Housekeeping Secrets of Swallows

Thanksgiving is a day to keep snug and warm, and so you might think that was what drew me to the southern-hemisphere summer of Argentina, this November.

But at our field site just outside the town of Ushuaia, cupped within the powerful shoulders of blue, snow-capped mountains, the clouds are ominously low. We’re at the tip of South America here—as far south of the equator as Newfoundland is north of it. An icy wind cuts through our clothes as my coworker, Becky Windsor, and I scramble into rain pants and jackets.

Today, Becky and I are visiting a peat farm to check nest boxes of Chilean Swallows, and to install a couple of miniature devices that will give our research project an inside look at secrets of incubation.

These swallows, like their close relatives the Tree Swallows, are migratory birds. Chilean Swallows breed in southern Chile and Argentina and then flee winter’s chill for sunny Bolivia, Brazil, Paraguay, or Uruguay in the north.

At our first nest box—which we come to after scrambling over a few wire fences and past a peat farmer’s vociferous guard dogs—a pair of handsome Chilean Swallows greets us. Their coloring is similar to other Tachycineta swallows—blue head, back, and wings, with white throat, belly, and rump—but these are a more brilliant, iridescent blue than many of their cousins.

The protective pair give us dirty looks when we slip open the box and peer into their nest.

Inside is a painstakingly layered straw-and-feather nest with a tidy little cup holding four perfect white eggs. So fastidious are swallow fathers in keeping house that when I let a feather fly by in the wind, within seconds he dives after it and restores the feather to his home.

Today, we are making our own contribution to the nest, installing coin-sized devices called iButtons. We use a bit of wire to slither one of the small metallic buttons into the nest’s cup, beside the eggs, and we tack a second against the box wall.

Each has been programmed to record the temperature every two minutes. After 48 hours we’ll replace the device and download its data. The buttons track both the ambient temperature of the nest box and the temperature next to the eggs, so we can deduce the incubation scedules of the parents. These data will eventually flesh out our understanding of incubation, which is a delicate balancing act between eggs that must stay warm enough to develop and parents that must be well-fed enough to stay alive.

The data we collect from these Chilean Swallows becomes one more clue in a larger evolutionary puzzle. By comparing results with other Tachycineta swallows living in South and Central America, the Caribbean, and north to Canada, we can start to learn what kinds of pressures have shaped the meticulous process of incubation in birds in general.

In just a minute or two, we finish with the box. The nervous parents have begun circling just above us, not letting our long predator fingers out of their sight.

“Good job,” Becky tells the swallows. “Keep up the good work!”

(Images courtesy Daisy Yuhas.)