 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.