PECULIAR INTERRELATIONS 127 



selects rabbit burrows for its nursery, will not tolerate the 

 presence of the rightful owners of the burrow. 



Without doubt the most interesting and most puzzling of 

 all the interrelationships displayed by birds relate to the 

 phenomena of parasitism which is manifested in two distinct 

 ways. 



The first of these is illustrated by birds which waylay 

 others for the sake of robbing" them of their food, and among 

 these the Skua-gulls and the Frigate-birds hold perhaps the 

 foremost place. " Woe to the Gull or other sea-bird," says 

 Brehm, " which seizes its prey within sight of a Skua! With 

 arrow-like swiftness he follows the fortunate possessor uttering 

 barking cries, dances, as if playfully, round him on all sides, 

 cunningly prevents any attempts at flight, resists all defence, 

 and untiringly and ceaselessly teases him till he gives up his 

 prize, even though it has to be regurgitated from his crop." It 

 is this habit of catching the food of the persecuted bird which 

 has given the Skua the unenviable name of dung-eater, the 

 common notion being that the bird in its fright voided ex- 

 crement! The Frigate-bird is a no less persistent robber. Dr. 

 C. W. Andrews, during his exploration of Chri.stmas Island 

 (Indian Ocean) had plenty of opportunities of watching these 

 birds. They "by no means depend for food," he says, "on the 

 fish they catch themselves, but systematically rob the Gannets 

 which feed in great numbers on the island. Towards sunset 

 many Frigate-birds may be seen sailing along the coast, watching 

 for the return of the Gannets full-fed from the fishing grounds. 

 The birds being well aware of what is in store for them, and 

 knowing that if they can reach the shelter of the trees they are 

 safe, approach the island at a great speed, flying as low down as 

 possible. Usually, while they are still at some distance, two 

 or three Frigate-birds give chase, and hunt the Gannet back- 

 wards and forwards, continually trying to get beneath it and to 

 cut off its retreat to the trees. The chase may last several 

 minutes, but at length the exhausted bird disgorges some of 

 the fish it had swallowed, and this is immediately caught in 

 mid-air by one of the pursuers." 



Before proceeding further it would be well to point out that 

 neither in the cases which have been cited, nor in those which 

 are to follow on the subject of parasitism, are these inter- 



