Short answers, pulled from the story.
Birds are warm-blooded vertebrate animals in the class Aves, defined by feathers, toothless beaked jaws, hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. There are over 11,000 living species split into 44 orders, and more than half are passerine or perching birds.
Birds evolved from earlier theropods, which makes them the only known living dinosaurs. In the modern cladistic sense they are reptiles, and their closest living relatives are the crocodilians, with both forming a clade called archosaurs.
Upon inhalation, 75% of fresh air bypasses a bird's lungs and flows into a posterior air sac, while 25% goes directly to the lungs. On exhalation the stored fresh air is forced into the lungs, so the lungs receive a constant supply of fresh air during both inhalation and exhalation.
Arctic terns were recorded travelling an average of 70,900 kilometres between Arctic breeding grounds in Greenland and Iceland and wintering grounds in Antarctica, with one bird covering 81,600 kilometres. Sooty shearwaters make annual round trips of 64,000 kilometres from New Zealand and Chile to the North Pacific.
Birds range in size from the 5.5 centimetre bee hummingbird to the 2.8 metre common ostrich. These extremes show the size range across the more than 11,000 living bird species.
Birds navigate using several methods. Diurnal migrants use the sun by day, correcting for its changing position with an internal clock, and use a stellar compass keyed to the constellations around Polaris at night. Some species also sense the Earth's geomagnetism through specialised photoreceptors.
Brood parasitism is when an egg-layer leaves her eggs in another individual's nest, and it is more common among birds than any other organism. One hundred species, including honeyguides, icterids, ducks, and the famous cuckoos, are obligate brood parasites that must lay their eggs in other nests.