Incubating the eggs
Birds now have to pay the penalty for being endothermic, for reptiles can bury their eggs and abandon them. Bird's eggs like the adults themselves, need to be kept at a constant temperature which is usually several degrees above ambient temperatures. Birds therefore incubate their eggs. Some birds just before egg-laying moult a group of feathers on their undersides and expose a bare patch of skin which becomes distended with minute blood vessels. The eggs are kept against this patch and kept at the same temperature as the parent bird. But not all birds produce this patch by moulting. Ducks and Geese mechanically pluck out their own feathers. The blue-footed Booby (Sula nebouxii; Sulidae), not only uses its feet for display but also uses them as insulators.
Bird Incubating eggs