Vireos
Red-eyed Vireo (and a Toco Toucan)
We think of Red-eyed Vireos as North American birds because they nest here, but they spend more time each year in South America, mingling with resident tropical birds like toucans.
Red-eyed Vireo (and a Toco Toucan)
We think of Red-eyed Vireos as North American birds because they nest here, but they spend more time each year in South America, mingling with resident tropical birds like toucans.
■ It has long been reported that birds have a system of tendons that “automatically” grips a branch when their legs are bent, but this is not true. A recent study found that there is no automatic perching mechanism, and that birds simply balance as they sleep. They sleep with their body shifted farther forward than when they are active, so their weight is balanced over the feet, with their toes loosely draped over the perch, not gripping tightly at all. The ability to balance on a slender, unsteady twig—while sleeping—is just something birds can do (see this page).
A Red-eyed Vireo in active posture (left) and sleeping posture (right)
■ Birds do have a tendon-locking mechanism in their toes, which works something like a plastic cable tie. In a bird’s toes, a rough bumpy surface on each tendon (blue) matches angled ridges on the inside of the tendon sheath (red). When the tendon is pulled tight, the toe curls, the bumps on the tendon lock into the ridges on the sheath, and the toe stays tightly curled with almost no further muscular effort. Raptors use this system to hold their prey, clamping on with an unbreakable grip that requires very little effort. Clearly they can release their grip easily, but exactly how they do that is still unknown.
The toes and tendons of a typical songbird
■ There is no green pigment in American birds. Much of what we perceive as greenish (on vireos, flycatchers, warblers, and others) is a combination of yellow and gray pigment. The three feathers here show the two pigments separately and combined. More intense green colors are produced by a combination of yellow pigment and blue structural color (see this page) or by structural color alone (see this page).
Gray and yellow pigments combine in the middle feather to form a greenish color.