Merlins are small birds, only slightly larger than Kestrel, but stockier overall and fly with speed and stability. Merlins seem easily agitated, harassing larger raptors seemingly for fun. Merlin is a widespread breeder across boreal habitats, often nesting near clearings around bogs and lakeshores from Alaska east through the Canadian Maritimes, and south through the US-Canada border region. They are also more common in urban areas in the northern U.S. Great Plains, around the Great Lakes, and New England area nowadays. They nest in conifer stands in abandoned crow and raptor nests, but sometimes on the ground north of the treeline.
The Taiga race occurs throughout most of North America but is the only race seen in the Eastern half of the United States. Prairie Merlins occur throughout the western half of the U.S. and southern Canada within the Great Plains, and winters from the breeding range into Central America. They are uncommon as far west as California, and rare in the East. Black Merlin breeds and winters in the Pacific Northwest forests from southern Alaska to Washington. Some winter along the Pacific Coast to southern California, inland to Utah. They are vagrants otherwise. Some Merlins winter within their breeding range. Merlin is typically silent, but vocal during aggressive encounters with other raptors or when nesting. Call is a rapid, high-pitched "kee-kee-kee-kee-kee." Female is larger than male, and appears longer winged.
Backyard birders confuse the streaky-plumaged juvenile Sharp-shinned Hawk for Merlin when seen in their yards. However, Merlin rarely frequents backyards, preferring open habitats instead. Merlins have shorter, boldly banded tails, a "moustache" mark, and much longer wings. They are also more heavily streaked on the underparts. All Merlins have dark brown eyes compared to the yellow eyes of juvenile Sharp-shinned Hawks, a telling giveaway.
Perched, appear small like Kestrel but bulkier and body tapers toward belly, with wing tips reaching just short of somewhat long tail, and do not bob their heads or tails like Kestrel.
More angular in shape than Kestrel, with broad-based wings, sharply pointed wing tips, shorter tail, and stocky chest.
Compared to larger falcons, Merlin has shorter wings, smaller head, and narrow, short tail that does not taper towards tip.
Tail typically square-tipped when folded, usually rounded on other falcons.
Wings held flat or slightly drooped at all times.
Steady, direct fliers.
Stiff, rapid, forceful wing beats compared to wimpy Kestrel, and fluid, whip-like Peregrine.
On migration, frequently soar high up with other raptors.
Merlin is dark-streaked underneath and uniformly darker on top. Almost all have dark tails with several narrow white bands. Sexes differ in adult plumage, but are similar in juveniles, which resemble adult females. Eyes are dark brown.
Three races of Merlin: Taiga (Boreal forest), Prairie (Great Plains), and Black (Pacific Northwest). Plumage ranges from very pale Prairie Merlin to very dark Black Merlin. Taiga overlaps with Prairie and Black at each end of the spectrum, some birds are not safely assignable to subspecies.
Taiga Merlin (F. c. columbarius) is the "standard" Merlin and most common, having well marked undersides and a perfectly banded tail with a white tip. Adult male is dark blue above with distinct tail bands, and creamy below with brownish or rufous-brown streaks and whitish throat. Wrists and leggings are yellowish. Adult female is slate-brown above, heavily streaked below, and paler throat. Juvenile is nearly identical to female, but browner above. Few adult female and juvenile Taigas lack tail bands.
Prairie Merlin (F. c. richardsonii) is pale overall with faint streaking below comparatively. Prairie Merlin has boldly defined tails bands with bold sub-terminal band. Adult male is very pale below with faint rufous streaking on body and yellow leggings. Topside is sky blue with blackish primaries. Adult female and juvenile are tan above and white below, with pale rufous-brown streaking, and pale heads. They are similar to female American Kestrel in appearance.
Black Merlin (F. c. suckleyi) is darkest race, often lacks spots on flight feathers, and shows reduced or no bands on tail. Black Merlin is dark-headed (less distinct "moustache" and pale throat than other races) and heavily marked underneath, looking blackish overall. Adult male is blue-black above with blackish primaries, and dark streaked below on a rufous wash. Adult female and juvenile are dark slate-black above and very heavily streaked below on a buffy tone.
During fall, concentrations occur (mid-September to late October) at Cape May, NJ, Kiptopeke, VA, Florida Keys, other Atlantic Coast sites, and along parts of Great Lakes shorelines.
The most reliable sites in West are Wasatch Mts., UT, Goshute Mts., NV, and Marin Headlands, CA.
Spring migration more dispersed than fall, and takes place from April through May. Moderate flights occur along Atlantic Coast and shorelines of Great Lakes.
Northern birds migratory from Sept. to Nov. in fall, and March to May in spring. Winter throughout the U.S. to northern South America.
(Map from Cornell Lab of Ornithology's All About Birds. For dynamic distribution maps, visit the eBird website.)