Because many water birds, birds of a response to living in a depth of water of different. And has the ability to capture prey in the depths of the water are not equal.
Because of the many kinds of water birds in the bird groups are responding to earn a living in the depths of water are different and have the ability to catch prey in the depths of the water.
as many types of birds, water birds in the group is to respond to the professional level in the depth of the water and have the ability to vary the level of prey in the depth of the water that is not equal.