GENERAL FACTORS CONDITIONING AGGREGATIONS 79 



that closely resemble close masses of bees. They swarm on the tall 

 grass, or, if this is lacking, they pack together in the low grass, on 

 stones or on the ground. At sunrise the swarm gradually breaks and 

 continues its migration. These night clusters are conspicuous objects 

 showing up as reddish-brown patches on the veldt. Man and pre- 

 sumably other animals take advantage of these aggregations to de- 

 stroy great numbers of the grasshoppers. The benefits accruing are 

 not known. Nikolsky (1925, vide Uvarov, 1928) thinks that they 

 conserve animal heat. Holmquist's observations on mass collection 

 of ants (1928a) suggest that they may at least slow down the rate of 

 change of temperature. 



The congregation of birds for sleeping has been widely observed 

 (Brewster, 1890; Davis, 1894; Bates, 1895; Widman, 1898, 1922; 

 Allen, 1925), particularly for martins, robins, grackles, and crows. 

 Many other birds are reported to gather in the roosts dominated by 

 martins and robins. Extreme cases of close crowding in these roosts 

 are reported by Baker (vide Allen, 1925) for the crested tree swift of 

 India. 



"On arriving at their proposed meeting place," Baker says, "they 

 fly round and round, gradually lowering their flight until one bird 

 makes a sweep and settles on some part of the tree near the top. This 

 is the signal for the rest to perch, and in a few minutes they are all 

 dotted about the higher branches. They then begin to close up with 

 the bird which first ahghted on the tree, finally collecting in a feath- 

 ery ball, one on top of the other. Sometimes this happens again and 

 again before they get settled, but at last the twittering stops and 

 they are asleep for the night. It is wonderful how compactly these 

 birds close up; a flock of eleven appeared not to take more than a 

 foot long by half that breadth." 



The Indian swallow shrike is said on the same authority to have a 

 similar habit. Sharp records that the colonies of mouse birds of 

 Africa, small birds resembhng parrots, roost in small parties that 

 cling together. 



It is well known that bats also gather into sleeping aggregations 

 (Goldman, 1920; Howell, 1920; Allen, 1921). They may congregate 

 in clusters comprising only a few individuals, or hundreds may hang 

 with bodies touching. The groups may be homotypic or heterotypic. 



