Birds have a lightweight skeleton made of mostly thin, hollow bones. The keel-shaped sternum or breastbone is where the powerful flight muscles attach to the body. Birds have a smaller total number of bones than mammals or reptiles. This is because many of their bones have fused together making the skeleton more rigid. That is also beneficial when a bird wants to take to the air. Birds also have more neck (cervical) vertebrae than most other animals; most have 13 to 25 of these very flexible neck vertebrae. Birds are the only vertebrate animals to have a fused collarbone called the furcula or wishbone and a keeled breastbone. One species that has this feature would be the Buff-breasted Sandpiper preening. The bird's skeleton is extremely lightweight, but strong enough to withstand the difficulties that a bird experiences when taking off, flying, and landing. The bird's hollow bones make the skeleton lighter for flying, and allow more oxygen to be taken in from the air into the blood for the extra energy that a bird needs. Bird bones usually contain air. Many bird bones are hollow, with struts or trusses called cross walls that crisscross for structural strength. However Some flightless birds like penguins have only solid bones.