24 THE RIDDLE OF MIGRATION 



quite usual to see a number of nests alongside each 

 other as though each space had provided a separate 

 stimulus to the nest-building instinct. (Fig. 4). 

 The little south American bird, (Furnarius cuni- 

 cularius) which makes a horizontal nesting burrow 

 in the ground sometimes six feet in length, has been 

 observed to burrow into a mud wall, only to break 

 through at the other side before the burrow had 

 attained its requisite length. Yet repetition upon 

 repetition fails to teach it the limitations of the 

 wall or the futility of its perseverance. 



So strong is instinctive behavior in birds that it 

 may actually lead to destruction . Many birds have 

 the habit of removing the faeces of the young from 

 the rim of the nest. As shown by Howard, if the 

 droppings are removed by human agency during 

 the absence of the parent birds, they will insist on 

 their return in removing something and if there is 

 nothing else it will actually be the material of the 

 nest. They might thus, by persistence, be induced 

 to destroy the very nest they have made for their 

 young while these are still in occupation. 



It has been experimentally shown that birds have 

 very retentive memories. Pigeons that have been 

 taught to thread a maze, even though the teaching 

 be slow, will remember the key for a year or more. 

 Canaries switched from an aviary A to an aviary B 



