Birds have a smaller total number of bones than mammals or reptiles. This is because many of their bones have fused together (like the middle to lower vertebrae), making the skeleton more rigid. Birds also have more neck (cervical) vertebrae than many other animals; most have 13 to 25 of these very flexible neck vertebrae (this helps them groom their feathers). Birds are are the only vertebrate animals to have a fused collarbone (the furcula or wishbone) or a keeled breastbone.