If you could take all of the spirit of a peregrine falcon, and compress it into a package a quarter of the size, it's likely that the result would be pretty close to a merlin. And as with so many other things, when you shrink it in size, you concentrate the energy. Join me to learn a bit more about these amazing little birds.
Juvenile Merlin (Falco columbarius)
There are three subspecies in North America: taiga (F.c. columbarius), prairie (F.c. richardsonii) and black (F.c. suckleyi). Taiga is the most widespread, seen across the country - they're the "Base Model". Adult males have breast streaked with dark brown, with back, wings and crown of blue-grey. Their tails are mostly dark with 4 narrow paler bands, and their faces are similar to a peregrine - large dark eye, a single malar stripe (mustache) - with the addition of a pale eyestripe. Adult females look similar, but the blue-grey color is limited to their rump area (very hard to see in the field under most circumstances) with the rest of the back and crown being a dark brown. Juvenile birds look essentially like adult females without the blue-grey rump. Juv males and females look alike and can only be told apart by size - males are roughly the size of a kestrel and females are about 1/3 larger in measurements (sometimes weigh twice as much). On young birds in fairly fresh plumage, you can sometimes see a "scalloped" pattern on the back, created by pale fringes on the juvenile feathers (the pale edges wear away over time).
Black Merlin in hand (left) and perched Prairie Merlin (right)
Black Merlins are found along the west coast, and are darker overall. They have heavier breast streaking, thicker mustache lines, and dark streaking in the cheeks (often giving the appearance of a solidly dark head) and have minimal light bands on their tail - sometimes the tail appears entirely dark. The blue-grey on adults is much darker, almost a deep slate color. Prairie merlins live in the central regions of the country and are much lighter - lighter brown (almost tan), and the adults' blue-grey is almost powder blue; the pale bands are largest on their tails.
Unlike their bigger cousins, the peregrines and prairies, merlins do turn up regularly in backyards during the winter to pick up a quick meal near the bird feeders. In this setting they can sometimes be confused with accipiters (one of the nicknames for them among my fellow banders is "dark-eyed sharpie"). A few quick pointers to tell your small backyard predators apart: Merlins have pointed wings in flight (vs. rounded on sharpies) fairly long wings compared to the tail (visible on many perched birds). The best clue, however is eye color - always dark brown/black on a merlin, and yellow (juveniles) to orange or red (adults) on a sharpie
Sharp-shinned Hawk (left) vs. Merlin (right)
Merlins are circumpolar birds, breeding in northern regions, using both open spaces (like tundra) and forest edges. Most of us only see them on migration or on their wintering grounds, where they range across the U.S. and as far south as northern South America. They follow their prey base, small songbirds and shorebirds, as they move.
Their hunting style combines the speed of big falcons, but with the low-to-the-ground maneuverability of accipiters. They don't have the mass to do the incredible stoops from above like peregrines, so they use their speed to hunt in open flight. I've watched them over mudflats and marshes and it's a memorable sight; hard to believe they can sustain that kind of speed for as long as they do. The best description I've seen is: "That's a merlin heading this way, wasn't it?"
They are also legendary for their agression toward other raptors. In the book "Hawks in Flight" one of the behavioral fieldmarks is described thusly: If a small raptor flies past a perched raptor and doesn't take a shot at it, it's not a merlin.
In recent years, I've been fortunate enough to encounter several of these wonderful birds each season when banding, and can say without a doubt: good things come in small packages.
(At right, a drawing that I did for a Golden Gate Raptor Observatory season summary report a few years ago - it was a big merlin year.)