Falcon

• 1. Introduction

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  Falcons are birds of prey in the genus Falco, a group of roughly 40 species that live on every continent except Antarctica. One of them, the peregrine falcon, has been recorded diving at 200 miles an hour, making it the fastest-moving creature on Earth. Falcons look like hawks and eagles at a glance, sharing a hooked beak and similar hunting habits, but the two groups are not closely related. The way a falcon kills, the origin of its name, and a series of recent surprises in its fossil record all set it apart from every other raptor. That difference starts with something small: the shape of a beak.

• 2. A Beak Built to Kill

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  A falcon kills with its beak, using a small hook on the side of the bill, called a tomial tooth, to sever the spine of its prey. Hawks, eagles and other large birds of prey belong to a separate, unrelated family called Accipitridae, and they rely on the talons on their feet instead. That single difference in hunting method is one of the clearest ways to tell a falcon from a similar-looking raptor.

  Adult falcons have thin, tapered wings built for speed and sudden changes of direction. Fledglings, in their first year of flying, grow longer flight feathers that give them a wing shape closer to a general-purpose bird such as a broadwing. That extra length makes early flight easier while the young bird is still learning the aerial skills adult falcons depend on to hunt.

  Some small falcons with long, narrow wings are known as hobbies. Others, ones that hover in place while scanning the ground below, are called kestrels. Both names describe a hunting style rather than a strict scientific category, and that same genus spans an enormous range in size, from one of the smallest raptors alive to one of the largest.

• 3. Extremes of Size and Sight

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  The gyrfalcon is the largest falcon alive, reaching up to 65 centimetres in length. At the other end of the scale, the pygmy falcon measures just 20 centimetres, barely bigger than a common songbird.

  As with hawks and owls, female falcons are typically larger than males, a pattern called sexual dimorphism. That size gap lets a mated pair hunt a wider range of prey between them than either bird could manage alone.

  The visual acuity of one falcon species has been measured at 2.6 times that of human eyes, sharp enough to spot small, fast-moving prey at a distance most predators could never match. That kind of eyesight raised a question for eighteenth-century naturalists trying to sort falcons from every other hook-beaked bird in the sky.

• 4. A Name From a Sickle

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  Carl Linnaeus, the Swedish naturalist, introduced the genus Falco in 1758, in the tenth edition of his Systema Naturae. He designated the merlin, Falco columbarius, as the type species that would define the group. The name Falco comes from Late Latin, drawn from falx, falcis, meaning a sickle, a reference to the curved shape of the bird's claws. In Middle English and Old French, the related word faucon was used more loosely, covering several species of captive raptor kept for sport. Falco itself is the largest genus within Falconinae, one of three subfamilies of Falconidae, the other two made up of caracaras and a handful of other birds that also carry the name falcon.

  The merlin was once known as the pigeon hawk, the American kestrel as the sparrow hawk, and the peregrine falcon as the duck hawk, older informal names that hint at each bird's favoured prey rather than its place in the genus.

  A male falcon has traditionally been called a tercel, or tiercel in American spelling. One explanation traces the word to the Latin tertius, meaning third, from an old belief that only one in three eggs in a clutch hatched a male. Other sources instead connect it to the male's size, since he typically weighs about a third less than the female. A falcon chick still in its downy stage, especially one raised for falconry, is called an eyas. That word arose from a mistaken split of the Old French un niais, itself descended from a presumed Latin word for nestling. Falconry, the practice of hunting with trained captive birds of prey, produced much of this specialized vocabulary, but the fossil record tells an even older story.

• 5. Three Family Branches

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  The first of falcons' three or four loose groups is the kestrels, generally small, stocky birds with brown backs, though three African species are grey instead. The fox kestrel and greater kestrel look almost identical at a glance and can really only be told apart by their tail colour. Kestrels feed mainly on rodents, reptiles and insects caught on the ground.

  The second group, the hobbies, carry considerable dark slate-grey in their plumage, with a black patch on the cheek, called the malar area, showing up in nearly every species. Unlike kestrels, hobbies feed mainly on smaller birds caught in flight.

  Third are the peregrine falcon and its relatives, larger and more powerfully built, usually with a black malar patch and often a black cap as well. Their tails stay a fairly uniform dark grey with faint banding and small white tips across the group, whatever the rest of their plumage looks like. These larger falcons hunt mid-sized birds and terrestrial vertebrates rather than insects or rodents.

  A fourth, closely related set, the hierofalcons, meaning hawk-falcons, carry more reddish and brown pigments called phaeomelanins, giving them a strongly patterned, hawk-like appearance. The saker falcon and the gyrfalcon both belong to this group, and their undersides show a lengthwise pattern of blotches, lines or arrowhead marks, in contrast to the horizontal barring typical of the peregrine group. Sorting out how these four groups actually relate to one another took mtDNA evidence, not just plumage, and it delivered some surprises.

• 6. An Ancient, Uncertain Lineage

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  For years, the oldest fossils assigned to the genus Falco dated only to the Late Miocene, less than 10 million years ago, a period when many modern bird genera were becoming recognizable in the fossil record. As of 2021, that estimate has been overturned. The oldest known falconid fossil is now thought to be about 55 million years old, pushing the genus far deeper into the past than researchers once assumed. Based on where both fossil and living Falco species turn up, the genus probably originated in North America, Africa, or possibly the Middle East or Europe.

  Despite their obvious resemblance to hawks and eagles, falcons are not closely related to any other bird of prey. Their nearest living relatives are parrots and songbirds, a connection that has little to do with how any of these birds actually look or hunt. That distant relationship sent researchers to mtDNA to work out how the falcons themselves are related to each other.

• 7. A Genetic Family Tree

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  A study of mtDNA in several kestrel species identified one distinct group, birds carrying a dark stripe across the cheek known as a malar stripe. The greater kestrel and lesser kestrel both lack that stripe and fall outside this tighter grouping. The American kestrel does have a malar stripe, but black feathers behind its ear, a feature never seen in the true malar-striped kestrels, tie it more closely to the hobbies instead. This clade appears to have split from its relatives roughly 2.0 to 2.5 million years ago, during a period called the Gelasian, and likely originated in tropical East Africa.

  The hierofalcons turned out to be a single, closely related group, and hybridization between the larger falcon species happens often. Early studies of mtDNA suggested the hierofalcons split off earliest of all living falcons, but the discovery of a stray nuclear copy of that mtDNA, called a NUMT, overturned the theory. The hierofalcons are actually a young lineage, arising around 2 million years ago, at roughly the same time as the main kestrel radiation, and very little of their fossil history survives.

  Today's American Falco species belong to the peregrine group, and the genus's broader spread across the world was likely Holarctic in origin, beginning somewhere around central Eurasia or northern Africa. At least one lineage had reached North America by the Early Pliocene. The major branches of the modern genus, the typical hobbies and kestrels, the peregrine-hierofalcon complex, and the aplomado falcon lineage, all trace back to a window between 2.4 and 5.3 million years ago. Some groups are far younger. The hierofalcon complex and the peregrine falcon-Barbary falcon superspecies both appear to have evolved within roughly the last 120,000 years. Not every branch of that family tree survived to be studied this way, and the fossil record holds a few outright mistakes as well.

• 8. Bones, Mistakes and Extinctions

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  Falco medius, unearthed in Ukraine and dated to the Late Miocene, is among the oldest fossils confidently placed in the genus. Falco bulgaricus, also from the Late Miocene, comes from Bulgaria, while the younger Falco antiquus turns up in Middle Pleistocene deposits in France. A Cuban kestrel, Falco kurochkini, is known from Late Pleistocene and Holocene remains found on Cuba, in the West Indies.

  A specimen called Sushkinia pliocaena, from the Early Pliocene of Kazakhstan, appears to be a falcon of some kind, though not every fossil once filed under Falco actually belongs there. The genus name Sushkinia was already taken by a prehistoric dragonfly relative, forcing scientists to rename the bird Psushkinia in 2015. A fossil once labelled Falco pisanus turned out to be a pigeon of the genus Columba instead. Another, called Falco readei, is now classed as a paleosubspecies of the yellow-headed caracara, a distant cousin within the wider falcon family rather than a true Falco.

  The Réunion kestrel, Falco duboisi, went extinct around the year 1700, a reminder that even a genus this successful has not survived everywhere it once flew.