Swifts are fast flyers that forage in the air for flying insects. They have streamlined bodies, long, narrow wings, and short, wide bills that they open wide when foraging. Their feet are very small, and all four toes face forward, making them unable to perch on twigs; thus they cling to vertical surfaces when not in flight. Social birds, many swifts forage and nest in groups. They nest in out-of-the-way cavities, chimneys, and crevices, and attach their half-saucer nests to the inside of these areas with sticky saliva. Clutch size is variable among species, and both parents care for the young.