Swallows

Swallows

Swallows have for long enjoyed harmonious association with man, often Nesting in and on his homes and other buildings. Whether they are mud huts or concrete skyscrapers. Because swallows are cat insects they are usually popular birds. In temperate lands, returning swallows are welcomed as the symbolic ending to the long, cold winter.

The family is almost cosmopolitan, occurring in open habitats from sea level to high mountains, from small forest clearings to extensive grasslands. Twenty-nine species, including I, g members of the largest genus Hirundo, are confined to Africa south of the Sahara; likely, the family originated here. Twenty-five species are found only in the New World, 5 in Australasia, and 4 in Asia. The rest are widespread in the Old World. Sand martins and Barn swallows (the latter known in Europe as the swallow) have the most extensive ranges, breeding in both North America and Eurasia.

There are no taxonomic differences between “swallows” and “martins,” the names being interchangeable. Europeans and martins, for example, are known as Bank swallows in North America. Although superficially similar to swifts, swallows, and martins form a distinct family. They are streamlined in appearance with a short neck: long, pointed wings with nine primary feathers; a short, broad, flat bill with a wide gape; small, weak feet; and, usually, a forked tail which makes them very maneuverable in flight.

African and White-eyed river martins, however, have large, robust bills and feet and are placed in separate subfamilies (Pseudochelidoninae) from other swallows (Hirundininae). The distribution of the river martins is unusual since the ranges of the two species lie 0, 000km (6.000 mi) apart. African and American rough-winged swallows are also distinctive; they have a hook-like thicken-in on the outer margin of the first primary feather, the function of which is unknown.

All swallows are insect-eaters, feeding almost exclusively while in flight. Tree swallows, however, also eat some seeds and berries (in particular the bayberry), especially in cold weather when insects are scarce. Swallows eat a wide variety of insects but some specialize in a particular size or type. The largest of the swallows, the New World martins, for example, consume moths, but-deerflies, and dragonflies. Different species of swallows living in the same area often have different diets so competition between themes is avoided.

Thus in Britain Barn swallow seats mainly very large flies such as blue bottles and hoverflies; Sand martins eat smaller flies and mayflies; while House martins feed on small flies and greenflies. Swallows in the tropics differ from their temperate counterparts in eating more fly-in ants and parasitic wasps but fewer flies and aphids. Similarly, when in their winter quarters Barn swallows also feed on ants rather than flies.

The feeding behavior of swallows changes with the weather. In cold, wet weather, when flying insects are scarce, swallows find it more difficult to find enough insects and have to spend longer feeding. The few insects which fly in bad weather occur mainly near ground level and over water, so this is where swallows concentrate their hunting. There is a saying that the weather will be fair when swallows fly high and wet when they fly low.

This is true of House martins which feed high up in warm weather, but not of Barn swallows which feed low over the ground whatever the weather. When swallows are flying in bad weather they often use a combination of flapping and gliding instead of flapping their wings all the time, since gliding uses up less energy.

Occasionally swallows catch nonaerialprey, especially in bad weather. They may pick up spiders, ants, and other insects from vegetation or the ground. Barn swallows also take certain moth caterpillars.

Swallows usually carry several insects ate time to their nestlings, compressing the insects into a ball which is carried in the throat. Barn swallows may bring some meals a day to a rapidly growing brood office, about 8,000 insects in all Since insects are scarce in winter at high latitudes, swallows of temperate zones have to migrate, whereas tropical species are resident all year.

Swallows are unusual among birds in postponing their post-breeding molt until they have reached their winter quarters. Once there, they often form flocks: many hundreds or even thousands of individuals may roost together in reed beds or, sometimes, on overhead wires incites. However, they do not breed in their wintering areas.

Swallows usually return each year to toothier old nesting sites, the oldest individuals arriving first. Birds in their first year, however, usually disperse though they remain within a few kilometers of their parents’ nest site. Old nests are often reused; mud nests may be strengthened with fresh mud. Temperate-zone swallows normally only live for about four years, rarely seven or eight, but a nest made of mud may outlast the occupants and subsequently may be bemused by a different pair. Burrowing swallows, however. Such as the Sand martin, usually make new nests because of the presence of parasites in the old one and because old burrows sometimes collapse.

Swallows do not hold exclusive feeding territories but they will defend a small area around the nest from other swallows. The radius of this area varies from a few centimeters in the colonial Cliff swallow to about film (i811) in the solitary Tree swale-low. Most species are solitary or nest in small groups, although where suitable nesting sites are scarce large numbers may nest together.

Only a few species are truly colonial: colonies of House and Sand martins may number hundreds, and Cliff swallows thousands of pairs, with nests built very close to each other. Nesting in colonies may enable these swallows to detect and deter predators more quickly; individuals may also find distant or scarce sources of food by following successful foragers.

All swallows are monogamous, although promiscuous mating does occur, especially in colonial species. However, the roles of the sexes vary. Usually, only the female incubates and broods the young nestlings, but in a few species, mainly those living in colonies, the male shares these duties. Both sexes feed the nestlings. Individuals from the first broods of Barn swallows and House martins have also been known to help their parents feed the second brood.

Swallows start to breed each year when their insect food has become sufficiently abundant in the spring, in temperate areas, or before the period of peak rainfall in the tropics since very wet weather prevents swallows from collecting enough food to raise abroad successfully. Eggs are laid at daily intervals but weather may delay laying for a day or more.

Clutches of temperate species are larger than those of tropical swallows. Island species lay the smallest clutches: the Southern martin of the Galapagos Islands has only one or two eggs whereas its close relative, the Purple martin of North America, lays up to eight.

Clutch size declines during the breeding season perhaps because the time good weather is available for feeding the nestlings also becomes reduced. Old females also usually have larger clutches than young birds. One or two brood years depending on the species and locality, and occasionally three in a favorable season. Some individuals, especially first-year birds, only have one brood; female House martins rearing one brood have a lower risk of mortality than double-brooded birds.

The growth of nestling swallows is strongly influenced by the prevailing weather and food abundance. Bad weather sometimes leads to nestlings starving to death, although they can survive a few days of adverse weather.

Most swallow populations have probably benefited from their close association with man, as more artificial nesting sites and more open spaces have extended suitable habitats for them. However, the intensification of agriculture, the use of pesticides, and industrial pollution have probably contributed to a decline in some areas.

In Britain air pollution in towns and cities has decreased since the Clean Air Act of 1956and the number of House martins has since increased in urban areas, although the presence of suitable nesting and feeding sites remains crucial. No species of the swallow is considered to be a serious pest, although large flocks of roosting swallows, especially the Brown-cheated martin in South America, may create local cleaning, health, and safety problems. Indeed, swallows are usually welcomed as they eat many insect pests such as greenflies and midges.

The biology of many swallows is still poorly known; the nests and eggs of some species have never been found. One swallow, the White-eyed river martin, was only discovered in 1968 but may already be closet extinction. Ten individuals were originally found at Lake Broached in Thailand in a reed bed roosting among other swallows. The breeding sites, however, are unknown. Since 1968 a few individuals have been seen but none were found during an intensive search in 1980-81.