For The Nest Generation: the Impacts
of
By: Andrea Nebhut
Unless we practice conservation, those who come after
us will have to pay the price of misery, degradation, and failure for the
progress and prosperity of our day.
Every year, dozens of bird species migrate from their winter
homes in the north to warmer grasslands and scrublands in the south, but each
year these flocks grow smaller. According to the 2016 State of the Birds Watch
List, temperate grassland bird populations from across America have declined by
33% since 1970 and are projected to continue to plummet over the next several
decades as expanding development and agriculture continue to destroy these
birds’ winter homes. In turn, birds have shifted from living in natural
ecosystems to living in areas that have been altered by human activity, such as
pastures, hayfields, and abandoned fields. These lands face frequent human
disturbance, such as mowing and the application of agricultural pesticides and
fertilizers, but the cumulative effect of these disturbances on the quality of
bird wintering habitats was not known. It is important that we understand which
management strategies promote suitable winter habitat for the birds, as
high-quality winter habitats allow the birds to return to their summer range
sooner and produce more surviving chicks. Therefore, a group of scientists from
the Smithsonian Conservation Biology Institute and George Mason University lead
by Dr. Amy Johnson set out to find out how habitat structure and the timing of
grassland management affect bird populations.
Overall, the scientists found that fields managed by humans
can be a suitable habitat for many bird species. While more bird species were
found in native warm-season grass fields such as switchgrass compared to
non-native cold-season grass fields like hayfields, there was some overlap in
which species were found in each habitat. Spatially varied, enclosed fields
that had been shaped through practices such as patch-burning and grazing
offered more shelter and food during the winter and were home to more bird
species than more uniform fields with short and open vegetation. In contrast,
most managed fields of non-native cold-season grasses are usually uniform in
habitat structure, which limits the number of bird species that can comfortably
reside there by simply narrowing down the types of potential homes and food
resources. In addition, Johnson’s team
found that fields managed in the winter were home to fewer species than fields
managed in the spring or summer growing seasons. They suggested that when
fields of non-native cold-season grasses are left undisturbed throughout the
growing season, their structure becomes similar to fields of native warm-season
grasses and, as a result, can sustain more bird species. Even so, relatively
uniform fields of spring and summer managed cold-season grasses make excellent
homes for grassland species and there are a handful of flexible bird species
that are suited to both varied fields of native warm-season grasses and uniform
cold-season grass fields. Simply put,
diversity in habitat structure welcomes in a diversity of bird species by
providing a wide variety of potential homes and food sources, which means that
people can support bird populations by planting native warm-season grasses and
managing their fields in the late spring or summer.
While these results may seem obvious, gathering the data to
produce the mathematical model comparing the two habitat types was no easy
feat. Over the course of three years,
Johnson’s team traveled to 25 properties around Virginia, categorized each plot
by the types of plants present and the season when the field plot was last
managed, and surveyed the area for birds. Of the 41 bird species observed, the
scientists selected 16 for analysis based on the available information about
their habitat preferences and the high number of sightings for each. In
addition, the group gathered vegetation information about each plot, including
plant height, the amount of visual obstruction, and the percentage of each plot
taken up by bare ground, grass, woody plants (e.g. trees and shrubs), and
forbs, which are non-woody flowering plants such as wildflowers. With this
data, they created a mathematical model for how likely a given species was to
appear in a field of either non-native cold-season grasses or native
warm-season grasses. Next, they expanded their model to include other variables
such as visual obstruction and percent groundcover. Finally, Johnson’s team
used their model to compare the number of species sighted between the two
habitat types and the different management seasons by simply taking an average
of the number of species estimated by the model for each combination of habitat
type.
With this new information, Johnson’s team recommended that,
whenever possible, farmers should defer their annual mowing to late spring or
summer and replace non-native cold-season grasses with native warm-season
grasses. Together, these two actions will produce a varied landscape with
abundant shelter and food that a diverse range of bird species can call home
following their annual migration south and, in turn, return to their breeding
grounds ready to produce abundant healthy offspring.
References
Cornell University. (2016). State of North America's Birds:
2016. Retrieved from http://www.stateofthebirds.org/2016/habitats/grasslands/
Johnson, A. E., Sillett, T. S., Luther, D., Herrmann, V.,
Akre, T. A., & McShea, W. J. (2019). Effects of grassland management on
overwintering bird communities. The
Journal of Wildlife Management.
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