Fast and furious: The real lives of swallows

For more on small drones inspired by birds and other flying animals, see SN’s feature “Flying animals can teach drones a thing or two.”

Last summer, biophysicist Douglas Warrick spent eight hours each day patiently sitting on the fence of an Oregon cattle farm. Pointing an alienlike metal wand toward a field, he waited. The rod was a special antenna designed to listen for very small radiotags. Earlier that week, Warrick and his team from Oregon State University had glued these tiny trackers, weighing less than a third of a gram, to 120 barn swallows (Hirundo rustica).

“These birds are the absolute extreme when it comes to flight performance,” Warrick says. “They’re at the cutting edge for what can happen with an avian body plan in terms of flight, yet so much of their behavior remains unknown.”

Barn swallows fly fast and evolved highly acrobatic skills to capture some of nature’s most elusive prey: flies. At the same time, the birds have turbo-charged metabolisms, which require 13-hour stints of hunting with few breaks. Speed, style and stamina wrapped into a small, 17- to 20-gram avian frame. Warrick has partnered with Ty Hedrick from the University of North Carolina at Chapel Hill to document how the barn swallows do it.

Each time a tagged barn swallow darted by Warrick’s hand-held radio antenna, it triggered three high-speed cameras to capture the intimate facets of the bird’s dynamic flying. Lab members are combing through the recordings for details on aerodynamics and energy expenditure, but behavioral patterns have already emerged. For example, individuals do not forage alone; they often travel in pairs. The birds may be gleaning hunting tactics from each other, Warrick suggests. Such advantages may define the species, but these evolutionary concepts have never been explored at this intimate level.

“The unit of natural selection is the individual, but we never measure that most of the time,” says Warrick. “The radiotags gave a sense of the identity of the individuals.”

The team also tracked and recorded five cliff swallows (Petrochelidon pyrrhonota) because of their evolutionary contrasts with barn swallows, says Bret Tobalske, a leader on the Oregon project and director of the University of Montana Flight Laboratory in Missoula. While barn swallows are smaller with long wings made for quick scooting close to the ground, cliff swallows have heavier cores with shorter, stubbier wings made for circling higher in the air in search of insect prey, he says.

Both are agile fliers, but cliff swallows deal with higher wind speeds, so their skillset differs from that of barn swallows. A few meters in front of Warrick’s fence roost, the team planted a second wand on the ground. Called an ultrasonic anemometer, it recorded wind speeds as the tagged swallows flew by.

Both birds likely harvest energy from the air currents, kind of like wind surfers, Tobalske says: “They cue and align themselves in ways that can help move [them] upstream. That energy becomes a bonus because it doesn’t burn calories to gain that speed.”

The researchers hope to use what they learn about swallow flight performance to inspire drones that, for instance, require less battery life to fly for long periods of time.

“Framing our questions from an evolutionary standpoint helps us understand how biophysics can shape these designs,” says Tobalske.