Birds

How do they live?

The nests of birds vary widely. They can be deep burrows, structures that hang from tree branches, a mere depression in the sand, a hole in a tree or rock, and others. Materials that are used to build and line the nest can consist of sticks, leaves, algae, rootlets, small stones, dirt, mud, and animal materials like horsehair, snakeskin, or feathers. Except for mound builders, all birds incubate their eggs, with period of incubation varying widely from 11 to 80 days.

Birds brood their eggs until hatching. Some bird species are helpless at birth, and must be fed by their parents till wings grow and they learn how to fly and hunt; such birds as these are said to be altricial. Precocial birds, are those that are born feathered, and are able to walk and to feed on their own shortly after hatching - ducks and chickens are well-known birds of this catagory.

Whether precocial or altricial, birds grow rapidly, reaching adult size within one year. A phenomenon that has been found in birds is altruism. In this species, the parents of hungry nestlings may be helped in feeding and guarding their young by other adult birds who have not bred that year and thus possess no young ones. This seems contrary to "survival of the fittest" concepts of evolution, in which species on lineages that do not maximize reproductive output are supposed to go extinct.

However, the "altruistic" helper birds are very often closely related to the parent birds and their offspring - which means that they share many genes with the young birds they are helping to raise. By helping the parents ensure the survival of their offspring, these helpers are thus ensuring that their own genes will be passed on to the next generation, expanding their own gene pool. In this way both the parents andhelpers benefit

The feeding habit of the earliest birds was insectivorous and many of today's birds have a very specialized feeding techniques. Some birds that catch insects on the wing, have wide gapes. There are woodpeckers who dig out wood-boring grubs, thrashers who use their bills to dig in the ground, and the raptorial varieties who feed on animals and who have evolved talons and hooked bills and spoonbills that flip fish out of the water.

From where did they evolve and when?

Birds evolved from reptiles in the Jurassic period (about 150 million years ago). It's ancestor was the Archaeoptryx.