Pity the poor gnats, midges, leafhoppers, moths, and other flying insects that can see well.
Unlike other insects that do not fly and whose visual capabilities are not well developed, flying bugs are hard-wired to jump when they perceive an approaching object that contrasts sharply with the background.
Several clever insect-eating birds — including American Redstart, Blue-gray Gnatcatcher, Northern Mockingbird, and Painted, Slate-throated, and other Myioborus redstarts — have evolved a strategy to take advantage of the behavior.
While foraging, each flashes its wings, tail, or rump, exposing conspicuous, bright plumage patches. The displays startle the insects, which immediately try to escape. When they do, the birds chase after them and often capture them in flight. Ornithologists call the practice “flush-pursuit foraging.”
Ronald L. Mumme, a professor at Allegheny College in Meadville, Pennsylvania, wondered if conspicuous white spots on the tail of Hooded Warbler play a similar role, so he devised a novel experiment to find out.
He used a broad-tipped marking pen to darken the spots of adult warblers that he captured in northwestern Pennsylvania and then later recorded how they foraged. He also set up high-definition video cameras at nests to document what type of food the warblers brought to their nestlings and how often.
The ink typically lasted only a few days. “After 5–7 days it was difficult to tell that a bird had been treated at all,” he writes in the April 2014 issue of The Auk. “Plumage manipulation also had no lasting effect on feather structure.”
The results provide compelling evidence that the warbler’s incessant tail-flicking helps it capture aerial prey, he says. Not only did experimentally marking birds reduce how often they attacked prey while hovering or flying, but the marked females delivered significantly fewer winged insects, and proportionally more insect larvae, back to the nest.
A version of this story appeared in the August 2014 issue of BirdWatching magazine. Subscribe.
Read the paper
Ronald L. Mumme, 2014, White tail spots and tail-flicking behavior enhance foraging performance in the Hooded Warbler, The Auk: Ornithological Advances 131: 141–149.