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For Baby Swallows, What You Eat Is As Important As How Much You Eat

By Andrew Dreelin

Purple Martin by Andrew Dreelin.
Male Purple Martin by Andrew Dreelin.

Andrew Dreelin (’17) majored in Biological Sciences at Cornell University. This  story about his experience as an undergraduate science student was made possible by the Cornell Lab of Ornithology Science Communication Fund, with support from Jay Branegan (Cornell ’72) and Stefania Pittaluga.

The male Purple Martin in my hand was gorgeous—glossy and purple-black, watching me with dark darting eyes. It felt powerful, like it was ready to burst out of my hand at any second. I worked quickly, attaching a postage-stamp-sized data logger to the bird’s back using a tiny harness, and let the bird go. Its scimitar-shaped wings flicked open, and the bird rocketed out over the shining dawn water.

That martin was one of 39 Tree Swallows, Barn Swallows, and Purple Martins I captured that summer in the Finger Lakes region of New York as part of my senior thesis. The swallows quickly won my heart, but choosing them as a research topic was motivated by concern as much as fascination.

Swallows and other aerial insectivores—the term applies to birds like nightjars, swifts, and flycatchers, too—are one of the most steeply declining groups of birds in North America, according to the State of the Birds Report 2014. Four species—Common Nighthawk, Chuck-will’s-widow, Least Flycatcher, and Bank Swallow—have each lost more than half their global population since 1970 and are listed as Common Birds in Steep Decline by Partners in Flight. Six of North America’s eight swallow species are declining. The causes aren’t clear, but work by my undergraduate adviser, David Winkler of Cornell University, and his students is starting to connect the ecological dots, pointing to problems in the food chain.

Female Purple Martin by Andrew Dreelin
Aerial insectivores divide up the airspace—Purple Martins, like this female, forage higher than many other species. Photo by Andrew Dreelin.

The common thread linking this disparate group of birds is their prey: flying insects. Each type of bird specializes in a different part of the airspace. Flycatchers and some nightjars catch insects near the treetops by “sallying” to and from a fixed perch. Swallows and swifts are aerialists; they “hawk” for insects, twisting and turning high in the air or low over meadows to nab their prey. And they’re incredibly good at it.

“Swallows are the most magical aerial creatures that we have,” says Winkler, who has studied them for more than 30 years. Once, he recalls, he watched a Tree Swallow sit on top of its nest box during a rainstorm, turning its head quickly from side to side. The bird was catching individual raindrops as they fell.

Swallows use these incredible reflexes every day to find flying insects. To make their task more complicated, any shift in temperature, air pressure, or wind speed can change how many insects are aloft and where they are. The aerial environment changes like the weather because that’s what it is.

Winkler and other ornithologists have decades of data on what swallows do in nest boxes, but there’s a lot less known about what they do in the air, beyond a few basics: warm temperatures bring more insects into the air; a run of cold, wet days can cause chicks to starve.

To learn more about their aerial ecology, I spent my summer putting data loggers on swallows and martins. The tags record the air pressure surrounding the bird once per minute, and that lets me calculate their altitude, helping me understand the aerial behavior of each species. Putting on the tags wasn’t hard—the main challenge was catching and recatching the birds to get the data.

Barn Swallow ready for release carrying a data logger. Photo by Andrew Dreelin
A Barn Swallow is ready for release with its data logger backpack. Photo by Andrew Dreelin.

Tree Swallows and Purple Martins were easy—they nest inside boxes, so I mainly needed patience and a plastic flap to cover the nest entrance once a bird went inside. Barn Swallows were tougher: we had to array mist nets across barn entrances and wait for the birds to fly into them. The birds’ sharp eyes could pick out the fine weave of the net even in the morning light. Despite my mounting frustration each time they made 180-degree turns to dodge the net at the last second, it was hard not to be impressed.

The main pattern from my analysis suggests that swallows and martins divide up the aerial environment into distinct bands like layers on a layer cake. Barn Swallows fly lowest, Tree Swallows in the middle, and Purple Martins highest. Just as flycatchers, nightjars, and swallows claim different parts of the airspace, these swallows subdivide their portion of the sky. This may even mean they have different diets, since many insects are also distributed in altitudinal bands in the air.

Different insects in a swallow’s diet could spell the difference between having hefty or scrawny chicks. Recent work has shown that what a swallow chick eats can be more important than how much it eats. Two of Winkler’s graduate students, Lily Twining and Ryan Shipley, explored this idea by focusing on omega-3 fatty acids, which are thought to be “critical for nervous tissue development, cardiac function, immune function, hormonal regulation, and more,” according to Twining.

These are the same kinds of oils we get by eating fish like salmon and mackerel. Swallows get them from eating flying insects that, as larvae, live in the water—such as mayflies, caddisflies, and dragonflies. These stream-living species contain more fatty acids than terrestrial insects do.

To test whether omega-3s had any effect on chick development, Twining and Shipley raised several broods of Tree Swallow chicks in the lab and fed each brood one of four distinct diets:

  • Lots of food that was high in omega-3s
  • Lots of food that was low in omega-3s
  • Smaller amounts of food that was high in omega-3s
  • Smaller amounts of food that was low in omega-3s

For two weeks, they got up early and stayed up late to feed the growing chicks promptly every 15 minutes from 6 a.m. to 10 p.m. Their hard work paid off, and the two researchers found that chicks fed on omega-3s developed faster than chicks without. Even the chicks that received less food overall still developed faster than chicks that got more food but lacked omega-3s.

The bottom line is that the amount of omega-3 fatty acids that chicks receive from their parents is likely make or break for Tree Swallow reproductive success. Taking their research a step farther, Twining and Shipley repeated the experiment with Eastern Phoebes, another aerial insectivore, and found similar results.

A logical thread emerges: if aerial insectivores need to provide their chicks with lots of omega-3s for them to develop properly, they probably need to catch a lot of aquatic insects. In that case, declines in wetlands and stream health could reduce aquatic insect populations and thereby bring down the breeding success of aerial insectivores. In the search for answers to the widespread declines of these species, could this be a lead? It’ll take more studies to find out, but I’m happy to have played a small part in researching the lives of our “most magical aerial creatures.”



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