I have been enjoying predictable but fascinating bird behavior in mid-August on the shores of Cobscook Bay in Lubec. Particularly in late afternoon, flocks of Bonaparte’s Gulls fly in large circles over the land adjacent to the water at heights of 50 to 100 feet or so. They are joined by Tree Swallows, Cliff Swallows and Barn Swallows.
The birds are in a feeding frenzy. But what are the birds eating? There are some dragonflies in the air, suitable for the gulls but not the smaller swallows. The food morsels of interest are flying ants.
Most ants in a colony are not reproductive. At times of the year, reproductive males (called drones) and reproductive females (queens) hatch with wings. They take to the air on nuptial flights, hoping to mate while airborne. If they do not become a meal for a bird, the queens will come back to earth, drop their wings and start a new colony or join an existing one.
The flight season for ants is short but provides largesse for birds that prey on flying insects. At a length of ¾ inch, a flying carpenter ant is a significant tidbit.
Swallows and other flying insectivores spend a good bit of their day in the air. This life style requires efficient flight.
Physics-savvy ornithologists have developed models to predict efficient flying speeds of birds. Field observations largely agree with the models.
The enemy of efficient flight is friction (drag in the parlance of physicists). Minimizing the friction of air or water passing over a moving object is important for race cars, Olympic swimmers and downhill skiers. This type of friction is called profile drag. It becomes increasingly costly the faster an object or person moves.
A less appreciated type of drag is called induced drag. For birds and airplanes, the shape of wings creates the upward force called lift that enables flight. However, air moving across the wing, particularly at the tips, becomes turbulent rather than smooth. These turbulent eddies increase friction and hence decrease flight efficiency. Unlike profile drag, induced drag decreases as flight speed increases.
Back to the swallows. These flying birds are on the horns of a dilemma. Flying fast minimizes induced drag but greatly increases profile drag. Flying slowly makes induced drag stronger but reduces profile drag.
What are birds to do? The flight models predict two solutions. One is to fly at a speed that maximizes the distance flown. This strategy is used by many migrating birds. The other viable solution is to fly at a speed that maximizes the time that can be spent aloft (fuel efficiency). This speed occurs where the sum of the induced drag and the profile drag are the smallest. That is what swallows and other flying insectivores do.
Our flying insectivores (nightjars, swifts, flycatchers, swallows) are the first of our breeding birds to depart in the fall. They need to leave before flying insect abundance declines as cool temperatures begin.
Sometimes, spectacular flocks of these birds can be seen. I’ve seen Tree Swallow flocks in the thousands in August. But a couple of memorable events for me involve Common Nighthawks. One August, I looked out our kitchen window in South China to see at least 500 migrating Common Nighthawks foraging over a hayfield. I had seen such a flock over 40 years ago in Baltimore at an Orioles game. I’m sure the lights of Memorial Stadium attracted flying insects that in turn provided food for the migrating nightjars.
We know more about Common Nighthawk migration now thanks to six GPS-fitted birds tracked by Peter Marra. The birds were tagged in western Canada, flew to Florida, then Cuba and on to Brazil for the winter. Their return flight did not follow the same route as the fall migration.