Pneumatization/honeycomb structure in bones allows for the birds to weigh less and be buoyant
Birds have lightweight feathers on the wings hence allowing for a higher lift to weight ratio.
Birds take in very little water in their bodies hence reducing on the carried weight. This is why birds excrete their waste as uric acid that requires less water to eliminate compared to ammonia
Instead of boned jaws, birds have beaks made of keratin which is lightweight
The breastbone of birds is enlarged allowing for the attachment of muscles involved in flight
