Common House Martin

Appearance

The adult Common House Martin of the western nominate race is 13 centimetres long, with a wing span of 26–29 centimetres and a weight averaging 18.3 grammes. It is steel-blue above with a white rump, and white underparts, including the underwings; even its short legs have white downy feathering.

It has brown eyes and a small black bill, and its toes and exposed parts of the legs are pink. The sexes are similar, but the juvenile bird is sooty black, and some of its wing coverts and quills have white tips and edgings. "D. u. lagopodum" differs from the nominate race in that its white rump extends much further onto the tail, and the fork of its tail is intermediate in depth between that of "D. u. urbicum" and that of the Asian House Martin.

Distribution

The subspecies "D. u. urbicum" breeds across temperate Eurasia east to central Mongolia and the Yenisei River, and in Morocco, Tunisia and northern Algeria, and migrates on a broad front to winter in sub-Saharan Africa. "D. u. lagopodum" breeds eastwards of the Yenisei to Kolyma and south to northern Mongolia and northern China; it winters in southern China and Southeast Asia.

Habitat

The preferred habitat of the Common House Martin is open country with low vegetation, such as pasture, meadows and farmland, and preferably near water, although it is also found in mountains up to at least 2,200 metres altitude. It is much more urban than the Barn Swallow, and will nest even in city centres if the air is clean enough. It is more likely to be found near trees than other Eurasian swallows, since they provide insect food and also roosting sites. This species does not normally use the reed-bed roosts favoured by migrating Barn Swallows.

Reproduction

The Common House Martin was originally a cliff and cave nester, and some cliff-nesting colonies still exist, with the nests built below an overhanging rock. It now largely uses human structures such as bridges and houses. Unlike the Barn Swallow, it uses the outside of inhabited buildings, rather than the inside of barns or stables. The nests are built at the junction of a vertical surface and an overhang, such as on house eaves, so that they may be strengthened by attachment to both planes.

Breeding birds return to Europe between April and May, and nest building starts between late March in North Africa and mid-June in Lapland. The nest is a neat closed convex cup fixed below a suitable ledge, with a narrow opening at the top. It is constructed by both sexes with mud pellets collected in their beaks, and lined with grasses, hair or other soft materials. The mud, added in successive layers, is collected from ponds, streams or puddles. House Sparrows frequently attempt to take over the nest during construction, with the House Martins rebuilding elsewhere if they are successful. The entrance at the top of the cup is so small once it is complete that sparrows cannot take over the nest.

The Common House Martin tends to breed colonially, and nests may be built in contact with each other. A colony size of less than 10 nests is typical, but there are records of colonies with thousands of nests. Four or five white eggs are usually laid, which average 1.9 x 1.33 centimetres in size, and weigh 1.7 grammes. The female does most of the incubation, which normally lasts 14–16 days. The newly hatched chicks are altricial, and after a further 22–32 days, depending on weather, the chicks leave the nest. The fledged young stay with, and are fed by, the parents for about a week after leaving the nest. Occasionally, first-year birds from the first brood will assist in feeding the second brood.

There are normally two broods each year, the nest being reused for the second brood, and repaired and used again in subsequent years. Hatching success is 90%, and fledging survival 60–80%. Third broods are not uncommon, though late nestlings are often left to starve. The average annual mortality for adults nesting in the Western Palaearctic is 40–60%, with most deaths outside the breeding season. A study of British breeders gave an average adult survival rate of just under 40%, but ranging from 25% to 70%. Rainfall in the African wintering grounds is a major factor in adult survival, although wet weather in the breeding areas has very little effect. Although individuals aged 10 and 14 years have been recorded, most survive less than five years. For weeks after leaving the nest the young congregate in ever-increasing flocks which, as the season advances, may be seen gathering in trees or on housetops, or on the wires with Swallows. By the end of October, most Martins have left their breeding areas in western and central Europe, though late birds in November and December are not uncommon, and further south migration finishes later anyway.

Once established, pairs remain together to breed for life; however, extra-pair copulations are common, making this species genetically polygamous, despite being socially monogamous. A Scottish study showed that 15% of nestlings were not related to their putative fathers, and 32% of broods contained at least one extra-pair chick. Extra-pair males, usually from nests where laying had already taken place, were often seen to enter other nests. The paired male initially ensured that his female spent little time alone at the nest, and accompanied her on flights, but the mate-guarding slackened after egg laying began, so the youngest nestling was the most likely to have a different father.

The Common House Martin has been regularly recorded as hybridising with the Barn Swallow, this being one of the most common passerine interspecific crosses. The frequency of this hybrid has led to suggestions that "Delichon" is not sufficiently separated genetically from "Hirundo" to be considered a separate genus.

Food

The Common House Martin is similar in habits to other aerial insectivores, including other swallows and martins and the unrelated swifts, and catches insects in flight. In the breeding areas, flies and aphids make up much of the diet, and in Europe, the House Martin takes a larger proportion of aphids and small flies than the Barn Swallow. As with that species, Hymenoptera, especially flying ants, are important food items in the wintering area.

This species hunts at an average height of 21 metres during the breeding season, but lower in wet conditions. The hunting grounds are typically within 450 metres of the nest, with a preference for open ground or water, the latter especially in poor weather, but the martins will also follow the plough or large animals to catch disturbed insects. On the wintering grounds, hunting takes place at a greater height of over 50 metres.

Predators

Although the Common House Martin is hunted by the Hobby, its aerial skills enable it to evade most predators. It is most vulnerable when collecting mud from the ground. This has therefore become a communal activity, with a group of birds descending suddenly on a patch of mud.

It is parasitised externally by fleas and mites, including the "House Martin Flea", "Ceratophyllus hirundinis", and internally by endoparasites such as "Haemoproteus prognei", which are transmitted by blood-sucking insects including mosquitoes. A Polish study showed that nests typically contained more than 29 specimens of ectoparasite, with "C. hirundinis" and "Oeciacus hirundinis" the most abundant.