 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.
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. Arguably, they might be considered distinct species, with gene flow having ceased at least a million years ago, but probably more.
By and large, color variation in either group independently follows Gloger's Rule. The Pacific temperate rain forest subspecies suckleyi's males are almost uniformly black on the upperside and have heavy black blotches on the belly, whereas those of the lightest subspecies, pallidus, have little non-dilute melanin altogether, with grey upperside and reddish underside pattern.
- Falco columbarius columbarius (Linnaeus, 1758) – taiga merlin, boreal merlin, tundra merlin
- Canada and northernmost United States east of Rocky Mountains, except Great Plains. Migratory, winters in S North America, Central America, the Caribbean, and N South America from the Guyanas to the northern Andes foothills. Rarely winters in the northern USA.
- Falco columbarius richardsonii (Ridgway, 1871) – prairie merlin
- Falco columbarius suckleyi (Ridgway, 1873) – black merlin, coastal forest merlin
- Pacific coast of North America, from S Alaska to N Washington state. Resident (some altitudinal movements).
- Falco columbarius/aesalon aesalon (Tunstall, 1771) – Eurasian merlin
- Northern Eurasia from British Isles through Scandinavia to central Siberia. Population of northern Britain shows evidence of gene flow from subaesalon. British Isles population resident, rest migratory; winters in Europe and the Mediterranean region to about Iran.
- Falco columbarius/aesalon subaesalon (C.L. Brehm, 1827) – Icelandic merlin, smyril (Faroese), smyrill (Icelandic)
- Iceland and Faroe Islands. Latter population has some gene flow with aesalon. Resident (some winter dispersal).
- Falco columbarius/aesalon pallidus (Sushkin, 1900) – pallid merlin, steppe merlin
- Asian steppes between Aral Sea and Altay Mountains. Migratory, winters in S Central Asia and N South Asia.
- Falco columbarius/aesalon insignis (Clark, 1907) – East Siberian merlin
- Falco columbarius/aesalon lymani (Bangs, 1913) – Central Asian merlin
- Mountains of eastern Kazakhstan and surrounding countries. Short-distance migrant.
- Falco columbarius/aesalon pacificus (Stegmann, 1929) – Pacific merlin
The merlin is 24–33 cm (9.4–13.0 in) long with a 50–73 cm (20–29 in) wingspan. Compared with most other small falcons, it is more robust and heavily built. Males average at about 165 g (5.8 oz) and females are typically about 230 g (8.1 oz). There is considerable variation, however, throughout the birds' range and—in particular in migratory populations—over the course of a year. Thus, adult males may weigh 125–210 g (4.4–7.4 oz), and females 190–300 g (6.7–10.6 oz). Each wing measures 18.2–23.8 cm (7.2–9.4 in), the tail measures 12.7–18.5 cm (5.0–7.3 in) and the tarsus measures 3.7 cm (1.5 in). 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.
The male merlin has a blue-grey back, ranging from almost black to silver-grey in different subspecies. Its underparts are buff- to orange-tinted and more or less heavily streaked with black to reddish brown. The female and immature are brownish-grey to dark brown above, and whitish buff spotted with brown below. 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. Nestlings are covered in pale buff down feathers, shading to whitish on the belly.
The remiges are blackish, and the tail usually has some 3–4 wide blackish bands, too. Very light males only have faint and narrow medium-grey 