Subspecies
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.
American group
- Falco columbarius columbarius – Taiga Merlin, Tundra Merlin
- Canada and northernmost USA 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 richardsoni – Prairie Merlin
- Great Plains from Alberta to Wyoming. Resident (some winter dispersal).
- Falco columbarius suckleyi – Coastal Forest Merlin, Black Merlin
- Pacific coast of North America, from S Alaska to N Washington state. Resident (some altitudinal movements).
Eurasian group
- Falco columbarius/aesalon aesalon
- 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 – smyril (Faroese), smyrill (Icelandic)
- Iceland and Faroe Islands. Latter population has some gene flow with aesalon. Resident (some winter dispersal).
- Falco columbarius/aesalon pallidus
- Asian steppes between Aral Sea and Altay Mountains. Migratory, winters in S Central Asia and N South Asia.
- Falco columbarius/aesalon insignis
- Siberia between Yenisei and Kolyma Rivers. Migratory, winters in continental East Asia.
- Falco columbarius/aesalon lymani
- Mountains of eastern Kazakhstan and surrounding countries. Short-distance migrant.
- Falco columbarius/aesalon pacificus
- Russian Far East to Sakhalin. Migratory, winters in Japan, Korea and nearby.
Read more about this topic: Merlin (bird), Systematics