Why does preening have to be done?

How do you tell if a swan is a male or female? What type of feathers are long and extend beyond the body for lift? Do birds have a gallbladder? Are Galahs Warm blooded? What part of the body of the Maya and lovebird use for eating and what kind of food? What is a word for Bird like? What is MD state bird? How many eggs can a light sussex sit on? How do you use the word katydid in a sentence? How do pigeons adapted urban arrears? What is a physycle? What do baby flamingos eat? What do herring birds eat? What birds DON'T prey? Do snakes have to lie on their eggs like birds sit on their eggs? When does a male ruby-throat hummingbird get the red band around the neck? What is the longest distance swan? How much do kingfishers weigh? Do humming birds have knees? What characteristics does a bald eagle share with other species in its population?