(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 bands, while in the darkest birds the bands are very wide, so that the tail appears to have narrow lighter bands instead. In all of them, however, the tail tip is black with a narrow white band at the very end, a pattern possibly plesiomorphic for all falcons. Altogether, the tail pattern is quite distinct though, resembling only that of the aplomado falcon (F. berigora) and (in light merlins) some typical kestrels. The eye and beak are dark, the latter with a yellow cere. The feet are also yellow, with black claws.
Light American males may resemble the American kestrel (F. sparverius, not a typical kestrel), but merlin males have a grey back and tail rather than the reddish-brown of the kestrels. Light European males can be distinguished from kestrels by their mainly brown wings. In the north of South Asia, wintering males may be confused with the red-necked falcon (F. chicquera) if they fly away from the observer and the head (red on top in F. chicquera) and underside (finely barred with black in F. chicquera) are not visible.
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. They are not very habitat-specific and can be found from sea level to the treeline. 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. During migration however, they will utilize almost any habitat.
Most of its populations are migratory, wintering in warmer regions. Northern European birds move to southern Europe and North Africa, and North American populations to the southern United States to northern South America. 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. The migration to the breeding grounds starts in late February, with most birds passing through the US, 