Cornell Lab of Ornithology
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Birds in Forested Landscapes

The Thrush Still Sings
by Stefan Hames

Hermit and Swainson's thrushes stake out territories near campsites, despite an abundance of predators

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.

Our collaborators in the U.S. Forest Service recruited volunteers at more than 150 National Forest campgrounds in 11 states. Using a protocol modified from the Lab's BFL citizen-science project, participants noted the presence of territorial male thrushes, using recorded thrush calls as necessary. 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).

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.

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 SwainsonÕs Thrush to be less abundant in intensively used campgrounds.

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 to attract 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.