Merlins are small, fierce falcons from the Northern Hemisphere. They are swift fliers and skilled hunters and have for centuries been well regarded as a falconry bird. 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 a whitish buff spotted with brown below. The eye and beak are dark, the latter with a yellow cere. The feet are also yellow, with black claws.
Merlins are found throughout the Northern Hemisphere. Most of the 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. Merlins inhabit the fairly open country, such as willow or birch scrub, shrubland, but also taiga forest, parks, grasslands such as steppe and prairies, sand dunes, deserts or moorland. 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.
Merlins usually lead a solitary life outside of the breeding season. However, in the winter they may roos communally or sometimes migrate in loose groups. They start migrating to the breeding grounds in late February. Migration to winter quarters at least in Eurasia peaks in August/September. Merlins are diurnal hunters and rely on speed and agility to hunt their prey. They often hunt by flying fast and low, typically less than 1 m (3.3 ft) above the ground, using trees and large shrubs to take prey by surprise. But they actually capture most prey in the air, and will "tail-chase" startled birds. Breeding pairs will frequently hunt cooperatively, with one bird flushing the prey toward its mate. Merlins communicate vocaly and are usually noisy near the nest and during the displays. When alarmed birds produce “kikiki” or “kekeke” shrills.
Merlins are carnivores and specialize in preying on small birds including sparrows, quail, larks, or pipits. Larger birds and other animals such as insects (especially dragonflies and moths), small mammals like bats and voles and reptiles complement their diet as well.
Merlins are serially monogamous and pairs form for one season only. Their breeding season occurs typically in May-June and during this time birds perform aerial courtship displays. Merlins do not build nests but mainly use abandoned crow or hawk nests which are located in conifer or mixed tree stands. In moorland-particularly in the UK- females usually make a shallow scrape in dense heather to use as a nest. Others nest in crevices on cliff-faces and on the ground, and some may even use buildings. The female lays 3 to 6 (usually 4 or 5) rusty brown eggs. She incubates them within 28 to 32 days while the male hunts to feed the family. Hatchlings weigh about 13 g (0.46 oz) and fledge after another 30 days or so. They are dependent on their parents for up to 4 more weeks. Sometimes first-year merlins (especially males) will serve as a "nest helper" for an adult pair. Merlins become reproductively mature at one year of age and usually attempt to breed right away.
The most serious threat to merlins is habitat destruction, especially in their breeding areas. These birds also suffer a collision with man-made objects, particularly during attacks, from pesticides and human disturbances.
According to the IUCN Red List, the total merlin population size is around 3,000,000 individuals. This includes 500,000-2,000,000 mature individuals. The European population is estimated at 32,000-51,600 pairs, which equates to 64,000-103,000 mature individuals. The North American population is estimated at approximately 1,300,000 individuals or 867,000 mature individuals. Overall, currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.