How to identifyThe house martin is glossy black above, completely white below, and has a white rump and a short, forked tail.
RSPB The house martin is a small bird with glossy blue-black upper parts and pure white under parts. It has a distinctive white rump with a forked tail and, on close inspection, white feathers covering its legs and toes. It spends much of its time on the wing collecting insect prey. The bird's mud nest is usually sited below the eaves of buildings. They are summer migrants and spend their winters in Africa. Although still numerous and widespread, recent moderate declines earn them a place on the Amber List.
Devon Birds Search the Devon Birds website for recent sightings of House Martins in Devon.
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https://commons.wikimedia.org/wiki/File:Delichon_urbicum.png (breeding range colour altered to bright green)
National Biodiversity Network Delichon urbicum
The NBN Atlas is a collaborative project that aggregates biodiversity data from multiple sources and makes it available and usable online. It is the UK’s largest collection of freely available biodiversity data.
Wikipedia The common house martin (Delichon urbicum), sometimes called the northern house martin or, particularly in Europe, just house martin, is a migratory passerine bird of the swallow family which breeds in Europe, north Africa and across the Palearctic; and winters in sub-Saharan Africa and tropical Asia. It feeds on insects which are caught in flight, and it migrates to climates where flying insects are plentiful. It has a blue head and upperparts, white rump and pure white underparts, and is found in both open country and near human habitation. It is similar in appearance to the two other martin species of the genus Delichon, which are both endemic to eastern and southern Asia. It has two accepted subspecies.
Both the scientific and colloquial name of the bird are related to its use of human-made structures. It builds a closed cup nest from mud pellets under eaves or similar locations on buildings usually in colonies.
It is hunted by the Eurasian hobby (Falco subbuteo), and like other birds is affected by internal parasites and external fleas and mites, although its large range and population mean that it is not threatened globally.
eBird Fairly common to common summer migrant from winter grounds in Africa. Breeds colonially under eaves of buildings, where builds mud nests; feeds over varied open and semiopen habitats, especially water bodies and adjacent fields. Distinctive plumage, with blue-black upperparts, big white rump patch, and white underparts; tail fork shallower than on Barn Swallow, and flight jerkier overall, less smooth and graceful. Juvenile in late summer–autumn is duller above, dingier below than adult.