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Swainson’s Hawks (Buteo swainsoni) occur throughout the western half of the
United States and are a Tier II species for the state of Nebraska (Schneider et
al. 2005). Swainson’s Hawks inhabit the grasslands of the west, which continues
to be converted into agriculture fields, which could, or could not, have a
negative effect on this species. Nebraska Prairie Partners (NPP), a cooperative
between Rocky Mountain Bird Organization (RMBO) and Nebraska Game and Parks
Commission (NGPC), wanted to what it was about western Nebraska that attracts so
many Swainson’s Hawks, and more specifically, what was it about Kimball County
and the Kimball Grasslands that attracts Swainson’s Hawks by the hundreds.
Swainson’s Hawks are the last raptors to arrive in the grasslands from their
wintering grounds in and around Argentina, which is over a 10,000 km trip
(England et al. 1997). Given that they are the last to arrive, this could affect
where they nest because potentially all the suitable areas are taken by other
raptors, or possibly other variables affect where they nest. Swainson’s Hawks
share these areas with several other raptors including Ferruginous Hawks (Buteo
regalis), Red-tailed Hawks (Buteo jamaicensis), Golden Eagles (Aquila
chrysaetos), and Great Horned Owls (Bubo virginianus). With the grasslands of
the west continuing to be turned into agricultural fields is there enough room
and habitat left to support Swainson’s Hawks, the smallest of the five raptors
mentioned? According to a northern Colorado study, only Swainson’s Hawk
regularly nested near cultivated lands (Olendorff 1973), which could be one
reason why Swainson’s Hawks can thrive amongst the larger raptors, when all the
other isolated, suitable habitats have already been inhabited.
After the Swainson’s Hawks’ long journey up to their breeding grounds, which can
number in the hundreds or even thousands (England et al. 1997), they have
several other local risks they must face. Some of these risks are weather
related and thus uncontrollable factors like severe thunderstorms, that are
common in the high plains, and high winds that can easily blow out the flimsy
nests that the Swainson’s Hawks usually build. Possibly the most important and
immediate risk that they face has to do with the land itself. Loss of habitat
and habitat fragmentation, associated with conversion of grasslands to
farmlands, not only affects the amount of habitat to support a breeding pair of
Swainson’s Hawks, but also affects nesting availability. Trees and previous
built nests could be available, but the surrounding areas might not be the
suitable habitat the breeding pair requires as was seen with Ferruginous Hawks,
where Schmutz (1984) suggested cultivation and human disturbances adversely
affects the nesting density of Ferruginous Hawks. It is possible that farmlands
do not affect Swainson’s Hawks breeding grounds as seen by Gilmer and Stewart
(1984), in North Dakota, where they stated that some pairs showed noticeable
tolerance to agricultural disturbance.
The purpose of this study was to address the nesting ecology of Swainson’s Hawks
and the landscape complementation in these areas. Western Nebraska appears to be
a focal point for Swainson’s Hawks, though they are found statewide, but are
found in steadily decreasing numbers to the east (Mollhoff 2001). With the
panhandle being near the edge of their range, this was an important study to see
what factors influence them in nest site selection in Western Nebraska. Several
factors were addressed in this study including, tree types selected (single,
clump of trees, or planted shelterbelts), cultivated lands vs. grasslands,
distance between conspecific active nest, and distance of nest from roads.
Historically, this area was all short-grass prairie and today a large portion of
this area has been converted to agricultural fields that have caused
considerable fragmentation in the landscape, better known as habitat
fragmentation. With agricultural fields and homesteads, more trees are planted,
which creates more nesting sites for raptors, so habitat fragmentation could
possibly be a benefit for Swainson’s Hawks depending on the amount of
disturbances that are acceptable to this species.
The study area was the Nebraska panhandle, with over 95% of the panhandle
privately owned, studies and future management practices in these areas are
extremely important for Swainson’s Hawks and other species. The bulk of the
study was done in the southwest portion of the panhandle in Kimball, Cheyenne,
and Banner counties. The focal point of the study occurred in what is called the
Kimball Grasslands, which is in the southwest corner of Kimball County. The
Nebraska Natural Legacy Project has recognized this area as a Biologically
Unique Landscape (BUL) (Schneider et al. 2005). These landscapes were selected
based on known occurrences of ecological communities and at-risk species
(Schneider et al. 2005), which the Kimball Grasslands not only contains
Swainson’s Hawks, a Tier II species, but contains several other Tier II and Tier
I species.
What we discovered in this two year study, 2007 & 2008, was type of tree
structure and distance from road had no effect on detectability, nor did it
affect where Swainson’s Hawks nested. Our detectability was high being around
70%, but was not perfect. In 2007, we had 107 active nests with 106 nests in
trees and 1 on a power pole, out of a 147 total nests, giving an apparent active
rate of 73%. In 2008, we had 95 active nests, all occurring in trees, out of 171
total nests, giving an active rate of 56%. Most of these nests were in
agricultural areas, with only a handful in surrounding grassland habitats, and
most occurred in Kimball County. Agricultural lands compared to grasslands,
being occupied by Swainson’s Hawks, showed no significant difference, being
almost identical in probabilities. The mean distance between two active nests,
in 2007, was 3372 meters, or 2.11 miles, while the mean distance between an
active and inactive nest was 2166 meters, or 1.35 miles. The mean distance
between two active nests, in 2008, was 3432, or 2.15 miles, and the mean
distance between an active and inactive nest was 2813 meters, or 1.76 miles.
Swainson’s Hawks are using shelterbelts to nest in and has been observed in
other studies as well (Gilmer and Stewart 1984). Shelterbelts are associated
with homesteads, which is one form of habitat fragmentation, and while
Swainson’s Hawks may be using shelterbelts, they may not be by choice. Since
they are the last raptor to arrive, they probably do not have too many places,
trees and habitat, to build their nests in. In both years shelterbelts made up
about 50% of all the active nests, but that could be simply because there are
more shelterbelts than other tree structures like clump of trees and single
trees. For example, in 2007, 34 trees contained Swainson’s Hawks nests and 26 of
them were active, which is 76%, while 72 nests were found in shelterbelts and 53
of those were active, which is 74%. What this demonstrates is, if more single
trees were available that could be their choice in tree structure to construct
their nests in, but given the situation of arriving last, they take what is
available, which happens to be shelterbelts.
Comparing the two years shows the number of active nests is down significantly
in 2008, with only around 56% of the nests being active compared to 73% that
occurred in 2007. One reason for the down year has to do with the construct of
their nests. Swainson’s Hawks build flimsy nests using tumble weeds and whatever
else they can find. These nests are poorly constructed and can be blown out of a
tree fairly easily, especially in high, strong winds that occur frequently in
the Nebraska panhandle. Swainson’s Hawks appear to possibly come back and use
the same nest year after year, but if their nest was blown down they will build
another one a couple of trees over, or maybe up to a mile away, but usually stay
within the area. This creates several new nests every year and therefore it
appears there numbers are down, when it probably has more to do with the old
nest being blown down and a new one was constructed near-by.
It appears available trees is at least one driving force in determining where
Swainson’s Hawks will nest and has been observed in several other studies
throughout the years (Schmutz 1984; Bechard et al.1990; Gilmer and Stewart
1984), but we feel it is not the only variable that figures in where they nest.
Another variable that we feel strongly influences where Swainson’s Hawks nest is
the distance between two active conspecific nests. The distance between two
active Swainson’s Hawks nests, in 2007 and 2008, was 2.11 miles and 2.15 miles,
respectively. This distance between the two conspecifics is essential what their
home ranges are and could be the key in determining where Swainson’s Hawks nest.
What we are suggesting is, the likelihood of a Swainson’s Hawk mating pair to
occur in Kimball County, regardless if it is mainly cropland or grassland, is
determined by being a little over 2 miles away from an active nest and an
available tree, is very probable. Inactive nest to an active nest mean distance
backs up this statement considering the mean distance from inactive to active
was 1.35 miles and 1.76 miles, in 2007 and 2008, respectively.
In conclusion, our detectability of Swainson’s Hawks was high, but not perfect,
being around 70%, as nest distance from the road had no effect and tree
structure also had no effect on detectability. Number of active nest was high in
2007, but lower in 2008, which was probably aided by the flimsy nests that
Swainson’s Hawks build, which can easily be blown out of a tree and cause them
to rebuild a new nest the next year, thus causing the old one to be inactive,
giving a higher number of inactive nests. There is no question Swainson’s Hawks
rely heavily on trees to nest in and are using shelterbelts 50% of the time in
western Nebraska, simply because there are significantly more shelterbelts than
any other type of tree structure that is not already occupied by another raptor.
Active conspecific nest distance might be the deciding factor on where this
species nest in western Nebraska, and more specifically Kimball County.
Therefore, two key variables that determine where Swainson’s Hawks nest in
Kimball County are distance between active conspecific nests and tree
availability.
Literature Cited
Bechard, M. J., R. L. Knight, D. G. Smith, and R. E. Fitzner. 1990. Nest sites
and habitats of sympatric hawks (Buteo spp.) in Washington. J. Field Ornithol.
61(2):159-170.
England, A. S., M. J. Bechard, and C. S. Houston. 1997. The Birds of North
America. The American Ornithologists’ Union. No. 265.
Gilmer, D. S., and R. E. Stewart. 1984. Swainson’s Hawk nesting ecology in North
Dakota. Condor 86:12-18.
Mollhoff, W. J. 2001. The Nebraska Breeding Bird Atlas 1984-1989. Nebraska Game
and Parks Commission, Lincoln, NE.
Olendorff, R. R. 1973. The ecology of nesting birds of prey of northeastern
Colorado. U.S. Int. Biol. Program, Grassland Biome, Tech. Report 211.
Schmutz, J. K. 1984. Ferruginous and Swainson’s hawk abundance and distribution
in relation to land use in southeastern Alberta. J. Wildl. Manage. 48:1180-1187.
Schneider, R., M. Humpert, K. Stoner, G. Steinauer. 2005. The Nebraska Natural
Legacy Project: A comprehensive wildlife conservation strategy. Nebraska Game
and Parks Commission, Lincoln, NE.
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Contact:
Bart Bly
Bart.Bly@rmbo.org
State: Nebraska
Funder:
- Nebraska Environmental Trus
- Nebraska Game and Parks Commission
- University of Nebraska - Lincoln
Cooperators:
Materials:
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