Landscaping
with native vegetation helps local bird species
Yards with plants that mimic native
vegetation offer birds "mini-refuges" and help to offset losses of
biodiversity in cities, according to results of a study published today in the
journal PLOS ONE.
"Native" yards support birds
better than those with traditional grass lawns and non-native plantings.
Researchers conducted the study through
the National Science Foundation's (NSF) Central Arizona-Phoenix Long-Term
Ecological Research (LTER) site, one of 26 such sites around the globe in
ecosystems from coral reefs to deserts, from forests to grasslands.
"To a desert bird, what's green is
not necessarily good," says Doug Levey, program director in NSF's Division
of Environmental Biology. "Arizona birds don't view lush urban landscapes
as desert oases. The foraging behavior of birds in greener yards suggests that
there's less food for them there than in yards with more natural
vegetation."
The research, led by scientists Susannah
Lerman and Paige Warren of the University of Massachusetts-Amherst, and Hilary Gan
and Eyal Shochat of Arizona State University, looked at residential landscape
types and native bird communities in Phoenix, Ariz.
It's among the first to use quantitative
measures and a systematic approach--including 24-hour video monitoring--in
yards to assess and compare foraging behavior of common backyard birds.
The scientists found that desert-like,
or xeric, yards had a more even bird community and superior habitat compared
with moist, or mesic, grass lawns.
"We already know that bird
communities differ, and that there are more desert birds found in a desert-type
yard," says Lerman.
"With this study, we're starting to
look at how different yards function--whether birds behave differently by yard
type. We're doing that using behavioral indicators, especially foraging, as a
way of assessing birds' perceptions of habitat quality between differing yard
designs."
Lerman and colleagues conducted the
experiment in 20 residential yards at least 1.8 miles apart, making it unlikely
that the same birds would visit more than one study yard.
Half the yards were desert-like, while
the others had green lawns.
From February through April 2010,
homeowners removed bird feeders before and during a 24-hour experimental data
collection period.
The researchers set up feeding
stations--seed trays--in each yard to simulate resource patches similar to ones
where birds feed in the wild. Plastic trays contained 0.70 ounces of millet
seed mixed into six pounds of sand. The trays were placed on low stools and
left out for 24 hours.
Later, Lerman removed the trays, sifted
out and weighed uneaten seed to the nearest 0.01 gram. The amount of seed
remaining quantified the giving-up densities (GUD), or the foraging decision
and quitting point for the last bird visiting a seed tray.
Trays were videotaped for the entire
24-hour experiment.
The experiment assumed that an animal
behaving optimally would stop foraging from a seed tray when its energy gains
equal the "costs" of foraging, Lerman says.
Costs include predation risk, digestion
and missed opportunities to find food elsewhere.
As time spent foraging at a seed tray
increases, so do the costs associated with foraging. When a bird first arrives
at the tray, seeds are easy to find, but that gets harder as the tray becomes
depleted.
Each bird makes a decision about whether
to spend time searching in the tray or to move on to a new patch in the yard.
The "giving up" point will be
different for different species and in different environmental conditions.
Birds visiting seed trays in yards with more natural food available will quit a
tray sooner than birds in resource-poor yards.
Since the method only measures the
foraging decisions for the last species visiting the seed tray, the researchers
devised a mathematical model for estimating the foraging decisions for all
visiting species.
Using the videotapes, they counted every
peck by every bird for each tray to calculate the relationship between the
number of pecks and grams of seed consumed for each seed tray. This was the
GUD-peck ratio for the last species visiting the seed tray.
They then estimated the seed
consumption--GUD ratio for all other species visiting the seed tray based on
the number of pecks per tray when each species quit.
"We know how many pecks each
species had and can put that number into the model and calculate the number of
grams at that point," Lerman says. This greatly enhances the GUD method by
expanding the ability to assess foraging decisions for all species visiting
trays.
In all, 14 species visited the trays, 11
of which visited both yard types. Abert's towhee, curve-billed thrasher (a
species unique to the Sonoran desert), house finch and house sparrow were the
most widespread tray visitors.
Species that visited trays in both yard
designs consumed more seed from trays placed in mesic yards, indicating lower
habitat quality compared with xeric yards.
Similarly, foragers in the desert-like
yards quit the seed trays earlier due to greater abundance of alternative food
resources in those yards, spending more time foraging in the natural yards and
less at the seed trays.
Lerman says that by videotaping the
trays, counting pecks and measuring giving-up points by species, the research
also advanced the GUD method, allowing researchers to disentangle some of the
effects of bird community composition and density of competitors, and how these
factors affect foraging decisions between two different landscape designs.
The results build upon evidence that
native landscaping can help mitigate the effects of urbanization on common
songbirds, she says.
-NSF-
Posts
