Par for the Course

A preliminary look at the breeding success of Tree Swallows and Eastern Bluebirds

By Tina Phillips and Caren B. Cooper

There are more than 16,000 golf courses across the United States -- open green areas that are attractive to aerial insectivores such as Eastern Bluebirds and Tree Swallows. In addition to feeding on insects around golf courses, cavity-nesting birds may find places to nest, thanks to the efforts of bird lovers who put up nest boxes. But are golf courses suitable for cavity-nesting birds, especially where pesticides may place them at greater risk of exposure to potentially harmful chemicals?

In 2001, The Birdhouse Network introduced the Pesticide Study to evaluate whether areas where pesticides are used, such as golf courses and agricultural fields, are good habitats for cavity-nesting birds to raise their young. The study asked participants to provide information about pesticide applications near nest boxes, in addition to data on breeding success. Given the documented negative effects of pesticides on a variety of animals, from frogs to birds to mammals, we expected that birds nesting in areas where pesticides were applied would have poorer nesting success than those in areas without pesticides, either directly from chemical-induced abnormalities or indirectly from lower prey abundance. However, a preliminary analysis of all reported habitats, including golf courses, indicates no significant differences in clutch size, brood size, or number of fledglings of Tree Swallows and Eastern Bluebirds nesting in areas with and without pesticides.

Participants collected data for the Pesticide Study between 2001 and 2004 from across the continent and reported on the nesting success of 13 cavity-nesting species. Because of the small sample sizes for many of the species, we limited this initial analysis to Eastern Bluebirds and Tree Swallows, two of the most prevalent cavity nesters inhabiting golf courses.

Photo by Diane L. Tessaglia-Hymes/CLO

More than 400 nest-box monitors provided information about the presence of pesticides within 100 feet of nest boxes, the type of pesticide used, the frequency and start of application, and the crop type, if any. Participants also reported standard data on breeding, such as the date when the first egg was laid and the number of eggs, chicks, and fledglings. We limited the analysis to nest attempts with a known first-egg date, a total of 1,399 attempts for Eastern Bluebirds and 866 attempts for Tree Swallows.

When we analyzed all habitats together, we found no differences in clutch size, brood size, or number of fledglings of Tree Swallows and Eastern Bluebirds in areas with and without pesticides. In this crude analysis, failing to detect any overall negative effects of pesticide application near nest boxes is not altogether surprising, because many factors influence breeding success. For the effects of pesticides to be obvious, we must detect variation above and beyond that caused by factors such as latitude, season, and habitat. Yet, even after controlling for latitude and season, we find equal reproductive performance in areas with and without pesticides.

No significant differences were found in hatch rates of Tree Swallows and Eastern Bluebirds among four different habitats where pesticides were used. Fledge rates showed a similar trend. The hatch rate is the proportion of eggs that hatch from nests with at least one nestling.

Habitat is an important confounding variable in interpreting results of the Pesticide Study. We looked at the reproductive performance of Eastern Bluebirds and Tree Swallows in four different habitats -- agricultural areas, golf courses, parks and fields, and residential areas. We found that fledge rates and hatch rates were slightly higher on golf courses. This indicates that the larger variation arising from differences in habitat may override the smaller variation potentially created by pesticide application.

Why do golf courses appear to be such good habitats for some cavity-nesting birds -- a finding noted by other researchers as well? A recent study conducted by Dan Cristol at the College of William and Mary in Virginia examined the nesting success of Eastern Bluebirds on and off golf courses and also found that overall productivity was about the same. Interestingly, in Cristol's study, golf course birds laid slightly more eggs and produced more fledglings, but also experienced more nest failures. A study by Mark Stanback at Davidson College in North Carolina found slightly, but not significantly, poorer reproductive performance of birds nesting on golf courses, with the exception of chick condition, which he found to be significantly poorer on golf courses. Although preliminary, our study spans the entire breeding ranges of Eastern Bluebirds and Tree Swallows and corroborates the findings of the local studies by Cristol and Stanback. On golf courses in any region, some combination of currently unidentified features may override significant detrimental effects of pesticides. Stanback suggested one feature that might make golf courses particularly attractive to Eastern Bluebirds: insects might be easier to find in the short grass of golf courses than in the tall grass of hayfields.

Another reason may have to do with predation. We found that in boxes with at least one egg laid, nearly 80 percent of all nest boxes occupied by Eastern Bluebirds on golf courses were equipped with predator guards, compared with only 50-60 percent in the other habitats. We found that nest boxes with predator guards in agricultural areas, parks, and residential areas fledged more young than those without guards. We could not make a similar comparison for golf courses because of the small sample size for boxes on golf courses without guards. These results underscore the importance of providing boxes with predator guards, especially in predator-prone areas such as parks. They also point to the difficulty of teasing apart the effects of predator guards and pesticides on golf courses, since there is little opportunity for comparison (most golf courses have pesticides and predator guards).

Before nest-box monitors hit the links with a cart full of nest boxes in tow, they should familiarize themselves with the golf course environment, starting with the golf course superintendent, who is interested foremost in providing the best experience possible for golfers. Golf course superintendents need to know the benefits of having nest boxes on the course, such as an increase in the number of insect-eating birds. Other practical aspects of golf course trails also need to be considered, such as placement of boxes and devising a monitoring schedule that minimizes interference with golfers. Although our preliminary results suggest that golf courses do provide suitable habitat for cavity-nesting birds, there are still many consequences that we are unable to examine. For instance, we do not know whether fledglings from golf courses have higher survivorship than those in other habitats, or whether disturbance from golfers has any effect on nesting success.

As urbanization continues to expand into once-natural areas, wildlife will by necessity have to make use of managed areas such as golf courses. For some species, golf courses may be the best available habitat to nest. The Birdhouse Network will continue to explore how habitats affect nesting succes. We ask that nest-box monitors continue to enter data about pesticide use into The Birdhouse Network's database. For instructions on how to do this, go to www.birds.cornell.edu/birdhouse/getting_started/quickinst.html.

Tina Phillips is project leader of The Birdhouse Network. Caren B. Cooper is a research associate in the Lab's Bird Population Studies program.

For permission to reprint all or part of this article, please contact Miyoko Chu, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-2451. email: mcc37@cornell.edu