AGGREGATIONS OF ANIMALS 111 



the least and the most highly specialized families 

 of that order have the same habit. With all 

 these insects, the collections formed over-night 

 tend to remain together on the following morning 

 until the day becomes warm. On dull, cool days, 

 the groups may be found together in a state of 

 lethargy well into the day. 



Many birds form such nightly collections. 

 Crows, robins, martins, blackbirds and chimney 

 swifts are notable examples. Several different 

 species may share the same roost for the night. 

 The birds may roost so closely together that they 

 actually pile upon each other; certainly on many 

 occasions they tend to occupy much less than the 

 total available and apparently equally desirable 

 space. 



Bats also come together in similar close- 

 packed roosts. Hundreds may hang themselves 

 to rafters or rocks so close together that their 

 bodies are touching. With many birds the 

 common roosts end with the breeding season or, 

 if continued, they are not occupied by the breed- 

 ing birds. With some, including robins, there is 

 evidence that even during the breeding period, 

 the males come together for their community 

 roosts. The bats have the sexes segregated, at 

 least during the period when the females are 

 carrying or caring for their young. 



