254 FLEAS, FLUKES AND CUCKOOS 



A stranger form of clepto-parasitism is practised by certain tropical 

 thrushes (Turdidae), particularly of the genus Aleihe, which are often 

 referred to as "ant-birds." They have developed the habit of following 

 parties of driver ants and, in addition to catching insects which may 

 be flushed by foraging columns, they actually rob the ants of the prey 

 which they are carrying. In the Belgian Congo, Chapin once observed 

 small parties of thrushes and bulbuls (Pycnonotidae) waiting alongside 

 a forest road and robbing the ants when they were forced to expose 

 themselves to view in the open while crossing from one side of the road 

 to the other. The frigate-birds (Fregatidae), of which there are five 

 species, are related to our gannets and cormorants. They range over 

 the tropical seas, sailing around throughout the day on motionless 

 wings, sometimes rising to great heights until they are mere specks in 

 the sky. At sundown they return to the shore and roost communally in 

 convenient trees. They never settle on the water and this particular 

 aversion may well be one of the factors contributing to the development 

 of the clepto-parasitic habit. If there are shoals of pelagic fish within 

 sight they swoop down and take their choice almost without ruffling 

 the surface of the sea. If such prey is scarce they pursue other birds and 

 force them to disgorge their food, either from their beaks or their crops. 



The skuas are brown, gull-like birds, which range across the northern 

 and southern oceans at all distances from the shore, spending most of 

 their lives at sea, and unlike the frigate birds do not return to land to 

 roost. They, also, obtain a large proportion of their food by robbing 

 other birds. There are four native species, but of these only the great 

 skua and the arctic skua breed in Britain. The long-tailed skua and 

 pomatorhine skua are passage migrants and seasonal visitors. 



The great skua is essentially a maritime bird and is rarely seen in- 

 land. It is a little larger than a herring-gull, dark brown in colour, with 

 a white patch at the base of the primary wing feathers. In the spring it 

 resorts to elevated moorlands and rough hilly pastures near the sea, 

 where it breeds in colonies. The nest is little more than a scrape lined 

 with heather and moss. As a rule it lays two eggs, which are usually 

 olive-grey or reddish-brown with dark brown spots and blotches. Both 

 sexes incubate the eggs and when the young hatch the male provides 

 their food while the female broods them. 



Most of this food is obtained by piracy. The great skua pursues a 

 number of different species of sea birds, chiefly gulls — even those which 

 are larger than itself, such as the greater black-backed gull. It attacks 



