nighthawk of North America flies in swift-like fashion on its long, slender, pointed wings. The common poorwill, on the other hand, flies low and perches low to the ground and will sally up into the air after insects. Opportunistic feeding Many other birds are known to engage in hawking as an opportunistic feeding technique or a supplemental source of nutrition: among these are the cedar waxwing, which mostly eats fruit but is also often observed hawking insects over streams; terns of the genus Chlidonias, such as the black tern, fly in search of insects, sometimes chasing after dragonflies in flight; and even large owls that normally feed on rodents will snatch flying insects when the opportunity arises. 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.