Seven common species of swallows are found within the limits of the
United States, four of which have abandoned to some extent their
primitive nesting habits and have attached themselves to the abodes of
man.
[Illustration: Fig. 7.--Barn swallow. Length, about 7 inches.]
In the eastern part of the country the barn swallow[16] (fig. 7) now
builds exclusively under roofs, having entirely abandoned the rock
caves and cliffs in which it formerly nested. More recently the cliff
swallow[17] has found a better nesting site under the eaves of buildings
than was afforded by the overhanging cliffs of earth or stone which it
once used and to which it still resorts occasionally in the East and
habitually in the unsettled West. The martin[18] and the white-bellied,
or tree, swallow[19] nest either in houses supplied for the purpose, in
abandoned nests of woodpeckers, or in natural crannies in rocks. The
northern violet-green swallow,[20] the rough-winged swallow,[21] and the
bank swallow[22] still live in practically such places as their ancestors
chose.
[16] _Hirundo erythrogastra_.
[17] _Petrochelidon lunifrons_.
[18] _Progne subis_.
[19] _Iridoprocne bicolor_.
[20] _Tachycineta thalassina_.
[21] _Stelgidopteryx serripennis_.
[22] _Riparia riparia_.
Field observation convinces an ordinarily attentive person that the food
of swallows must consist of the smaller insects captured in mid-air or
picked from the tops of tall grass or weeds. This observation is borne
out by an examination of stomachs, which shows that the food is made up
of many small species of beetles which are much on the wing; many species
of flies, together with large quantities of flying ants; and a few
insects of similar kinds. Most of these are either injurious or annoying,
and the numbers destroyed by swallows are not only beyond calculation but
almost beyond imagination.
Unlike many other groups of birds, the six species of swallows found in
the Eastern States extend in a practically unchanged form across the
continent, where they are reinforced by the northern, or Pacific-coast,
violet-green swallow.
It is a mistake to tear down from the eaves of a barn the nests of a
colony of cliff swallows, for so far from disfiguring a building they
make a picturesque addition to it, and the presence of swallows should
be encouraged by every device. It is said that cliff and barn swallows
may be induced to build their nests in a particular locality, otherwise
suitable, by providing a quantity of mud to be used by them as mortar.
Barn swallows may also be encouraged by cutting a small hole in the gable
of the barn, while martins and white-bellied swallows will be grateful
for boxes like those for the bluebird, but placed in a higher situation.