The most species rich order among birds and of all terrestrial vertebrates, they are normally small and colorful, varying in relation to the habitat and ecological niche they occupy. They have a short beak and an unique adaptation on their legs for perching. When the leg bends, a tendon at the back of the leg that connects the muscle behind the tibiotarsus to the bottom of the toes will automatically pull and tighten, causing the foot to curl and stiffen when the bird lands on a limb. This allows passerines to snooze without sliding off their perch. Anisodactyl arrangement refers to a passerine's foot's three forward-pointing toes and one backward-pointing toe. The long rear toe, or hallux, connects to the leg at a point about parallel to the front toes. They have complex muscles in their voice box or syrinx allowing them to sing songs.