The Peregrine Falcon (Falco peregrinus), also known simply as the Peregrine, and historically also as "Duck Hawk" in North America, is a cosmopolitan bird of prey in the family Falconidae. It is a large-sized falcon, females being larger and about the size of a large crow, with a blue-gray back, barred white underside, and a black head and "mustache". About seventeen subspecies are recognized, which vary in appearance and range.

The breeding range includes land regions from the Arctic tundra to the Tropics. Essentially, this species can be found everywhere on Earth, except in the polar regions, on very high mountains, in deserts, and most tropical rainforests making it the world's most widespread falcon, and in fact the most widespread bird of prey. The only major ice-free landmass from where it is entirely absent is New Zealand. Both the English and scientific names of this species mean "wandering falcon" and refer to the migratory habits of some populations of this widespread species.

It feeds almost exclusively on medium-sized birds, but will occasionally hunt small mammals. It reaches sexual maturity at one year, and mates for life. It nests in a scrape, normally on cliff edges or, in recent times on tall man-made structures. The Peregrine Falcon became an endangered species due to the use of pesticides, especially DDT. Since the ban on DDT from the beginning of the 1970s onwards, the populations recovered, supported by large scale protection of nesting places and releases to the wild.

The Peregrine Falcon has a body length of 34–50 cm (13–20 in) and a wingspan of around 80–120 cm (31–47 in). The male and female have similar markings and plumage, but as in many birds of prey the Peregrine Falcon displays marked sexual dimorphism in size, with the female measuring up to 30 percent larger than the male. Males weigh 440–750 g, and the noticeably larger females weigh 910–1500 g; for variation in weight between subspecies, see under that section below.

The back and the long, pointed wings of the adult are usually bluish black to slate grey with indistinct darker barring (see "Subspecies" below); the wingtips are black. The underparts are white to rusty and barred with thin clean bands of dark brown or black. The tail, colored like the back but with thin clean bars, is long, narrow and rounded at the end with a black tip and a white band at the very end. The top of the head and a "mustache" along the cheeks are black, contrasting sharply with the pale sides of the neck and white throat. The cere is yellow, as are the feet, and the beak and claws are black. The upper beak is notched near the tip, an adaptation which enables falcons to kill prey by severing the spinal column at the neck. The immature bird is much browner with streaked, rather than barred, underparts, and has a pale bluish cere.

The scientific name Falco peregrinus, means "wandering falcon" in Latin. Indeed, the species' common name refers to its wide-ranging flights in most European languages. The Latin term for falcon, falco, is related to falx, the Latin word meaning sickle, in reference to the silhouette of the falcon's long, pointed wings in flight.

The Peregrine Falcon belongs to a genus whose lineage includes the hierofalcons and the Prairie Falcon (F. mexicanus). This lineage probably diverged from other falcons towards the end of the Late Miocene or in the Early Pliocene, about 8–5 million years ago (mya). As the Peregrine-hierofalcon group includes both Old World and North American species, it is likely that the lineage originated in western Eurasia or Africa. Its relationship to other falcons is not clear; the issue is complicated by widespread hybridization confounding mtDNA sequence analyses; for example a genetic lineage of the Saker Falcon (F. cherrug) is known which originated from a male Saker producing fertile young with a female Peregrine ancestor some 100,000 years ago.[ref]

Today, Peregrines are regularly hybridized in captivity with other species such as the Lanner Falcon (F. biarmicus) to produce the "perilanner", a somewhat popular bird in falconry as it combines the Peregrine's hunting skill with the Lanner's hardiness, or the Gyrfalcon to produce large, strikingly-colored birds for the use of falconers. As can be seen, the Peregrine is still genetically close to the hierofalcons, though their lineages diverged in the Late Pliocene (maybe some 2.5–2 mya in the Gelasian).

The Peregrine Falcon lives mostly along mountain ranges, river valleys, coastlines, and increasingly in cities. In mild-winter regions, it is usually a permanent resident, and some individuals, especially adult males, will remain on the breeding territory. Only Populations that breed in arctic climes typically migrate great distances during the northern winter.

The Peregrine Falcon is often stated to be the fastest animal on the planet in its hunting dive, the stoop, which involves soaring to a great height and then diving steeply at speeds of over 322 km/h (200 mph) hitting one wing of its prey, so as not to harm itself on impact. A study testing the flight physics of an 'ideal falcon' found a theoretical speed limit at 400 km/h (250 mph) for low speed flight and 625 km/h (390 mph) for high speed flight (Tucker 1998). Despite these theoretical values, measurements of real stoops by using radar resulted in maximum diving speeds of only 140 km/h (87 mph).

The life span in the wild is up to 15.5 years. Mortality in the first year is between 59–70%, declining to between 25–32% in adults. Apart from anthropogenic threats like collision with man-made objects, the Peregrine may be killed by large eagles or large Owls. The Peregrine Falcon is host to a range of parasites and pathogens. It is a vector for Avipoxvirus, Newcastle disease virus, Falconid herpesvirus 1 (and possibly other Herpesviridae), and some mycoses and bacterial infections. Endoparasites include Plasmodium relictum (usually not causing malaria in the Peregrine Falcon), Strigeidae trematodes, Serratospiculum amaculata (nematode), and tapeworms. Known Peregrine Falcon ectoparasites are chewing lice Ceratophyllus garei (a flea), and Hippoboscidae flies (Icosta nigra, Ornithoctona erythrocephala).

The Peregrine Falcon feeds almost exclusively on medium sized birds such as doves, waterfowl and songbirds. Very rarely will it hunt small mammals, including bats, rats, voles, hares, mice and squirrels; the coastal populations of the large subspecies pealei feed almost exclusively on seabirds. Insects and reptiles make up a small proportion of the diet, which varies greatly depending on what prey is available. In urban areas, the Peregrine Falcon catches common city birds such as pigeons and Common Starlings. In many cities, it has been credited with controlling the numbers of birds that are often considered pests, hence alleviating the need for controversial methods such as poisoning or hunting.

The Peregrine Falcon hunts at dawn and dusk, when prey are most active. It requires open space in order to hunt, and therefore often hunts over open water, marshes, valleys, fields and tundra. It searches for prey either from a high perch or from the air. Once prey is spotted, it begins its stoop, folding back the tail and wings, with feet tucked. The air pressure from a 200 mph dive could possibly damage a bird's lungs, but small bony tubercles in a falcon's nostrils guide the shock waves of the air entering the nostrils (compare intake ramps and inlet cones of jet engines), enabling the bird to breathe more easily while diving by reducing the change in air pressure. Prey is struck and captured in mid-air; the Peregrine Falcon strikes its prey with a clenched foot, stunning or killing it, then turns to catch it in mid-air. The Peregrine will drop it to the ground and eat it there if it is too heavy to carry. Prey is plucked before consumption.

The Peregrine Falcon is sexually mature at the end of the first year of age but in healthy populations they breed after two to three years of age. The pair mates for life and returns to the same nesting spot annually. The courtship flight includes a mix of aerial acrobatics, precise spirals, and steep dives. The male passes prey it has caught to the female in mid-air. To make this possible, the female actually flies upside-down to receive the food from the male's talons. The Peregrine Falcon is territorial during the breeding season; nesting pairs are usually more than 1 km (0.6 miles) apart, and often much farther, even in areas with large numbers of pairs. The distance between nests ensures sufficient food supply for pairs and their chicks. Within a breeding territory, a pair may have several nesting ledges; the number used by a pair can vary from one or two to seven in a 16 year period. The pair defends the chosen nest site against other Peregrines, and often against eagles or ravens.

The Peregrine Falcon nests in a scrape, normally on cliff edges or, today regularly in many parts of its range, on tall buildings or bridges. Cliff nests are generally located under an overhang, on ledges with vegetation, and south-facing sites are favored. In some regions, as in parts of Australia and on the west coast of Northern North-America, large tree hollows are used for nesting. Before the demise of most European peregrines, there was a large population of peregrines in central and western Europe using the disused nests of other large birds . The female chooses a nest site, where she scrapes a shallow hollow in the loose soil, sand, gravel, or dead vegetation in which to lay eggs. No nest materials are added. In remote, undisturbed areas such as the Arctic, steep slopes and even low rocks and mounds may be used as nest sites. The man-made structures used for breeding closely resemble the natural cliff ledges that the Peregrine prefers for its nesting locations.

Mostly three to four eggs (range 1-5) are laid in the scrape. The eggs are white to buff with red or brown markings. They are incubated for 29 to 33 days, mainly by the female. The male also helps with the incubation of the eggs over day, but at night only the female incubates. The date of egg-laying varies according to locality, but is generally from February to March in the Northern Hemisphere, and from July to August in the Southern Hemisphere (the Australian subspecies macropus may breed as late as November and equatorial populations may nest anytime between June and December). The female generally lays another clutch if the eggs are lost early in the nesting season, though this is extremely rare in the Arctic owing to the short summer season. As a result of some infertile eggs and natural losses of nestlings, the average number of young found in nests is 2.5, and the average number that fledges is about 1.5.

After hatching, chicks are covered with creamy-white down and have disproportionately large feet. The male, which is called the "tiercel", brings food to the female and chicks, but the chicks are fed by the female, which stays at the nest and watches the young. The hunting territory of the parents can extend a radius of 19 to 24 km (12-15 miles) from the nest site. Chicks fledge 42 to 46 days after hatching, and remain dependent on their parents for up to two months.

The Peregrine Falcon became an endangered species because of the use of pesticides, especially DDT during the 1950s, 60s, and 70s. Pesticide biomagnification interfered with reproduction, thinning eggshells and reducing the number of eggs that survived to hatching. The organochlorine build-up in the falcon's fat tissues would result in less calcium in the eggshells, leading to flimsier, more fragile eggs. In several parts of the world, such as the eastern USA and Belgium, this species became extinct as a result. Peregrine eggs and chicks are often targeted by black marketeers and unscrupulous egg collectors, so it is normal practice not to publicize unprotected nest locations.

The Peregrine Falcon was used in falconry for more than 3,000 years, beginning with nomads in central Asia. Due to its ability to dive at high speeds, it was highly sought-after and generally used by experienced falconers. Peregrine Falcons are also occasionally used to scare away birds at airports to reduce the risk of bird-plane strikes, improving air-traffic safety. The Peregrine Falcon is featured on the Idaho State Quarter, in recognition of its efforts in its recovery.

In the USA, Canada and Germany, Wildlife services in Peregrine Falcon recovery teams breed the species in captivity. The chicks are usually fed through a chute or with a hand puppet mimicking a Peregrine's head, so they cannot see to imprint on the human trainers. Then, when they are old enough, the rearing box is opened, allowing the bird to train its wings. As the fledgling gets stronger, feeding is reduced forcing the bird to learn to hunt. This procedure is called hacking back to the wild. To release a captive-bred falcon, the bird is placed in a special cage at the top of a tower or cliff ledge for some days or so, allowing it to acclimate itself to its future environment.

Worldwide recovery efforts have been remarkably successful. The widespread restriction of DDT use eventually allowed released birds to breed successfully. The Peregrine Falcon was removed from the U.S. Endangered Species list on August 25, 1999.

Many Peregrine Falcons have settled in large cities, including London, Ontario, Derby, Brisbane and Cologne, and all across the U.S., where they nest on cathedrals, skyscraper window ledges, and the towers of suspension bridges. At least 18 pairs nested in New York City proper in 2005. In Virginia, state officials working with students from the Center for Conservation Biology of the College of William and Mary in Williamsburg successfully established nesting boxes high atop the George P. Coleman Memorial Bridge on the York River, the Benjamin Harrison Memorial Bridge and Varina-Enon Bridge on the James River, and at other similar locations. Thirteen new chicks were hatched in this Virginia program during a recent year. Over 250 falcons have been released through the Virginia program. In the UK, there has been a recovery of populations since the crash of the 1960s. This has been greatly assisted by conservation and protection work led by the RSPB. Peregrines now breed in many mountainous and coastal areas, especially in the west and north, and nest in some urban areas, capitalizing on the urban pigeon populations for food.