Birds evolved through natural selection to exploit opportunities in the environment. As they did so, they developed abilities to produce sounds which were suitable for their life cycle and
Consider the marine environment. Penguins feed at sea and then come to land to breed and rest. Penguins nest in colonies where pairs and young are closely packed. When adults return from the sea
to feed their young, they must pick them out from a mass of tens of thousands. These birds produce slight individual variations on calls so that parents and offspring can locate each other. Similarly, terns depend
on aquatic food and form breeding colonies on land, and they too can recognize individuals by voice.
Some calls communicate or function across species. In forests, flocks of mixed species feed together with each species making its own contact calls. A mixed flock might include a downy woodpecker,
chickadees, titmice, kinglets and warblers. The effect of the calls is that the flock will move together through the forest with each species typically feeding in a slightly different part of the
Some species sing duets, in a way like humans who finish each other's sentences. In winter and spring, a pair of Great Horned Owls may hoot from different places in their territory.
blend together in two part harmony. Duets are a reliable way to keep contact with a mate in dense forest, or at night, when visual contact is not possible.