Blue-footed Boobies, flocks, Tree Swallows
Answers to readers' questions about foot colors, flying, and ground-foraging
Published: April 25, 2008
Why do Blue-footed Boobies have blue feet? — Christine Strickland, San Diego, California
Blue-footed Boobies perform an elaborate courtship ritual that includes a high-stepping dance and a foot-flashing landing in which the males show off their feet to prospective mates. Females of most bird species use plumage to evaluate the condition of males. It's similar for boobies: Males with bright blue-green feet attract females.
How does foot brightness tell a female that one male is a better mate choice than another? A 2006 study by researchers from Spain and Mexico published in the journal Oecologia determined that the foot coloration was derived from a combination of the structure of the collagen (the protein in connective tissues) just below the outer layer of skin and yellow carotenoid pigments.
Carotenoids are responsible for most yellow, red, and orange plumage in birds, and they are also powerful antioxidants. Birds in poor health utilize carotenoids to boost their immune systems, leaving the pigments unavailable to brighten plumage or soft parts, such as feet and bills. Since pigments must be obtained through food, bright carotenoid-based colors can indicate good nutrition and health.
The investigators reported that males' foot color became duller after only 48 hours without food. The feet quickly regained their original brightness once feeding resumed, confirming the notion that foot color signals good health.
The researchers also experimentally altered males' foot color after their mates had laid one egg. The females paired with dull-footed males laid a smaller second egg than females whose mates had normally colored feet, suggesting that females are constantly evaluating their mates' condition and adjusting their reproductive investment accordingly.
Read more about where feather colors come from.
I've long marveled at how large flocks of birds fly in unison, making turns and reversals of direction simultaneously. How do flocks fly with such precision? — Max Van Ordsel, Granite Bay, California
It was once thought that birds in flocks took cues, following a leader in their elaborate maneuvers. But high-speed photography and computer modeling have determined that no single bird acts as a leader.
Careful observation of wheeling flocks, often as they elude predators, shows that the movements appear fluid and wave-like, beginning at one side of a flock and flowing to another -- a fact that at first puzzled researchers. If one bird is taking a cue from another, a time lag should occur between each individual, and the lag should multiply bird by bird. It was later confirmed not only that birds take cues from adjacent birds, but also that they can somehow see or sense the approach of a direction change from farther away. A move begins with a few birds and accelerates rapidly.
Computer modeling suggests that a "geometry of flight" is at work in which each individual bird operates by rules governing the distance to another bird, whether birds turn toward the center of the flock or not, the presence of a predator, the weather, and atmospheric conditions. The rules help account for some of the amazing synchronicity of flocks, but we still don't understand flocks completely. In his epic documentary series The Life of Birds, naturalist David Attenborough described the coordination of flocks as "one of the unsolved mysteries of ornithology."
I saw Tree Swallows foraging on the ground, apparently picking up insects. Is this unusual? — Patrick Collins, Newton, New Hampshire
Insect-eating swallows are great aerialists and consequently do most of their foraging in the air. But most North American species have been observed gathering food on the ground. (The exception is the Cave Swallow, for which I could not find published reports of ground foraging.)
On cool mornings, small flying insects become grounded, and few fly until the temperature rises. Swallows simply take advantage of the easy pickings. Ground foraging appears to be most common in Tree Swallows, which have a wider diet than other swallows. (They'll eat seeds and berries in addition to insects.)
However, other species can be opportunistic. My friend Dick Wolinski, chair of the Michigan Bird Conservation Initiative, reported in the Wilson Bulletin that he observed a Northern Rough-winged Swallow on a beach feeding on fly larvae that had amassed on a dead fish.
Swallows have also been known to fly through or brush past vegetation to flush insects or dislodge caterpillars, which then dangle from silk, enabling the birds to swoop back and catch them. They may also perch on or hover in front of plants to pick off insects.