SPRING 2002/VOLUME 16, NUMBER 2
By STEFAN HAMES
Hermit and Swainson's thrushes stake out territories near campsites, despite an abundance of predators
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Swainson's Thrush
George West, Birchside Studios, www.birchsidestudios.com |
Each year hundreds of thousands of people visit United States National
Forests for recreational opportunities such as hiking, fishing, biking,
river rafting, bird watching, and camping. Although numerous studies have
documented the effects of forest management for timber production, few
have addressed the impact of recreational activities on breeding forest
birds. The Birds in Forested
Landscapes (BFL) Recreation Study, a joint project of the Lab of Ornithology
and U.S. Forest Service, seeks to determine whether campgrounds provide
suitable habitat for breeding thrushes and whether the influence of campgrounds
discourages breeding at nearby forested sites.
Campgrounds could have a negative impact on forest birds in a number of
ways. The normal noise generated by traffic and campsite activities might
interfere with breeding activities of secretive species. Altering vegetation
at the campground and introducing invasive exotic plants might make the
area less suitable for ground- and shrub-nesting birds. The numbers of
nest predators such as crows, jays, chipmunks, and squirrels might reach
artificially high levels when these animals are subsidized with food from
improperly stored garbage, table scraps, or intentional feedings. Pet
dogs that are allowed to roam free may also disturb the area's birdlife.
All of these activities have the potential to reduce the nesting success
of birds breeding in the vicinity of campgrounds. The BFL Recreation Study
tests whether they actually do.
The study compared the rates of attempted breeding by thrushes at three
points of interest: in the center of the campground, at the edge of the
campground in the forest 500 feet (150 m) from the campground's edge,
and at a control site in the same forest patch between 1,500 feet (460
m) and 5 miles (8 km) from the edge site. All three sites were matched
for forest type, elevation, and presence or absence of surface water.
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| Nest predators
such as squirrels were found more often around campsites than at control
points.
Cindy Page
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To help pinpoint potential causes of reduced breeding, the volunteer citizen scientists who carried out the surveys identified and counted predators and gathered data on vegetation and forest fragmentation. Forest Service personnel recorded information on factors such as campground size, intensity of usage, use of garbage containers, and presence of invasive exotic plants.
Although the project is ongoing and not all of the 2001 data are in yet, interim analyses have yielded a few surprises. Because Swainson's and Hermit thrushes are more likely to breed farther away from forest edges and in larger forest patches, we expected to find fewer thrushes in and around campgrounds. However, we found no effect of location (point type) on the number of territorial Hermit and Swainson's thrushes. Territorial males were found at about 30 percent and 38 percent respectively of all points, regardless of whether they were within campgrounds, edge habitat, or the relatively undisturbed habitat of the controls (Figure 1).
As expected, however, we found that nest predators tended to be present at more campground sites than at controls (Figure 1). We also found trends, though not statistically significant (0.05<p<0.10), showing that the Hermit Thrush was less abundant as the number of campsites increased, and for the Swainson's Thrush to be less abundant in intensively used campgrounds.
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| Figure
1. Percentage of study sites with at least one territorial male Hermit Thrush during 2001. The percentage of sites was not significantly different among the three different site types. Lines show the percentage of sites at which mammalian or avian predators were detected. The percentage of sites with predator detections varied significantly depending on the site types. |
These preliminary results provide little evidence to support the hypothesis that campgrounds in National Forests discourage thrushes from attempting to breed. It is also worth noting that almost all of the study sites are in heavily forested regions with little fragmentation, and that under such circumstances, thrushes may be found in most forest patches. However, the study showed that both mammalian and avian nest predators are more abundant in campgrounds, a factor that could potentially contribute to decreased nesting success.
We have begun using playbacks of mobbing calls toattract birds so that
we can count fledglings and adults carrying food, a better measure of
breeding success. We also hope to get a clearer picture of the impact
of campgrounds on breeding efforts of thrushes by studying more National
Forests located near metropolitan landscapes. Additionally, increasing
our sample size will improve our power to detect differences among point
types. These methods will help increase our certainty about our results,
but based on our interim analysis, it looks like you can bird watch and
camp in our National Forests without unduly upsetting the thrushes breeding
there.
Suggested citation: Hames, Stefan, The Thrush Still Sings. Birdscope, newsletter of the Cornell Lab of Ornithology, Spring 2002. <www.birds.cornell.edu>