Birds are encountered on basically any island in the world of Salt. They come in 6 varieties: White, Black, Red, Yellow, Green, and Blue. The Blue variety is known as Old Blue, is bigger than the other varieties and has a lot more health.
Birds only drop 1-2 feathers of their respective colour, which can be turned in to a Sea Collector for rare items.
Due to their low health (2 hp), it is best to hunt birds using Avian Arrows, as they're cheap and deal exactly the amount of damage to kill a bird in a single shot.