Passerine birds are the most diverse and abundant group of birds. Containing the majority of living bird species, they inhabit every continent on Earth except Antarctica. What unites them is their tendency to perch and a body form to match. Passerine birds have three toes forward and one backward, which allows them to hold onto perches. Whether perching on branches, cliffs, rocks, fences, or other surfaces, rough skin on the bottom of the foot provides traction. A tendon on the back of each leg tightens to bend the bird's foot so that it curls around the perch. Many passerines spend the night on perches without falling off. Stiff tail feathers help them balance on vertical perches such as tree trunks. Their ability to perch makes a huge range of habitats available to them. A robin in New York City and a bird of paradise in a forest in New Guinea are both passerines.