Earth

Earth is the fifth largest planet in the solar system, third in order of distance from the Sun. It is the largest of its planetary system's terrestrial planets and the only place in the universe known to support life.

The most prominent features of the earth's climate are its two large polar regions, two relatively narrow temperate zones, and a wide equatorial tropical to subtropical region. Precipitation patterns vary widely according to location, ranging from several meters of water per year to less than a millimeter. About 70 percent of the surface is covered by salt-water oceans. The remainder consists of continents and islands, with the vast majority of the inhabited land in the Northern Hemisphere.

Earth has evolved through geological and biological processes that have left traces of the original conditions. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans, which at least several times have changed relatively quickly. The interior of the planet remains active, with a thick layer of molten Earth mantle and an iron-filled core that generates a magnetic field

Birds

Many species of bird undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviors including cooperative hunting, cooperative breeding, flocking and mobbing of predators. Birds are primarily socially monogamous, with engagement in extra-pair copulations being common in some species-other species have polygamous or polyandrous breeding systems. Eggs are regularly laid in a nest and incubated and most birds have an extended period of parental care after hatching.

Birds are economically important to humans: many are important sources of food, acquired either through hunting or farming, and they provide other products. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120-130 species have become extinct as a result of human activity since 1600, and hundreds more before this. Currently around 1,200 species of birds are threatened with extinction by human activities and efforts are underway to protect them.