392 THE ARCHITECTURE OF SWALLOWS 



of comparatively receut naturalization among as, attracted at 

 length by conveniences for nesting. The Bank Swallow, the 

 most cosmopolitan species of the whole family, seems to retain 

 its wonted ways most sturdily of any — a fact perhaps explained 

 by the highly exceptional character of its nidification. 



To all this there is no parallel among our birds. Various 

 si)ecies, indeed, now regularly accept the artificial nesting- 

 places man provides, whether by design or otherwise. Such is 

 notably the case with several kinds of Wrens, with at least one 

 kind of Owl, with the Bluebird, the Pewit Flycatcher, and 

 especially the House Spairow. Various other birds occasion- 

 ally avail themselves of like i)rivileges, still retaining In the 

 main their original habits. But in no other case than that of 

 Swallows is the modification of habit so profound, or so nearly 

 without exception throughout an entire family. Next after 

 the Swallows, the Wrens appear to yield the most readily, and 

 all of our rpecies will probably become modified in due time 

 after the fashion of the House Wren. But the geographical 

 distribution of no other birds than Swallows has been thus 

 far afiected by increased facilities for nesting. 



As architects, our Swallows are separable into several cate- 

 gories, 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 betheprimitivecustom ; 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 wit- 

 nessed in the Barn Swallow, which builds by preference in 

 corners, in any anghi between converging plane surfaces. 

 The most remarkable extreme is that the Cliff Swallow fur- 

 nishes, where we can i'.race the most elaborate retort-shaped 

 mud receptacles, back through less perfected purse-like struc- 

 tures, to the early idea of the bird, which was simply the wall- 

 ing-up of chinks and crannies on the face of cliffs, or the 

 formation of a cup like projection. In the material employed, 



