They are swift fliers and skilled hunters which specialize in preying on small birds in the size range of sparrows to doves and medium-sized shorebirds.

The merlin was described and illustrated by the English naturalist Mark Catesby (as the "pigeon hawk") in his Natural history of Carolina, Florida and the Bahama Islands published in 1729–1732.

[4][5] Based on this description, in 1758 Carl Linnaeus included the species in the tenth edition of his Systema Naturae and introduced the present binomial name Falco columbarius with the type locality as "America".

[6] The genus name is Late Latin; falco derives from falx, falcis, a sickle, referring to the claws of the bird.

[8] Thirteen years after Linnaeus's description Marmaduke Tunstall recognized the Eurasian birds as a distinct taxon Falco aesalon in his Ornithologica Britannica.

Indeed, the merlin seems to represent a lineage distinct from other living falcons since at least the Early Pliocene, some 5 Ma (million years ago).

As suggested by biogeography and DNA sequence data, it might be part of an ancient non-monophyletic radiation of Falco species from Europe to North America, alongside the ancestors of forms such as the American kestrel (F. sparvierus), and the aplomado falcon (F. femoralis) and its relatives.

Known from an almost complete right coracoid (specimen UMMP V29107) and some tarsometatarsus, tibiotarsus and humerus pieces (V27159, V57508-V57510, V57513-V57514), this prehistoric falcon was slightly smaller than a merlin and apparently a bit more stout-footed, but otherwise quite similar.

After adapting to its ecological niche, ancient merlins would have spread to Eurasia again, with gene flow being interrupted as the Beringia and Greenland regions became icebound in the Quaternary glaciation.

[10][14][19] That the merlin has a long-standing presence on both sides of the Atlantic is evidenced by the degree of genetic distinctness between Eurasian and North American populations.

They are probably best considered distinct species, with gene flow having ceased at least a million years ago, but probably more;[10][17] but more study is required, particularly in the less-sampled subspecies, before a formal split can be made.

[22][23] Such sexual dimorphism is common among raptors; it allows males and females to hunt different prey animals and decreases the territory size needed to feed a mated pair.

Besides a weak whitish supercilium and the faint dark malar stripe—which are barely recognizable in both the palest and the darkest birds—the face of the merlin is less strongly patterned than in most other falcons.

[24] Merlins inhabit fairly open country, such as willow or birch scrub, shrubland, but also taiga forest, parks, grassland such as steppe and prairies, or moorland.

In general, they prefer a mix of low and medium-height vegetation with some trees, and avoid dense forests as well as treeless arid regions.

In the milder maritime parts of its breeding range, such as Great Britain, the Pacific Northwest and western Iceland, as well as in Central Asia, it will merely desert higher ground and move to coasts and lowland during winter.

Migration to winter quarters at least in Eurasia peaks in August/September, while e.g. in Ohio, just south of the breeding range, F. c. columbarius is typically recorded as a southbound migrant as late as September/October.

[9][25] The merlin will readily take prey that is flushed by other causes, and can for example be seen tagging along sharp-shinned hawks (Accipiter striatus) to catch birds that escape from this ambush predator into the open air.

In the Cayman Islands (where it only occurs in winter), bananaquits were noted to die of an apparent heart attack or stroke, without being physically harmed, when a merlin went at them and they could not escape.

[26] Larger birds (e.g. sandpipers, flickers,[29] other woodpeckers,[30] ptarmigan, other grouse,[27] ducks[31] and even rock doves[32] as heavy as the merlin itself) and other animals—insects (especially dragonflies, moths, grasshoppers, butterflies and beetles[33][34]), small mammals, (especially bats,[35] shrews,[36] rabbits,[33] voles, lemmings[37] and other small rodents[38]) reptiles (such as lizards and snakes)[27] and amphibians[39]—complement its diet.

Quoting from one popular raptor watching reference,[40] "An observer may use this aggressive tendency for identification purposes and as a means of detection.

[9][41] John James Audubon illustrated the merlin in the second edition of Birds of America (published in London, 1827–38) as Plate 75, under the title, "Le Petit Caporal – Falco temerarius".

William Lewin illustrates the merlin as Plate 22 in volume 1 of his Birds of Great Britain and their Eggs, published 1789 in London.

In medieval Europe, merlins were popular in falconry: the Book of St. Albans listed it as "the falcon for a lady", where it was noted for classic "ringing" (circling rapidly upward) pursuits of the English skylark.

Quoting from one popular falconry book on the eagerness of merlins to chase a swung lure, "Every stoop, outrun, dodge, and aerial maneuver of a hard flight to real quarry can be duplicated with no risk of loss of the falcon.

It is listed on CITES Appendix II and on a local level protected as other birds of prey; while some countries allow to capture merlins, e.g. for falconry, international trade requires an export permit.

Ground-nesting populations in moorland have a preference for tall heather, and are thus susceptible to overmanagement by burning vast tracts instead of creating a habitat mosaic containing old and new growth.

Still, the merlin is rather euryoecious (adaptable to various conditions) and will even live in settled areas, provided they have the proper mix of low and high vegetation, as well as sufficient prey (which is usually the case) and nesting sites (which is a common limiting factor).