42 BRITISH SEA) BIRDS 
a terror to the Gulls and Terns; merciless robbers 
of the hard-won spoil of more weakly species ; 
destroyers even of the eggs and helpless young of 
other sea birds. All the four species of northern 
Skuas are visitors to the British Seas, but only two 
of them are indigenous to our islands. The first 
of these to be noticed here is the Great Skua, 
Stercorarius catarrhactes, one of the most local of 
British birds during the breeding season, as its only 
known nesting places in our area are on Unst 
and Foula, two small islands of the Shetland 
Group. Except during the breeding season, the 
Great Skua is mostly oceanic in. its habitat, 
wandering long distances from land in quest of 
prey, attending vessels and fishing fleets, only 
drawing landwards by stress of weather or un- 
usual abundance of food. This Skua is practically 
omnivorous. During its summer sojourn near and 
on the land it repeatedly raids the colonies of other 
sea fowl, to prey upon exposed eges or unguarded 
young; it captures the smaller Gulls, notably the 
Kittiwake: it also picks up any stranded fish or 
other carrion; and is constantly on the watch to 
chase any Gull or Tern that catches a fish, 
following the poor bird with fatal persistency 
until, terror stricken, it disgorges its food, which is 
promptly seized by the voracious Skua. The call 
note of this Skua is very similar to that of the 
Lesser Black-backed Gull, but when under the 
excitement of chasing other birds, or of seeking 
