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P.(Paul) Hendricks.

Surveys for grassland birds of the Malta Field Office-BLM, including a seven-year study in north Valley County (Volume 2008) online

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vegetation density and did not cluster with any
other SOC bird.



Bird Associations with Vegetation
Structure



26



■a

I

o

(0

o



o
O



n



22



<u "

U)
(0

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♦ BAB ♦ GRSP



C*L?fPI



♦ MCLO
♦ LBCU



14

35 40 45 50 55

Mean Vegetation Height (cm)

Figure 6. Ordination of nine Species of Concern birds on
vegetation structure in north Valley County, Montana, based
on mean values for 2001-2007 combined. Species acronyms:
BAIS (Baird's Sparrow), BOBO (Bobolink), BRSP (Brewer's
Sparrow), CCLO (Chestnut-collared Longspur), GRSP
(Grasshopper Sparrow), LARB (Lark Bunting) , LBCU (Long-
billed Curlew), MCLO (McCown 's Longspur), SPPl (Sprague s
Pipit).

When data across the seven years of study was
combined, points on which each SOC bird was
detected differed in vegetation structure from
points where each was not detected (Table 7).
Substantial differences in vegetation height
were evident for all species but Lark Bunting.
Vegetation height was shorter where Long-billed
Curlew, McCown's Longspur, and Chestnut-
collared Longspur were detected and the variation
in vegetation height for these points was relatively
narrower. Long-billed Curlew tended to occupy
the shortest vegetation (mean = 36.7 cm), followed
by McCown's Longspur (mean = 37.4 cm) and
Chestnut-collared Longspur (mean = 39.7 cm).
The remaining six species occupied points where
vegetation tended to be taller and the variation in
vegetation height for these points was relatively



wider. Bobolink showed a tendency to occupy
the tallest grassland vegetation (mean = 48.7 cm),
followed by Brewer's Sparrow (mean = 47.8 cm)
and Grasshopper Sparrow (mean = 46.8 cm); in
this group Sprague 's Pipit tended to occur where
vegetation was shortest (mean = 40.8 cm), although
it merits emphasis that the variation around mean
vegetation height for occupied points was at least
8 cm for all species, and there was overlap in
occupied and unoccupied points for all species.

Substantial differences in mean vegetation density
near the ground (contacts 0-1 dm) were evident
for all species but Brewer's Sparrow (Table 7)
for the combined seven years of data. Vegetation
density was less on occupied than unoccupied
points for Long-billed Curlew, Brewer's Sparrow,
Lark Bunting, and McCown's Longspur, and
greater on occupied points for the remaining five
species. Lark Bunting and Long-billed Curlew
occupied points with the least dense near-ground
vegetation (< 15 contacts) among the nine SOC
birds, while Bobolink, Grasshopper Sparrow, and
Baird's Sparrow occupied points with the densest
near-ground vegetation (> 20 contacts). As with
vegetation height, there was substantial variation
in near-ground vegetation density for all points
occupied by each species, with overlap in occupied
and unoccupied points.

There was substantial (Kruskal-Wallis ANOVAs, P
< 0.05) between-year variation in mean vegetation
height, near-ground vegetation density (contacts
0-1 dm), or both, on occupied points for all nine
SOC birds except Brewer's Sparrow (Table 8).
Differences were less substantial (P > 0.05) among
years for one of the two measures of vegetation
structure on occupied points for three bird species:
mean vegetation height for Grasshopper Sparrow
and Bobolink, and near-ground vegetation density
for Long-billed Curlew.

Among the nine SOC birds. Long-billed Curlew
and McCown's Longspur tended to occupy points
across all years with relatively short (< 40 cm) and
less dense near-ground vegetation (Table 8). The
opposite was the pattern for Grasshopper Sparrow,
Baird's Sparrow, and Bobolink. Sprague's Pipit
and Chestnut-collared Longspur tended to occupy



13



Table 8. Mean vegetation height and near-ground vegetation density (contacts 0-1 dm) for points occupied by nine Montana SOC birds. Standard deviations in parentheses.
P-values (Kruskal-Wallis ANOVAs) are for among-year comparisons, with those <0. 05 in bold.





2001


2002


2003


2004


2005


2006


2007


P


Long-billed Curlew


















height (cm)


35.1(8.1)


33.3 (5.4)


37.6(5.3)


37.3 (6.3)


39.4(6.3)


32.6 (6.9)


46.2 (7.5)


0.0229


contacts 0-1 dm


15.2(5.8)


11.8(6.2)


12.3(3.1)


14.0(4.1)


17.4(6.8)


14.4(6.5)


18.6(6.1)


0.2831


Sprague's Pipit


















height (cm)


39.6(10.8)


36.3(8.8)


44.1(8.9)


38.5 (6.9)


40.6(7.8)


39.3 (9.2)


47.3 (6.4)


<0.0001


contacts 0-1 dm


19.1(9.3)


16.0(6.3)


16.1(6.8)


18.4(7.0)


16.7(6.0)


20.0(11.1)


20.5(10.1)


<0.0001


Brewer's Sparrow


















height (cm)


45.5(16.0)


43.5 (8.9)


48.5 (8.0)


44.5(11.1)


51.9(6.8)


39.6(12.2)


53.6 (9.2)


0.2885


contacts 0-1 dm


15.4(5.2)


15.0(2.4)


19.3 (9.2)


15.2(7.2)


10.7(1.5)


13.5 (7.5)


19.5 (7.2)


0.2232


Lark Bunting


















height (cm)


43.8(13.6)


36.5 (9.3)


45.4(7.8)


38.9(8.3)


43.2(11.5)


39.8(11.1)


49.8 (7.8)


<0.0001


contacts 0-1 dm


14.4(5.1)


12.7(4.9)


12.7(5.2)


17.1(7.4)


14.4(5.6)


16.3 (6.0)


15.8(5.8)


0.0031


Grasshopper Sparrow


















height (cm)


52.0(14.4)


44.6(12.1)


46.8 (9.0)


47.6(11.5)


45.8(10.6)


44.6(13.0)


50.7 (6.9)


0.0532


contacts 0-1 dm


19.0(7.4)


14.5 (6.7)


18.1(8.0)


19.0(7.4)


19.1(7.1)


24.5(13.1)


24.5 (13.4)


0.0024


Baird's Sparrow


















height (cm)


43.1(11.1)


38.5(8.6)


46.6(9.5)


41.6(8.4)


43.0(9.1)


42.5(11.3)


48.2(7.1)


<0.0001


contacts 0-1 dm


21.2(8.2)


17.9(6.5)


17.1(6.2)


20.4 (6.2)


18.8(7.0)


23.2(11.3)


23.2(11.0)


<0.0001


McCown's Longspur


















height (cm)


34.4(7.9)


32.2(5.3)


44.1(7.9)


35.5 (6.9)


34.5 (6.0)


34.9 (6.0)


47.2(5.4)


<0.0001


contacts 0-1 dm


16.2(6.6)


12.6 (4.4)


15.0(6.5)


16.8(7.9)


14.3(5.8)


16.5 (7.0)


16.1 (4.3)


0.0188


Chestiiut-coUared Longspur


















height (cm)


37.2(9.6)


34.7(7.1)


43.7(8.8)


37.4(6.3)


39.8(8.1)


37.7(8.9)


46.8 (6.8)


<0.0001


contacts 0-1 dm


18.1(8.6)


15.4(5.8)


15.7(6.5)


17.9(6.8)


17.4(7.0)


18.8(9.1)


20.3 (9.4)


<0.0001


Bobolink


















height (cm)


too few data


too few data


46.9(8.3)


58.0(10.5)


44.3 (2.7)


52.0(16.4)


51.9(4.4)


0.1957


contacts 0-1 dm


too few data


too few data


17.1(8.5)


20.0 (4.4)


20.5 (9.5)


34.0(15.8)


39.7(19.2)


0.0067



points with vegetation of intermediate height and
near-ground density, while Brewer's Sparrow and
Lark Bunting occupied points with taller but less
dense vegetation. An unusual amount of April-
May precipitation in 2007 (Table 4) led to robust
growth of prairie grasses (see Figure 3), tending to
narrow the differences among species in vegetation
height on occupied points; the 2007 effect was less
noticeable for near-ground vegetation density.

Although there was substantial among-year
variation for vegetation structure on occupied
points for most of the nine SOC birds (Table 8),
significant (P < 0.05) within-year differences
between occupied and unoccupied points were not
always evident (Figure 7, Table 9). Long-billed
Curlew had the fewest significant within-year
comparisons, occupying points with significantly
shorter vegetation only in 2003 and 2007, although
it tended to occur on points where the vegetation
was shorter and less dense every year but one
(2005). Sprague's Pipit tended to occur on points
where vegetation was slightly taller (significantly
so in four years) and denser near the ground
(significantly so in four years). Conditions on
points occupied by Brewer's Sparrows remained
relatively consistent among years, although
vegetation was significantly taller during three
years, and near-ground vegetation was significantly
less dense during one of those years; these
patterns may be a result of small sample sizes (40



occupied points across seven years). Lark Bunting
occupied points where the vegetation tended to
be less dense near the ground (significant in four
years), but differences in vegetation height (taller
on occupied points in all years) were consistently
less pronounced. Grasshopper Sparrow was
found in most years on points where vegetation
was significantly taller (2003 being the only year
with a weaker difference) and often denser near
the ground. Only Baird's Sparrow displayed
significant differences between occupied and
unoccupied points in all years for both measures
of vegetation structure, occurring on points where
the vegetation was significantly taller and denser
near ground level. Vegetation on points occupied
by both longspur species tended to be shorter
than on unoccupied points. However, McCown's
Longspurs occupied points where the near-ground
vegetation was also less dense (significant in four
of seven years), while Chestnut-collared Longspur
occupied points where near-ground vegetation
was denser (significantly so in four years) than on
unoccupied points, just the opposite pattern from
its congener. Comparisons for Bobolink suffered
from small sample sizes (32 occupied points across
seven years), but this species consistently occurred
on points where vegetation was taller (significantly
so in three of five years with data sufficient to
calculate means and standard deviations) and
denser near the ground (significantly so in two of
five years).



15



50 -,



Long-billed Curlew




Long-billed Curlew




Sprague's Pipit




Sprague's Pipit




60 1



Brewer's Sparrow




Brewer's Sparrow




Figure 7. Vegetation height and vegetation density (number of rod contacts) for sites occupied by nine Montana bird Species of
Concern in north Valley County during 2001-2007. Black bars are for points where species was detected, open bars for points
where species was not detected.



16



Lark Bunting




Lark Bunting




Grasshopper Sparrow




Grasshopper Sparrow




Baird's Sparrow




? 20-

o

« 15-

1 10-

c

o ^
°


Baird's Sparrow




_




1










1






1


^




















~


2 u ^

12 3 4 5 6 7

Year





Year



Figure 7. (Continued) Vegetation height and vegetation density (number of rod contacts) for sites occupied by nine Montana bird
Species of Concern in north Valley County during 2001-2007. Black bars are for points where species was detected, open bars
for points where species was not detected.



17



McCown's Longspur




McCown's Longspur




Chestnut-collared Longspur



Chestnut-collared Longspur





Bobolink



Bobolink




?


50


■n




^


40


o




(0


30






(>






2U


c




o
o


10



rJiJrJ^



Year



4
Year



Figure 7. (Continued) Vegetation height and vegetation density (number of rod contacts) for sites occupied by nine Montana bird
Species of Concern in north Valley County during 2001-2007. Black bars are for points where species was detected, open bars
for points where species was not detected.



18



VO



Table 9. Mean vegetation height and density index (contacts 0-1 dm) comparisons for points w
(see Figure 7). P-values are based on Wilcoxin Rank-Sums tests; values <0.05 are in bold.


here nine Montana SOC birds were detected and not detected




2001


2002


2003


2004


2005


2006


2007


Long-billed Curlew
















height


0.5682


0.3589


0.0189


0.7621


0.8664


0.0168


0.652


density


0.2718


0.119


0.0874


0.2255


0.7328


0.1185


0.9865


Sprague's Pipit
















height


0.0007


0.0245


0.4717


0.0475


0.2737


0.0463


0.4079


density


0.0002


0.009


0.0276


0.0149


0.8708


0.2548


0.7276


Brewer's Sparrow
















height


0.1634


0.0493


0.3715


0.1602


0.0175


0.8761


0.0097


density


0.4336


0.7816


0.5379


0.5102


0.0435


0.1837


0.7693


Lark Bunting
















height


0.0944


0.6532


0.4218


0.4589


0.4282


0.2819


0.0751


density


0.1535


0.0001


0.0161


0.8181


0.0167


0.0625


0.0018


Grasshopper Sparrow
















height


<0.0001


0.0002


0.0988


0.0003


0.0001


0.0002


0.0036


density


0.176


0.8436


0.003


0.5171


0.0159


0.0001


0.0322


Baird's Sparrow
















height


<0.0001


<0.0001


0.0037


<0.0001


0.003


<0.0001


0.0278


density


<0.0001


<0.0001


0.004


<0.0001


0.0013


<0.0001


0.0001


McCown's Longspur
















height


0.08


0.0028


0.8439


0.0663


<0.0001


0.0012


0.7784


density


0.2846


0.0046


0.6214


0.4572


0.0033


0.0245


0.0008


Chestnut-collared Longspur
















height


0.1431


0.1399


0.0053


0.1976


0.116


0.0104


0.1134


density


0.0083


0.0123


0.2181


0.0493


0.018


0.4825


0.4923


Bobolink
















height


too few data


too few data


0.389


0.0067


0.0824


0.0444


0.0261


density


too few data


too few data


0.6408


0.6408


0.3403


0.0035


0.0004



Discussion



The prairie grasslands of north Valley County,
Montana support several SOC birds endemic to
the northern Great Plains, as well as other SOC
grassland birds that breed more widely (Table 2).
Nine SOC birds were detected on point counts each
year of our seven-year study, and 1 1 other bird
species also were observed on our counts every
year (Table 3). Especially noteworthy is several
species of conservation concern were also some of
the most abundant breeding species on this prairie
landscape. The bird occurring on the greatest
percentage (79.7-89.9%) of our point counts was
Chestnut-collared Longspur, a Montana SOC bird;
this species was previously recognized as the most
abundant breeding bird across the northern Great
Plains grasslands (Kantrud and Kologiski 1982).
Next in order of relative abundance were Horned
Lark, Western Meadowlark, Sprague's Pipit (SOC),
and Baird's Sparrow (SOC); each species was
detected every year on at least 30% of our count
points. The percentages presented in Table 3 are
generally conservative (compare with Table 3
in Hendricks et al. 2007), as most species were
detected on additional point counts beyond the
boundary of our 100 m fixed-radius count circles,
data not included in the present report.

The nine SOC birds tended to segregate themselves
across the north Valley County landscape on the
basis of vegetation structure, as has been noted
among grassland birds in other studies (Kantrud
and Kologiski 1982, Dale et al. 1997, Davis et al,
1999, Madden et al. 1999, Madden et al. 2000,
Johnson and Igl 2001, McMaster and Davis 2001).
When data was pooled for all years of our study,
mean vegetation structure on occupied points
differed significantly from unoccupied points in
both mean height and near-ground density for
seven species, and in one of these two traits for the
other two species (Table 7). A simple ordination
using the two vegetation structure variables as axes
(Figure 6) showed the nine SOC birds segregated
across space into distinct species pairs or groups
tending to occupy points sharing similar vegetation
structure. Where these species fell on a relative
vegetation-structure scale during our study in
north Valley County are consistent with the



studies previously noted, although only Kantrud
and Kologiski (1982) addressed the full range of
species we found, and their research focused on
species distributions in relation to grazing and soils
rather than directly on vegetation structure.

Comparison of vegetation structure on occupied
and unoccupied points within years showed
significant differences in one or both vegetation
metrics for all SOC birds in at least two years,
and usually four or more years (Table 9, Figure
7). Stronger differences might have been evident
in more years if we had sampled vegetation from
a larger area of each point-count circle, rather
than just the core. Nevertheless, the patterns
we detected by our sampling method tended to
be consistent with prior studies, an encouraging
result. In future studies using similar methods for
sampling vegetation associated with bird presence,
we suggest using a protocol that includes obtaining
vegetation-structure data from random locations
within each point-count circle.

Local climate conditions played a role in the
annual variation in abundance of grassland bird
species in north Valley County. In particular, the
amount of April and May precipitation appears
to have influenced the abundance of some
Species of Concern, such as Long-billed Curlew,
Baird's Sparrow, Chestnut-collared Longspur,
and Bobolink (Figure 5), probably mediated
through the effects of spring precipitation on
vegetation structure (Table 5, Figure 4), although
this contention was not uniformly supported by
correlating annual species abundance with mean
vegetation structure from all points (Table 6). Total
precipitation for April and May was chosen as most
likely to influence the vegetation conditions birds
find when they commence nesting in this region
(Davis 2003). Three species (Baird's Sparrow,
Chestnut-collared Longspur, and Bobolink)
associate with sites where vegetation is taller and
denser than average or at least where vegetation
provides more nest cover than random sites (Davis
et al. 1999, Johnsgard 2001, Dieni and Jones 2003).
Long-billed Curlew showed a negative response to
increased spring precipitation, an expected pattern



20



because this species selects sites with low-stature
grass for nesting (P am push and Anthony 1993,
Johnsgard 2001, Dugger and Dugger 2002).

The remaining Species of Concern birds detected
every year of our study (Table 3) had the weakest
abundance correlations with April and May
precipitation. The association of Brewer's Sparrow
with shrub cover (Best 1972, Bock and Bock 1987,
Paige and Ritter 1999) might contribute to a weaker
abundance-precipitation relationship, as the short
term structural response of shrubs to precipitation
is likely less obvious through the kinds of
measurements we recorded. Lark Bunting is more
nomadic than the other species across a large
region, and dramatic population fluctuations in
north Valley County (Figure 2) are probably in part
a response to drought conditions elsewhere (Yackel
Adams et al. 2006, Lenard et al. 2006, Hendricks
et al. 2007); Lark Bunting is also sometimes
associated with a low density of shrubs (Feist 1968,
Bock and Bock 1987, Johnsgard 2001). The weak
relationship of Sprague's Pipit and Grasshopper
Sparrow abundances with spring precipitation
was unexpected. These species occur in denser
and taller than average vegetation (Tables 7 and 9,
Figure 7), but also relatively shrub-free (Johnsgard
2001), so a correlation similar to Baird's Sparrow



or Bobolink (Figure 5) was anticipated in the case
of Grasshopper Sparrow, and to Chestnut-collared
Longspur in the case of Sprague's Pipit. However,
as Kantrud and Kologiski (1982) allude, Sprague's
Pipit appears to occur in relatively diverse
conditions, and this flexibility may be reflected in
the nearly flat abundance response to the variation
in spring precipitation we noted in our study.

All species recorded are likely to breed on or
near the project area, as field inventory occurred
during the breeding season and observations
included singing males and territorial displays in
appropriate breeding habitat. Nest observations
were not part of the survey protocol and have not
been summarized at this time, but we confirmed
breeding (nests with eggs or nestlings, or recently
fledged dependent young) for the SOC birds
detected each year on our point counts except
Bobolink. This information has been loaded into
the Montana Bird Distribution database (Lenard
et al. 2003) and is available online through the
Montana Natural Heritage Program Tracker
website ( http ://nhp.nris.mt. gov/Tracker ). This
database confirms breeding or indicates indirect
evidence of breeding for the project area in Valley
County for most of the species observed during our
project.



21



Conclusions



North Valley County is an exceptional example
of native northern Great Plains grassland,
representing some of the most intact remaining
prairie in Montana (Redmond et al. 1998) and the
entire North American continent. The area, which
lies adjacent to Canada's Grasslands National
Park, is recognized nationally and internationally
for its importance to prairie-dependent species
(Cooper et al. 2001, Smith Fargey 2004). Seven
of the ten most abundant species recorded each
year during this project (Table 3, Figure 2) are
state Species of Concern (MTNHP and MTFWP
2006). Five are Great Plains prairie endemic
birds (Samson and Knopf 1996) and are present
at greater abundances here than other regions in
Montana (Cooper at al. 2001). The size of this
large block of public rangeland in north Valley
County makes it especially important to species
with very limited breeding distributions, such as
McCown's Longspur (With 1994), Baird's Sparrow
(Green et al. 2002) and Sprague's Pipit (Robbins
and Dale 1999), and those species dependent
upon larger expanses of land, such as Baird's
Sparrow, Bobolink, and Marbled Godwit (Gratto-
Trevor 2000, Johnson and Igl 2001). The relative
importance of this area of Montana to the nine
grassland Species of Concern, as well as Mountain
Plover, and Greater Sage-Grouse is also highlighted



in the composite image of predicted distributions
for these species shown in Appendix C.

While the historical combination of bison,
pronghorn, elk, locusts, and fire created a
wide range of site conditions long past, land
management activities can mimic characteristics
of this historically heterogeneous landscape by
creating areas with low, sparse vegetative structure
to those with taller, denser vegetation (Gillihan and
Hutchings 2000). Each of these habitat conditions
is suited to individual prairie-dependent species.
Primary prairie endemics generally have more
restricted breeding ranges with less flexibility
in habitat requirements than more generalist
species (Davis et al. 1999). It is important to
recognize the subtle differences in foraging and
breeding habitat requirements for each species
when land management activities are considered
(see Appendix B in Lenard et al. 2006), but our
study also showed bird species occupy grasslands
where vegetation structure can vary relatively
substantially from one year to the next (Tables 5, 8
and 9, Figure 7). Providing suitable foraging and
breeding sites to a diverse assemblage of avifauna
will require a mosaic of vegetation characteristics
likely to be beneficial to a host of other prairie
species.



22



Summary and Recommendations



The present diversity and abundance of prairie-
endemic bird species in north Valley County
indicates habitat elements in this area may be closer
to historical conditions than other areas in the state.
In addition, the different habitat requirements
favored by each SOC bird recorded annually during
this study (Figure 6) indicate a diversity of habitats
is available. Given modern rangeland management
objectives to remove approximately half of the
annual growth through grazing (Adams et al. 2004,
Holechek et al. 2003), it is not surprising that
Sprague's Pipits and Chestnut-collared Longspurs,
which show a preference for areas with moderate
grass cover and litter, (Hill and Gould 1997,
Robbins and Dale 1999) were two of the most
commonly encountered birds during this study.
It is encouraging to note, however, both Baird's
Sparrows, which require denser grass and litter
(Green et al. 2002), and McCown's Longspurs,
which require sparse short grass and bare ground
(With 1994), also were two of the most numerous
bird species detected on points each year (Table 3,
Figure 2).

The current manageable activity affecting the
remaining native grasslands is grazing by livestock.
In contrast to concerns that current grazing
practices do not provide a range of habitats,
management in north Valley County appears to be
providing habitat suitable to a diversity of prairie
endemic species. At primary issue, however, is
whether current management activities address
grassland bird population objectives and if current
conditions are sufficient to sustain these prairie
species in the long term. Although this landscape
was shaped by thousands of years of grazing
by bison, the effect of domestic cattle grazing


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