The Eurasian crag martin is a small perching bird in the swallow family. It has ash-brown upperparts and paler underparts and has a broader body, wings, and tail than any other European swallow. The tail is short and square, with white patches near the tips of all but the central and outermost pairs of feathers. The underwing and undertail coverts are blackish, the eyes are brown, the small bill is mainly black, and the legs are brownish pink. The sexes are alike, but juveniles have buff-brown tips to the plumage of the head, upperparts, and wing coverts.
Eurasian crag martins breed in mountains from Iberia and northwesternmost Africa through southern Europe, the Persian Gulf, and the Himalayas to southwestern and northeastern China. Many European birds are resident, but some northern populations and most Asian breeders are migratory, wintering in northern Africa, the Middle East, or India. Crag martins breed on dry, warm, and sheltered cliffs in mountainous areas with crags and gorges. They can also be found in cultivated areas and around human habitations.
Eurasian crag martins are gregarious outside the breeding season and may form sizeable flocks where food is abundant. During migration, they may also sometimes join with flocks of Dusky crag martins and roost communally on ledges of cliffs or buildings. Eurasian crag martins feed by day catching insects in their beak in flight; they will also occasionally take prey items off rocks, the ground, or a water surface. When breeding, birds often fly back and forth near a rock face hunting for insects, feeding both inside and outside the nesting territory. Crag martins typically exploit the area close to the cliff when they hunt, relying on their high maneuverability and ability to perform tight turns. At other times, they may hunt flying above streams or alpine meadows. Crag martins communicate between each other with short high 'pli', and 'piieh' and 'tshir' calls resembling those of the linnet and the House martin respectively.
Crag martin pairs nest alone or in small colonies, usually containing fewer than ten nests. The birds build their nests on average 30 m (98 ft) apart and each pair aggressively defends its breeding territory against other crag martin and most other bird species. Nesting takes place from May to August, and usually, two broods are raised. The nest, built by both adults, is an open half-cup made of mud and lined with soft material such as feathers or dry grass. It is constructed under an overhang on a rock cliff face, in a crevice or cave, or on a man-made structure. It takes one to three weeks to build and is re-used for the second brood and in subsequent years. The female lays a clutch of 2-5 white with brownish blotches eggs and incubates them for 13-17 days to hatching. The chicks remain in the nest from 24 to 27 days until they fledge. Both parents feed the chicks bringing food every two to five minutes, and the young are usually fed for 14-21 days after fledging.
Eurasian crag martins do not face any known major threats at present.
According to the IUCN Red List, the total population size of the Eurasian crag martin is 1,210,000-2,280,000 mature individuals. In Europe, the breeding population consists of 182,000-342,000 pairs, which equates to 363,000-685,000 mature individuals. The population in China has been estimated at around 100-100,000 breeding pairs and around 50-10,000 individuals on migration. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.