Xttt 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-headud 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 are 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 
