THE SKUAS 223 



secret of the separate existence of the three. Much of our knowledge 

 of their at-sea distribution is obscured by the difficulty of identification 

 at a distance: immature birds are almost, and sometimes quite, 

 indistinguishable to the observer; they lack the long central tail 

 feathers by which the adults may be specifically identified. Only the 

 heavy thick-bodied immature great skua cannot be mistaken for any 

 other bird, being too dark for an immature gull, and too short in the 

 tail for another skua. The adult arctic skua can be recognised by the 

 two long straight feathers which project from the centre of the wedge- 

 shaped tail; these are distinct from the much broader and twisted 

 features of the tail of the somewhat larger and heavier pomarine 

 skua. The graceful long-tailed skua may be recognised by its buoyant 

 swallow-like flight, and the very long and streamer-like central tail 

 feathers. Both arctic and pomarine skuas have dark and light 

 phases (p. 52) ; in the arctic skua the dark form is more numerous in 

 the southern part of its breeding range. The dark form is scarcer in 

 the pomarine. 



At the Fair Isle Bird Observatory, Williamson (1951) found that 

 the phases of the arctic skua could be divided into dark, intermediate 

 and pale (white-bellied) types: D, I, and P. In eight nests under 

 observation the following matings were recorded in 1951 : Dx I, P x I, 

 PxD, Ixl, Pxl, Dxl, Dxl, Dxl. Two adults, ringed as non- 

 breeders in 1950, were again present but not breeding in 1951, arguing 

 a non-breeding period of at least three years in this species. 



The Fair Isle arctic skuas obtain most of their food by forcing the 

 abundant kittiwakes to disgorge the sillocks and other small fish 

 which are the staple kittiwake food in summer. All skuas pursue 

 other birds at sea in an attempt, not always successful, to make them 

 deliver their fish catches. This food, often partly digested in the vic- 

 tim's crop, is secured by the skua with a sudden swoop before it reaches 

 the water. The great skua in normal flight appears sluggish and slow, 

 as it forges along with laboured movement of the broad wings, but 

 when attacking another bird it displays great aerial power, although 

 in a deceptively leisurely manner, following the twists and turns of its 

 quarry with ease. It has been seen to grab a gannet by the wing-tip or 

 the tail and force its victim to disgorge, and even to retain hold until 

 the gannet has crashed into the sea. It will even pursue a fulmar; 

 on 16 June 1935, when about ten miles E.S.E. of the Westmann 

 Islands, Iceland, we saw a great skua persistently harry fulmars 

 following our ship; this does not seem, however, to be a habit, and 



