THE ARCHITECTURE OF SWALLOWS 393 



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; secur- 

 ing 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 

 (Cotylesiud Stelgidopteryx) is peculiar. It is the rule with weak- 

 billed and small-footed hole-in habiters that they accept natu- 

 ral 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 Wood- 

 peckers 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 (heir readi- 

 ness 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 

 fact that the Bank Swallow habitually excavates a hole for 

 itself, instead of accepting a ready-made retreat, renders intel- 

 ligible the fact that it still maintains its primitive ways, 

 instead of yielding, like the others, to modifying surroundings. 

 But the Bough-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 fur- 

 nished with soft material : Cotyle riparia, Stelgidopteryx serri- 

 pennis. 



2. Holes in trees or rocks not made by the birds, fairly fur- 

 nished with soft material: Progne subis, Tachycineta bicolor, 

 Tachycineta thalassina. 



3. Holes, or their equivalents, not made by the birds, but 

 secured through human agency, and more or less fully fur- 

 nished with soft material, according to the shallowness or 

 depth of the retreat. (Formerly, no species ; now, all the species 

 excepting Cotyle riparia.) 



4. Holes constructed by the birds, of mud, plastered to sur- 



