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 necessarily 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 Tyrannidae 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