SYMBIOSIS 25 



in an adjacent tree. Unlike most herons they do not leave the roost at 

 sunrise but wait until cattle are released from the compound. As many 

 as sixty-eight cattle ticks have been taken from the egret's crop, but 

 they are by no means confined to this type of diet. Frogs and other 

 aquatic animals are taken freely if they come across them. 



The only British breeding bird which takes advantage of the winged 

 insects swarming on domestic animals is the yellow wagtail which 

 hawks for blood-sucking horse-flies and clegs (Tabanidae) round cattle 

 grazing in the fields. Its vernacular name in France means "little 

 herdsman" and at least two African tribes designate it as "goatherd." 

 The degree with which these species of birds associate with cattle seems 

 to vary in different districts and even with different individuals. De- 

 lousers will often take advantage of any unusual circumstance which 

 disturbs insects from their hiding places. Thus cattle-egrets keep just 

 ahead of bush fires and mynahs follow sudden inrushes of water — 

 intercepting the insects which are disturbed by the flood and flames. 

 One species of ani or tick-bird {Crotophaga) has developed the interesting 

 and curious habit of following columns of the dreaded legionary ants 

 [Dorylinae) and feeding upon the insects which are "flushed" by the 

 ants as they drive relentlessly through the forest. 



In south-east Africa Swynnerton has made a long and careful study 

 of mixed bird parties and has come to the conclusion that although 

 sociability and the protection of the weaker species concerned may 

 account in part for these flocks, their principal function is co-operative 

 hunting. They are in all probability drives. He has watched, for 

 instance, the different species in a mixed party searching the vegetation 

 at various levels, while others again such as drongos (Dicruridae) and 

 flycatchers moved along with them and only took insects on the wing 

 which were disturbed by the "beaters." The role of the drongos was 

 apparently that of clepto-parasites but Swynnerton surmised that their 

 aggressive nature and readiness to attack made them very welcome 

 additions, and no doubt greatly added to the "mobbing" force of the 

 whole party. 



Little is known about the function — if indeed there is one — of the 

 various winter flocks of birds in Britain. It is not infrequent to meet 

 mixed parties of tits numbering a hundred or more. Recently it has 

 been shown that blue tits search trees and bushes at higher levels than 

 great tits, and it is quite possible that co-operative hunting is one of the 

 objects of these flocks. The association of feeding starlings and lapwings 



