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The avian immune system is the system of biological structures and cellular processes that protects birds from disease. The avian immune system resembles that of mammals since both evolved from a common reptilian ancestor and have inherited many commonalities. [1] They have also developed a number of different strategies that are unique to birds.
The hummingbird family has the third-greatest number of species of any bird family (after the tyrant flycatchers and the tanagers). [ 9 ] [ 1 ] Fossil hummingbirds are known from the Pleistocene of Brazil and the Bahamas , but neither has yet been scientifically described, and fossils and subfossils of a few extant species are known.
Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions. [294] Many homeowners erect bird feeders near their homes to attract various species. Bird feeding has grown into a multimillion-dollar industry; for example, an estimated 75% of households in Britain provide food for birds at some point ...
As an example, the great tit, a European songbird, uses such a signal to call on nearby birds to harass a perched bird of prey, such as an owl. This call occurs in the 4.5kHz range, [66] and carries over long distances. However, when such prey species are in flight, they employ an alarm signal in the 7–8 kHz range.
The number of cells in these groups vary with species; it has been estimated that the human body contains around 37 trillion (3.72×10 13) cells, [7] and more recent studies put this number at around 30 trillion (~36 trillion cells in the male, ~28 trillion in the female).
Bats are more agile in flight than most birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal , is Kitti's hog-nosed bat , which is 29–34 millimetres ( 1 + 1 ⁄ 8 – 1 + 3 ⁄ 8 inches) in length, 150 mm (6 in) across the wings and 2–2.6 g ...
Most birds have approximately 175 different muscles, mainly controlling the wings, skin, and legs. Overall, the muscle mass of birds is concentrated ventrally. The largest muscles in the bird are the pectorals, or the pectoralis major, which control the wings and make up about 15–25% of a flighted bird's body weight.
Ornithology is a branch of zoology that concerns the study of birds. [ 1 ] Several aspects of ornithology differ from related disciplines, due partly to the high visibility and the aesthetic appeal of birds. [ 2 ] It has also been an area with a large contribution made by amateurs in terms of time, resources, and financial support.