332 SWALLOWS. 
Wren.. But the geographical distribution of no other birds than Swallows has been 
thus far affected by increased facilities for nesting. 
“As architects, our Swallows are separable into several categories, according to the 
order of the structures they build. It will be remembered that they are weak-billed, 
feeble-footed birds, without the instruments required for the weaving of intricate fabrics, 
or the elaboration of ornate and highly finished dwelling places. So varied is their 
workmanship, so curious are some specimens of their art, that we can hardly refer their 
structures to any single plan; yet, in general, we may discern in their nesting the 
tendency, usually seen in weak-footed birds, to retire into holes, into which are brought, 
and artlessly arranged, the materials that are to receive the eggs. At least, such seems 
to be the primitive custom; and it is still the wont of the Purple Martin and of the 
Violet-green and Rough-winged and Bank Swallows. A modification of this is witnessed 
in the Barn Swallow, which builds by preference in corners, in any angle between con- 
verging plane surfaces. The most remarkable extreme is that the Cliff Swallow furnishes, 
where we can trace the most elaborate retort-shaped mud receptacles, back through less 
perfected purse-like structures, to the early idea of the bird, which was simply the 
walling-up of chinks and crannies on the face of cliffs, or the formation of a cup-like 
projection. In the material employed, we likewise see the progressive steps by which, 
from the mere deposit of soft material in a natural hollow, the Swallows began to 
project their fabrics out beyond the base of support; securing the required coherence of 
the materials, not by weaving, which to them was impracticable, but by- plastering 
with mud. Thus we gradually reach those wonderful structures which the Cliff Swallow 
builds entirely of mud, with only a little hay or a few feathers for a lining.” The case 
of the Bank Swallows (Clivicola and Stelgidopteryx) is peculiar. It is the rule with weak- 
billed and small-footed hole inhabiters that they accept natural cavities ready made to 
their purposes. Yet some of the feeblest of the Swallow tribe dig their own holes in the 
ground, exactly as the Kingfisher excavates its gallery, or the Woodpeckers chisel out 
their nesting-places in trees. The fact, then, that Swallows are naturally hole breeders, 
taking possession of such cavities as come to hand, serves to explain their readiness to 
accept the convenient artificial nesting-sites that man provides. Even the Cliff Swallow, 
when nesting on buildings, selects a site under eaves, and thus in a quasi cavity. The 
faé& that the Bank Swallow habitually excavates a hole for itself, instead of accepting 
a ready made retreat, renders intelligible the fat that it still maintains its primitive 
ways, instead of yielding, like the others, to modifying surroundings. But the Rough- 
winged Swallow, which normally breeds quite like the Bank Swallow, is already yielding 
to the times, and in the East now generally nests in or about buildings, such as bridges 
and piers; and we may confidently anticipate the time when the Bank Swallow shall 
go and do likewise. 
“The nesting of our Swallows now presents the following categories of method: 
“1. Holes in the ground, dug by the bird itself, slightly furnished with soft 
material: Clivicola riparia, Stelgidopteryx serripennis. 
“2. Holes in trees or rocks not made by the birds, fairly furnished with soft 
material: Progne subis, Tachycineta bicolor, Tachycineta thalassina. 
“3. Holes, or their equivalents, not made by the birds, but secured through human 
