THEIR REPRODUCTION (NESTS) 275 



nest-building these structures should never show 

 any variation from a certain type. But we 

 know that this is not so^ for birds not only 

 change their materials but their mode and situ- 

 ation of nest-building. Many birds have done 

 so within the measure of historical record. 

 Swallows have changed their mode of nidifica- 

 cation, and deserted rocks or caves in certain 

 civilised countries for buildings ; the House 

 Sparrow and the Starling have done the same. 

 Then we have instances of birds changing their 

 mode of nesting to escape some threatened 

 danger, as for instance the Moorhen building in 

 trees, in spots liable to sudden floods, or certain 

 Penguins forsaking exposed open sites for their 

 nests and selecting covered situations to guard 

 them against the depredations of pigs. Many 

 other examples might be quoted to show, be- 

 yond all reasonable doubt, that birds are ever 

 ready to take advantage of any favourable cir- 

 cumstances, and to alter their habits accord- 

 ingly. The materials of nests are also constantly 

 being changed, and instances are on record 

 where the most unsuitable articles have been 

 fabricated into receptacles for their eggs. It 

 would be most unfair then to attribute to birds 

 such a poor substitute for reason as blind 



