Hummingbirds seem to defy gravity. These tiny fliers can stop in mid-flight, hover, fly backwards, or zip away so fast it appears they simply vanish into thin air, like fairies. To do so, they flap their small but strong, flexible wings at a dizzying rate of 80 beats per second. So fast, that you can hear the characteristic hum of wings cutting through air, but not actually see them move.
The main reason for the hummingbirds' aerial efforts is food. Hummingbirds are nectar specialists, feeding on the sugary, high-energy liquid that plants secrete in their flowers. Hovering gives hummingbirds the ability to efficiently sip nectar where no perches exist. Flying fast between flowers minimizes time between meals, an important factor for an animal that must eat more than one-and-a-half times its weight per day to meet its metabolic demands.
A team of researchers at the U.S. Environmental Protection Agency (EPA) led by Dr. Timothy Lewis, a senior ecologist at the EPA's National Center for Environmental Assessment, are keeping an eye on hummingbird feeding activity to gain insights into how air pollution, particularly exposure to elevated ozone levels, might affect hummingbirds and other kinds of wildlife.
"Hummingbirds are excellent subjects for studying ozone exposure because, in proportion their body size, they have very large lungs. This makes them efficient little air samplers," explains the research team’s Allen Ledbetter, an engineer at the EPA's National Health and Environmental Effects Research Laboratory, located in Research Triangle Park, NC. According to Ledbetter, changes in hummingbird feeding activity could be an indicator of something significant going on with ozone exposure, a red flag to environmental scientists.
Ledbetter and fellow research engineer George Hudson devised an innovative way to track hummingbird feeding activity, using infrared sensors to monitor the tiny birds at feeding stations in North Carolina and Wisconsin. The sensors measure the number of visits hummingbirds make to the feeding stations, and records the amount of time each bird spends feeding. The data are then transmitted to a remote "data logger" for collection and future analysis by a team of EPA statisticians and ecologists.
Setting up field stations in both North Carolina and Wisconsin helped the team determine whether changes in feeding activity are due to high ozone levels or hot weather. Because hummingbirds are territorial and tend to stick to the same general area, statistical analysis from the study contained less measurement error than similar efforts that measured air pollution exposures in people.
What the researchers find from monitoring hummingbirds will help the EPA's overall efforts to study and monitor air quality. The team plans to build on their initial study this summer by determining the size of the birds' feeding range, which may also be influenced by air quality. Whatever the team discovers in the future may have important implications for human health.