Birds can be helpful to each other in many ways. Among the most familiar form of cooperation is when an owl or hawk appears in the neighborhood, causing every songbird within a quarter of a mile to sound alarm calls and even dive at the predator. Perhaps the most interesting form of help is when young birds of the first brood of the year help their parents raise the second brood of the year. There have been many reports of juvenile eastern bluebirds carrying food to the nesting houses containing their parentsí second brood.
American crows and Florida scrub-jays demonstrate some of the same cooperative behavior, when sexually immature one-year-olds help their parents gather nesting material, protect the territory, and carry food for the next generation.
An even more extreme example of cooperation occurs among acorn woodpeckers of the Far West, when groups of up to 10 birds live together, year-round, jointly defending the communal territory, gathering food, and together incubating eggs and feeding the nestlings of each pair in the group.
Sometimes birds of different species lay eggs in the same nest. This has happened among American robins and northern cardinals. The two species take turns incubating eggs, sometimes sitting side-by-side, and jointly feed the young when they hatch.
There are also adoptions. For example, a male Carolina wren started feeding an incubating female house wren. When the eggs hatched, the male Carolina continued by feeding the babies, causing both of the true parents to desert, leaving the rearing of their young to the foster father.
-- George H. Harrison