NORTHERN CLIFF SWALLOW 471 



over a phoebe's nest and siiccossfully reared their brood there. Wright 

 (1924) cites a similar case in which the young of a fallen nest were 

 placed in a tin can nailed to the barn. The birds returned at once and 

 built a neck of a nest over the opening of the can. 



Cliff swallows are not averse to nesting in the proximity to the 

 nests of other larger birds. Forbush (1929) records a case where 

 the nests were built among the outer sticks of a great blue heron's 

 nest. Coues (1878), Taverner (1919), and Forbush (1929) cite 

 cases where nests of the cliff swallow were built on the cliffs about 

 prairie falcons' nests. Taverner also found them about a duck 

 hawk's nest. Apparently they lived in harmony and were not dis- 

 turbed by these larger predacious species. 



It has been observed by Taverner (1928) and others that the nests 

 of large colonies of cliff swallows located on cliffs are situated in 

 sites least affected by heavy rains. Mr. Taverner's account of such a 

 colony located on the cliffs of the Red Deer River, Alberta, Canada, 

 is as follows: 



This river bed is sunk from two to three hundred feet below the main 

 prairie level and in many cases the valley walls approach the river bank in 

 irregular, and more or less sheer, cliffs. Many wall spaces thus formed are 

 sites of large colonies of nesting cliff swallows that plaster their gourd-like 

 nests closely together making continuous mud-incrustations over considerable 

 surfaces. The boundaries of these aggregations of nests are often very definite 

 but to the casual observer often erratically arbitrary in outline and extent. 

 Many groups of nests are obviously under sheltering overhangs, but others 

 seem well out into the open and subject to the inclemencies of every weather. 

 Often there is no apparent reason why nests should be huddled closely together 

 as if space were very precious and then cease at an imaginary line beyond 

 which conditions seem equally, or even more, desirable. 



When the rains come, however, darkening the exj>osed faces of the cliffs 

 with their wetness, much of the mystery is explained. In practically every 

 case it is then seen that the nest colonies occupy only the dry spots of the ir- 

 regular, and generally wet, surfaces and that the soluble, fragile nest-struc- 

 tures often cease almost on the line of moisture. Most of these colonies seem 

 to be occupied only for a single season or a short series of years and new sites 

 are selected at frequent intervals; consequently there are everywhere old and 

 deserted nest groups in various stages of delapidation whose obvious age be- 

 speaks their permanency and the good judgment with which they were founded 

 as regards prevailing weather, wind and rain. One mild shower and wetting 

 would be suflacient to dissolve their clayey structure into its constituent gumbo 

 to drop with unctuous splash to the talus below or to flow away in stalactites 

 of sluggish mud. 



It does not seem that this safety of situation is generally achieved by a 

 system of trial and error for few ruins of recent and obvious errors are noted 

 as would be were that the case. By some means Cliflf Swallows after nesting 

 under such condition for countless generations have evolved methods of nest- 

 site selection that nearly unerringly pick out amid the multitudinous wall ex- 

 posures and tricky wind currents of the canyons the safe situations. There is 

 no necessity here at least to defer nest making until a mud-making rain sup- 



