AGGREGATIONS OF HIGHER ANIMALS 1 I 3 



firmed. They suggest one mechanism, that of mutual 

 stimulation to mating, which may have operated to 

 produce social nesting among birds, and which seems 

 capable of giving added survival value to the larger 

 colonies, once the habit of collecting into breeding 

 flocks is established. We have here a suggestion that 

 these social colonies of birds have evolved far enough 

 so that there has come to be a threshold of numbers 

 below which successful mating does not take place. 

 The numbers that constitute this threshold probably 

 vary under a variety of conditions. 



In one case, when only two pairs were present, 

 nests were built but no eggs were laid, while in a 

 more favorable season, with three pairs, eggs were 

 laid and one chick out of eight that hatched lived 

 through the downy stage. 



I saw the laughing gulls myself at Woods Hole 

 last summer; and I also found a paper by Gross giv- 

 ing the case of another almost extinct population 

 which could not be revived. The heath hen, prob- 

 ably a representative of an eastern race of the prairie 

 chicken, was formerly very abundant in Massachu- 

 setts, and may have been distributed from Maine to 

 Delaware, or perhaps even further south. It was grad- 

 ually isolated by the killing of birds in the intermedi- 

 ate region and was driven back, until about 1850 it 

 was found only on Martha's Vineyard and the near-by 



