Lightweight bird skeletons minimize the energy required for flight. Other adaptations like fused bones and internal truss supports also improve skeleton stiffness and strength, other features necessary for flight. Soaring birds have the lightest skeletons and are most likely to have feathers that are heavier than their skeleton.
Flying wears flight feathers down and they need to be replaced. This process, called molting, is symmetrical across all bird’s wings and regulated to maintain balance and the bird’s ability to fly.
Hairballs for all cats result from grooming and swallowing hair. This hair is normally past through the digestive system, but sometimes can accumulate and result in a hairball.
Both dromedary (one hump) and Bactrian (two hump) camels can open and close their nostrils to keep sand out. They also sport bushy eyebrows and long eyelashes to help keep the desert elements at bay.