrub cover were
important vegetation features for all of the specialized species. An integrated summary of
species’ habitat use indicates that probability of occurrence of these species on Lostwood
NWR may be uniformly greatest (.50%) at roughly 0.5–1.5 dm visual obstruction and 40–
60% grass cover (or its approximate inverse in this study, 10–20% shrub cover). Using these
conditions as a central foundation around which to promote a mosaic of grassland types,
managers of similar northern mixed-grass prairies can encourage or inhibit more specific
vegetation components on their respective landscapes by adjusting treatment type, intensity,
and timing. On Lostwood NWR, for example, management to maintain moderately-dense
grass cover as a dominant (albeit dynamic) landscape component provides, by default, for
abundant forbs, native grasses and broad-leaved, exotic grasses (or warm season, native grass
species they replaced) with which several species of grassland birds are associated.
Acknowledgments.
—This study was funded by the Non-Game Migratory Bird Program and Refuges
and Wildlife Division of the U.S. Fish and Wildlife Ser vice, Region 6. We thank B. Johnson, L. Rawinski
and N. Fahler for field assistance. K. A. Smith, manager of Lostwood NWR, along with staff of Des Lacs
NWR Complex, provided advice and logistical support. J. R. Rotella and P. Munholland shared ideas
and suggestions for data analyses. Comments by I. J. Ball, S. T. Jones, M. L. Morrison, R. B. Renken,
R. Schroeder and two anonymous reviewers improved the manuscript.
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UBMITTED
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2000
... Grassland bird populations have a large range of habitat requirements, and diverse vegetation structure is thought to provide varied environments for many different species (Madden et al. 2000). Identifying specific habitat features that correlate with species' occurrence can offer information that can be used in habitat management decisions. ...
... Of particular interest was determining how different suites of environmental variables, including not only components of vegetation structure (e.g., litter, vegetation height) but also components of the landscape, such as anthropogenic features (e.g., roads, energy development) and management elements (e.g., range health), shape species' abundance within this subregion. We predicted that vegetation structure elements would be important for each species because structure has previously been shown to be important for grassland birds (Madden et al. 2000, Hovick et al. 2014. Of the vegetation structure covariates we modeled, we predicted that litter and vegetation height would be important predictors of abundance for each species, although the relationship would vary by species (e.g., positive for Sprague's Pipit and negative for Thick-billed Longspur). ...
... Two of our grassland species had a positive relationship between predicted abundance and vegetation height, whereas two had a negative relationship. Sprague's Pipit predicted abundance peaked between 9 and 12 cm, preferring taller vegetation, which is consistent with other findings , Madden et al. 2000, Lusk and Koper 2013. Baird's Sparrow predicted abundance increased over the range of vegetation height seen in our study area, consistent with other findings , Lusk and Koper 2013. ...
Article
Full-text available
Declining grassland bird populations across North America continue to be a concern. Understanding local relationships between grassland bird abundance and vegetative and landscape characteristics will enable more prescriptive recommendations to be made to land managers. We used point count survey data collected by the MULTISAR (Multiple Species At Risk) program along with field measurements of habitat and landscape characteristics on 15 ranches in the Dry Mixed-grass Subregion in southern Alberta to improve our understandings of habitat relationships for five grassland bird species: Baird’s Sparrow (Centronyx bairdii), Sprague’s Pipit (Anthus spragueii), Thick-billed Longspur (Rhynchophanes mccownii), Chestnut-collared Longspur (Calcarius ornatus), and Grasshopper Sparrow (Ammodramus savannarum). We used generalized linear mixed models to examine the relationship between the predicted abundance of a species and covariates that represented vegetative structure (e.g., litter), management (e.g., range health), and anthropogenic features (e.g., energy development) of habitat site selection. Model results demonstrate four vegetation structure covariates were of most importance for predicting abundance, including litter, vegetation height, bare soil, and shrub cover. Quadratic relationships were found with litter amounts for the predicted abundance of Baird’s Sparrow, Chestnut-collared Longspur, and Grasshopper Sparrow. Contrastingly, higher amounts of litter reduced the predicted abundance of Thick-billed Longspur. The relationship of vegetation height was quadratic for Sprague’s Pipit and was positive for Baird’s Sparrow, but negative for Thick-billed Longspur. As bare soil percentage increased, the predicted abundance of Baird’s Sparrow and Chestnut-collared Longspur decreased, with Sprague’s Pipit showing a quadratic association. Negative relationships were found with increased amounts of shrub cover for Chestnut-collared Longspur, Sprague’s Pipit, and Thick-billed Longspur. Our results help to further understand individual grassland bird species’ habitat requirements, enabling us to provide land management recommendations for maintaining, improving, or creating the heterogenic environments needed for a variety of grassland birds in the Dry Mixed-grass Subregion.
... We found significant correlations between biomass and several other vegetation covariates including: VOR (r = 0.93), heights of live grass (r = 0.87) and residual grass (r = 0.80), and the proportional coverage of exotic grasses (r = 0.77). VOR is a commonly used index of herbaceous biomass and an important determinant of grassland bird abundance [34,[46][47][48]. Because biomass is a more common condition informing rangeland management, we retained biomass and removed VOR and the heights of live and residual grass from further analyses [49]. ...
... We evaluated two candidate model sets, one set containing biomass and one set containing exotic cover with all other uncorrelated variables. Shrub cover was correlated with shrub height (r = 0.85); we retained shrub cover as a predictor variable and excluded shrub height due to previously observed associations of grassland birds with shrub coverage [26,[46][47][48]51]. Litter cover was inversely correlated with bare ground cover (r = -0.71); ...
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Grassland birds are declining faster than any other avian guild in North America and are increasingly a focus of conservation concern. Adaptive, outcome-based management of rangelands could do much to mitigate declines. However, this approach relies on quantitative, generalizable habitat targets that have been difficult to extrapolate from the literature. Past work relies heavily on individual versus population response, and direct response to management (e.g. grazing) versus response to outcomes. We compared individual and population-level responses to vegetation conditions across scales to identify quantitative targets of habitat quality for an imperiled grassland songbird, the chestnut-collared longspur (Calcarius ornatus) in northern Montana, USA during 2017–2018. We estimated nest density and nest survival within 9-ha survey plots using open N-mixture and nest survival models, respectively, and evaluated relationships with plot- and nest-site vegetation conditions. Plot-scale conditions influenced nest density, whereas nest survival was unaffected by any measured condition. Nest-site and plot-scale vegetation measurements were only weakly correlated, suggesting that management targets based on nest sites only would be incomplete. While nest survival is often assumed to be the key driver of bird productivity, our results suggest that nest density and plot-scale conditions are more important for productivity of longspurs at the core of the breeding distribution. Habitat outcomes for grassland birds should incorporate nest density and average conditions at scale(s) relevant to management (e.g. paddock or pasture).
... We used recent grassland conversion as another measure of conversion risk, identifying areas of grassland in 2001, which we defined as the grassland/herbaceous, pasture/hay, and emergent wetland cover classes from the National Land Cover Dataset (NLCD; Yang et al., 2018), that were classified as cropland in 2016. Our final measure of conversion risk was quantification of potentially undisturbed grasslands (PUDL; Fields & Barnes, 2019), which occur independently of ownership, are generally composed of native plant species that have greater biological value and are at lower risk of conversion than disturbed or restored areas (Hendricks & Er, 2018;Madden et al., 2000;Wimberly et al., 2017). ...
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Spatially explicit models are an important component of systematic conservation planning, enabling the depiction of biodiversity metrics across landscapes and objective evaluation of candidate sites for conservation delivery. However, sites considered “best” for conservation are typically viewed from the standpoint of biological value and may not be the most effective or efficient when risk of habitat loss, cost of conservation, intended conservation treatments, and overall conservation strategy are considered. We evaluated risk of habitat loss, land cost, and landscape context for geographic areas harboring most‐dense to least‐dense population quartiles for 16 species of grassland birds in the US northern Great Plains. Differences in land cost, risk of grassland conversion, and landscape context among quartiles and species indicated that a minimum‐area strategy may be inefficient and even ineffective. Priority zones for western species were generally associated with lower agricultural land cost, more protected land, and landscape characteristics associated with intact grasslands; eastern species were generally associated with higher agricultural land cost, tillage probability, grass loss, cropland, development, forest, Conservation Reserve Program grasslands, and distance to grass. Our results indicate that addressing areas outside of population cores increases conservation options and may provide substantial benefits to portions of populations that are most vulnerable to habitat loss or other stressors.
... When integration occurs on a spatial level, the benefits of diversity can be increased across the landscape (Hilimire, 2011;Lemaire et al., 2015;Sulc & Tracy, 2007). Perennial pastures often provide greater diversity than croplands due to naturalized species invasion over time and can be key habitats for pollinators (Sanderson, 2016) and birds (Derner et al., 2009;Madden et al., 2000). Recent emphasis on multiple-species annual cover crops can be another avenue for increasing plant diversity and could foster the integration of livestock into the operation if grazed by cattle (Blanco-Canqui et al., 2020). ...
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Contemporary agricultural systems can be generalized as highly specialized operations that are fueled by mechanization; supplied with external nutrients to maximize production; crops protected by petrochemicals to fight against weed, disease, and insect pressures; and livestock protected by therapeutics to ward off virus and bacterial infections when managed in confinement. Such specialized systems have led to low levels of diversity, elevated environmental risks from contamination, loss of soil organic matter, ecological instability, and limited adaptability to climate change. More diversified farming systems are possible, but research required to characterize them in a holistic manner as an alternative to contemporary, specialized systems remains challenging to fund and sustain over time, primarily because they require more labor, management skills, and accessible markets to achieve additional ecological, environmental, and social goals. We share some perspectives as to (1) how specialized systems became the norm and (2) what changes could be made to reverse some ecological risks and environmental declines associated with specialization, acknowledging there is no panacea. Strong evidence exists for perennial forages to restore soil organic carbon (C) and nitrogen, but system‐level analyses of the net balance in greenhouse gas emissions remain to be characterized in the myriad of potential integrated crop–livestock systems that might be deployed across the diversity of edaphic, environmental, and socio‐economic conditions. We suggest there are abundant opportunities for more sustainable agricultural production to sequester soil organic C, reduce greenhouse gas emissions, and develop more climate‐resilient agricultural systems that will be needed in a future dominated by climate change issues.
... An important factor contributing to the continued decline of grassland birds is our limited understanding of the mechanisms through which habitat structure affects bird populations, which can reduce the effectiveness of management actions and result in inconsistent patterns of bird-habitat relationships among studies. For example, Grasshopper Sparrows (Ammodramus savannarum) have been documented preferentially selecting grasslands dominated by native bunch grasses (Whitmore, 1981) and grasslands primarily composed of exotic rhizomatous grass (Johnson and Schwartz, 1993;Madden et al., 2000). Similarly, patch size, edge proximity, and landscape composition have varying effects on nest survival of grassland birds (Winter et al., 2006;Benson et al., 2013), despite generalized assumptions that such effects are consistent. ...
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Due to consistent population declines across the continent, grassland birds have become a guild of high conservation and management interest. Despite a large number of studies investigating grassland bird habitat associations, we know relatively little about the mechanisms through which habitat characteristics may impact grassland birds, as these mechanisms are often assumed rather than directly tested. For this study, we estimated whether the effects of habitat structure on breeding Field Sparrows are mediated through changes in predator (snake and raccoon) abundance, alternative prey availability, or arthropod biomass using structural equation models. We found no evidence of nest survival or nest density of Field Sparrows being directly influenced by nest predator abundance, alternative prey, or arthropod biomass, although habitat characteristics associated with increased nest survival were also associated with greater arthropod biomass and reduced predator abundance. We suggest that habitat structure in our study area primarily impacts breeding Field Sparrows through direct means, such as influencing nest concealment or foraging efficiency. Our results also suggest that nest success and nest density are decoupled in our study area, so Field Sparrows may be preferentially selecting nest sites with structural characteristics that do not increase nest survival. Ultimately, our findings from this study indicate that while predator avoidance and food provisioning likely play an important role in determining nest survival for grassland birds, predator abundance and arthropod biomass may not necessarily predict predation risk and foraging efficiency to the extent that is often assumed.
... Under varying intensities, grazing can create distinct vegetative conditions to which grassland bird species respond. Some species such as horned lark (Eremophila alpestris) show preferences for short and sparse vegetation (Martin & Forsyth, 2003;Rotenberry & Wiens, 1980), while other species such as clay-colored sparrow (Spizella pallida) prefer vegetation with a shrub component (Madden et al., 2000). Furthermore, the responses of individual species shape the response of the community. ...
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Temperate grassland ecosystems are one of the most threatened ecosystems worldwide, and their loss endangers the grassland songbirds that rely upon them. This guild of birds has shown long‐term declines in North America. At the same time, American bison (Bison bison) are becoming more common through reintroductions, and they may make significant modifications to grassland songbird habitat. To support conservation for this guild, we sought to understand the importance of bison grazing and ecosystem productivity to the species richness, occupancy, and abundance of this avian community. We conducted dependent double‐observer bird counts, measured bison grazing intensity with patty counts, and used remote‐sensed normalized difference vegetation index (NDVI) data to measure ecosystem productivity. Our work took place in the National Bison Range near Moiese, Montana and in Yellowstone National Park in Wyoming. We found that species richness was positively correlated with patty counts and had a weak negative correlation with NDVI. Occupancy probability for six of seven grassland songbird species was positively correlated with patty counts, and for six of seven species was negatively correlated with NDVI. Abundance of vesper sparrow (Pooecetes graminueus) and western meadowlark (Sturnella neglecta) were positively correlated with patty counts, although for western meadowlark, this trend became less positive with increasing patty counts. Our work suggests that managers may want to encourage a broad range of bison grazing intensities to ensure that vegetative conditions related to bison grazing are present for all species.
... Our results for grasslandassociated species are less consistent with those of previous research, but we are lacking sufficiently detailed vegetation data to fully interpret these results at our study site. We found western meadowlarks to be more abundant in sagebrush-steppe habitat with shrubs compared with habitat without shrubs, despite their typical positive association with grass and forb cover (Wiens and Rotenberry 1981;Madden et al. 2000) and negative association with woody vegetation at breeding sites (Bakker et al. 2002;Davis 2004;Grant et al. 2004). However, in British Columbia, Canada, they were found to tolerate shrubs up to 1 m in height (Schwab et al. 2006). ...
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Sagebrush communities, covering millions of hectares in the western United States, are among our most imperiled ecosystems. They are challenged by various anthropogenic stressors, including invasion by nonnative grasses that degrade habitat quality and alter ecosystem function. Sagebrush restoration efforts are underway to improve habitat conditions to benefit a wide range of sagebrush-dependent species. Because birds are good indicators of habitat quality, monitoring avian metrics is an effective way to measure progress of sagebrush restoration. We compared avian community composition and individual species abundance among three sagebrush–steppe habitat types with varying degrees of invasion by nonnative crested wheatgrass Agropyron cristatum at the Camas National Wildlife Refuge in southeastern Idaho, USA. Sagebrush-obligate birds, such as sage thrasher Oreoscoptes montanus and sagebrush sparrow Artemisiospiza nevadensis, were most abundant in sagebrush habitats with an understory of native grass. Community composition was similar between sagebrush habitats with native and nonnative grasses, but quite different from bird communities occupying crested wheatgrass. The Habitats and Populations Strategies database, a conservation planning tool, predicts that restoration of crested wheatgrass sites to sagebrush in poor or fair condition will increase the density of sagebrush-obligate bird species. Taken together, these results suggest that restoration of crested wheatgrass near-monocultures back to sagebrush will improve habitat value for much of the bird community whether or not the understory can be converted to primarily native grasses, or a mix of natives and nonnatives. Of the sagebrush bird species of concern, Brewer's sparrow Spizella breweri occupied sagebrush habitats with native vs. nonnative understory at similar abundances, and this species could serve as a metric of intermediate restoration success. However, sagebrush sparrow and sage thrasher, which were significant indicators of sagebrush with native grasses, will likely benefit most from full restoration of a native herbaceous understory. Grassland-obligate birds such as horned lark Eremophila alpestris and grasshopper sparrow Ammodramus savannarum were most abundant at crested wheatgrass–dominated sites and may not benefit from restoration back to shrubland; managers should understand potential trade-offs.
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Although species composition of birds of prey (raptors) that nest in the northern Great Plains is being altered due to land use, detailed case histories of such change are lacking, few long-term data exist to help understand population dynamics of raptors in prairie areas, and implications of changes for other prairie wildlife on which raptors prey are poorly understood. I studied mechanisms and implications of land use impacts on raptors on the Missouri Coteau of northwestern North Dakota by (1) tracing change during the past century in raptor species composition and habitat on the 108-km2 Lostwood National Wildlife Refuge (LNWR), (2) assessing current (1981-90) population stability, annual reproductive success, and habitat relationships of raptors on LNWR and comparative species' nesting densities on an adjacent area of different land use, and (3) determining prey needs of common, large (>700 g) raptor species on LNWR and prey use in relation to habitat on an area of contemporary land use. Northern harriers (Circus cvaneus), Swainson's hawks (Buteo swainsoni), ferruginous hawks (B.regalis), and burrowing owls (Athene cunicularia) comprised the community of raptors that nested on LNWR before Euro-American settlement in the early 1900s, great horned owls (Bubo virginianus) were rare but gradually increased over the last 40-50 years, red-tailed hawks (B. jamaicensis) pioneered about 30 years ago, and Cooper's hawks (Accipiter cooperii) pioneered in the past decade. Today, red-tailed hawks and great horned owls have replaced Swainson's and ferruginous hawks as dominant large raptors, coinciding with succession from mixed grass prairie to aspen parkland with brush-dominated uplands. Harrier abundance probably has changed little, but nesting burrowing owls have been absent >40 years. I suggest the most fundamental causes of change in the raptor community were altered susceptibility of prey to foraging behaviors of specific raptor species and decreased abundance of certain key prey species. During the 1980s red-tailed hawks and great horned owls exhibited high, stable nesting densities (mean, 0.23 and 0.13 occupied nests/km2) but erratic and low annual productivity (mean, 0.9 and 0.7 young/occupied nest); both species nested most in areas with highest densities of tree clumps, and the owl was associated with wetlands. Nearly all Swainson's hawk nests occurred on LNWR's boundary, and an adjacent area (93 km2) of different land use had twice as many occupied Swainson's hawk nests/km2 as LNWR. Great horned owl diets were studied during late spring and Swainson's hawk diets during summer, 1986-87 on areas of mixed land use; 2,900 and 1,284 prey items were recorded, respectively. Diet varied among families of both species. The owl relied heavily on avian prey from wetlands (total wetland prey: 57% of overall frequency and 76% biomass of diet) especially ducks and used less Leporid prey than reported elsewhere. Great horned owls depredated about 2-4 ducklings/km2 and 1.3-1.4 adult ducks/km2 during mid-May through June; adult females of several duck species were more vulnerable to owl predation than males. Swainson's hawks used many prey from wetlands (49% overall frequency, 42% biomass); mammals were the most important prey Class and overall diet was more diverse compared to study findings elsewhere. Variation in use of several important prey among Swainson's hawk families was mostly explained by nesting area habitat. A cursory survey of great horned owl and red-tailed hawk diets on LNWR suggested these raptors relied on prey from wetlands, especially ducks. Land use practices that favor nesting red-tailed hawks and great horned owls in prairies of the northern Great Plains are not amenable to raptor species that nest mainly in the region and perhaps not to many other species of indigenous migratory birds.
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To more effectively manage remaining native grasslands and declining populations of prairie passerine birds, linkages between disturbance regimes, vegetation, and bird abundance need to be more fully understood. Therefore, we examined bird-habitat relationships on mixed-grass prairie at Lostwood National Wildlife Refuge (NWR) in northwestern North Dakota, where prescribed fire has been used as a habitat management tool since the 1970s. We sampled bird abundance on upland prairie at 310 point count locations during 1993 and 1994 breeding seasons. We also measured vegetation structure and composition at each location. Complete fire histories were available for each point, with over 80% having been burned one to four times in the previous 15 years Post-fire succession generally transformed vegetation structure from short, sparse, and grassy with few forbs and low litter immediately after fire, to increasing and moderate amounts of forbs, litter, and shrubs two to eight years postfire, to tall, dense, shrubby prairie with little forb, grass, or litter understory when fire was absent (> 80 years). Most grassland birds (six of nine species examined) at Lostwood NWR were absent from prairie untreated with fire. Species richness and abundances of Baird's Sparrows (Ammodramus bairdii), Bobolinks (Dolichonyx oryzivorus), Grasshopper Sparrows (A. savannarum), Le Conte's Sparrows (A. leconteii), Sprague's Pipits (Anthus spragueii), and Western Meadowlarks (Sturnella neglecta) were positively related to an index of amount of fire, and these species were absent from unburned units. In contrast, Common Yellowthroats (Geothlypis trichas) and Clay-colored Sparrows (Spizella pallida) both reached highest abundance on unburned prairie. To provide maximum grassland bird diversity, managers of mesic, mixed-grass prairie generally should provide areas with short (2-4 year), moderate (5-7 year), and long (8-10 year, or more) fire return intervals. Because long-term rest may create habitat unfavorable for most species of grassland passerines in mesic, northern mixed prairie, periodic defoliations by disturbances such as fire should be considered essential to restore and maintain native biodiversity.
Chapter
Grasslands have figured prominently in our North American heritage. Prairies first provided significant barriers to westward expansion, then offered both economic and sociological opportunity, as well as heartache, for settlers. Many artists have gained significant inspiration from the beauty as well as the harshness of these regions and its biota. And, because of ideal climate and soil conditions, these grasslands have provided the agricultural foundation upon which much of the growth and stability of the United States economy rests. Yet, many see North America prairies as beautiful only when manipulated or exploited--green croplands or manicured park lawns are attractive, whereas native grasslands are “those ugly weeds.” In the past, plowing virgin prairie could be easily defended on both economic and sociological grounds. And historically, North American prairies must have seemed threatening in both their wildness and vastness. But preservation of these prairies is now an urgent need. This book describes the ecology of the North American prairie and urges conservation measures to protect the remaining North American grasslands. It provides non-economic arguments for the value of prairies, presents a current synthesis of prairie ecology to facilitate the best possible management, and deftly summarizes conservation and management issues, pointing out the costs and benefits of alternative actions. By approaching its subject from a variety of perspectives, including ethical and aesthetic considerations, the book will appeal to environmentalists and conservationists as well as to ecologists, botanists, and conservation biologists.
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As the century nears its end and demand for food and competition for land escalate, a most important issue facing conservationists will be the preservation of a mosaic of habitats in which can be preserved a representative cross-section of native species. The need to resolve this issue is emphasized in the Global 2000 Report to the President (Council on Environmental Quality 1980) which predicts that, worldwide, 500,000 to 2 million species will become extinct by the year 2000 and that the rate will increase from one per day in 1980 to one per hour by century's end (Myers 1979). Although these extinctions will largely occur in developing countries (Norman 1981), over 500 species and subspecies of flora and fauna have become extinct in North America since the Puritans arrived at Plymouth Rock in 1620 (Spinks 1979). This most critical need, to preserve habitat so that floral and faunal diversity can be maintained, rests not only on the loss of genetic diversity and scientific-medical properties, but on the long term consumptive, nonconsumptive, and social values of plants and wildlife to mankind.
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