Hawking (birds) - Physical Adaptations

Physical Adaptations

Hawking insects, like any feeding strategy, must provide a bird with sufficient nourishment to make the expenditure of energy worthwhile. The strategies and tactics for feeding on airborne insects are inextricably related to the adaptations and lifestyles of the birds that employ them.

Flight, especially flight driven by the muscle-powered flapping of wings, is a strenuous physical activity. Although a sally from a perch may look like a single, rapid movement to the human eye, actually the bird must perform several moves: it begins its take-off by pushing with its feet to get into the air, it flaps its wings to generate forward motion (thrust), pursues the prey item, turns in the air, flies back, and, with a final flurry of wings, lands on its perch. When a bird hawks insects, the prey must be substantial enough to pay off in terms of a biological energy budget. In other words, the bird must take in more energy in food than it is using up in the pursuit of food. Therefore, flycatchers tend to prefer insect prey of moderate size, such as flies, over smaller insects like gnats.

For birds that live in a forest habitat or other setting where short bursts of flight are used in sallies or for getting from tree branch to tree branch, their short, rounded wings are suitable for the rapid flapping required to maneuver in tight spaces. Birds in more open settings that sally after larger insects like bees, such as kingbirds and bee-eaters, benefit from longer, more pointed wings, which are more efficient because they generate more lift and less drag. Swallows and swifts, which glide about in totally open spaces, have even longer wings. Another function of long, pointed wings is to enable these birds to turn quickly and smoothly in mid-glide. The wingtips create little vortices of air, within which the low air pressure creates additional lift on the wingtips. Furthermore, long, forked tails provide additional lift, stability, and steering ability, which is important for flying at slower speeds (swifts, though capable of flying very fast, actually must fly relatively slowly to intercept airborne insects). In fact, swifts have bodies so well adapted for flying that they are unable to perch on branches or land on the ground, and so they nest and roost on precipices such as rocky cliffs, behind waterfalls (as the Black Swift of North America and the Great Dusky Swift of South America are known to do) or in chimneys, as in the case of the Chimney Swift.

Bill size and shape is also important. Compared to the bills of birds specialized for gleaning, a relatively larger, broader bill is ideal for catching sizeable insects such as bees and flies. The presence of bristles near the bill (rictal bristles) in some flycatchers may be an adaptation for hawking insects; scientists are not sure of the function but they may help protect the eyes or they might actually help provide the bird sensory information as to the location of the prey. Swallows, swifts, and nightjars do not have large bills, but they have wide-gaping mouths. Some nightjars also have bristles around the bill (the Common Poorwill does, the Common Nighthawk does not).

When different kinds of birds have the same adaptations, such similarities are not necessaily indicative of any familial relationship between bird species. Rather, they are the result of convergent evolution. Consider, for example, the marked resemblance in body size, shape, and coloration between flycatchers of several families, though these species are not closely related: the Asian Brown Flycatcher (of the Muscicapidae or Old World flycatcher family), Acadian Flycatcher (of the Tyrranidae or Tyrant flycatcher family) of the New World, and Slaty Monarch (of the Monarchidae or Monarch flycatcher family), endemic to Fiji. All three use flycatching to acquire some or all of their food. But these three families belong to separate branches of the evolutionary tree of songbirds, which diverged in two branching events some 60 and 90 million years ago and continued to evolve independently in different parts of the world. Likewise, the similarities of swifts and swallows once lead naturalists to conclude they were related, but it is now established that they are unrelated, and that the same lifestyle has led to the same adaptations.