Xiii INSTINCT AND REASON 341 



spirit of economy. The North American species lays 

 only one egg in one nest. Our Cuckoo in the perfec- 

 tion of the adaptation of its structure and habits seems 

 to surpass them all. 



Piracy. 



The White-headed Eagle watches the Osprey, and, 

 when the latter has secured a fish, pursues and 

 threatens him till he drops his prey, which, making a 

 swoop downward, he catches as it falls. The Robber 

 Tern lives wholly by the plunder of other birds. The 

 British Avifauna boasts four pirates, two that breed 

 here, besides two that visit us. All these are Skua 

 Gulls, and by far the commonest is the Arctic or 

 Richardson's Skua, intermediate in size between a 

 Kittiwake and a Herring Gull. The Great Skua, 

 which breeds in two islands of the Shetland group 

 and nowhere else in the British Isles, is a much larger 

 bird. When a Gull or a Tern has secured a fish, the 

 Arctic Skua will pursue him with a velocity that makes 

 escape impossible. When he has overtaken the 

 fugitive, he flies over and under him with a menacing 

 air. It is evident that he will brook no refusal, and his 

 victim drops the fish or allows it to be taken from his 

 beak, sometimes crying plaintively the while. Whether 

 the Skua ever finds it necessary to resort to actual 

 violence, I do not know. As far as I have been able 

 to see, threats arc sufficient, but the whole scene 

 passes with such rapidity that it is difficult to make 

 out the details of the action. It is certainly probable 

 that he uses his beak with effect, since he, is known to 



