WATCHING GULLS AND SKUAS 125 



other bird circled at a still greater height, and never 

 once joined in the attack. The height, I may say, 

 from which the birds swoop is not, as a rule, very 

 considerable. The above does not apply equally 

 to the Arctic skua at least in my own experience 

 for though often the two birds would attack, yet in 

 the greater number of cases only one of them did 

 so. Now the Arctic skua, as I have mentioned 

 elsewhere, is one of those birds which employs 

 strategy (begging here the question for the sake of 

 brevity) as well as force to defend its young, and 

 it occurred to me that here might be a case of co- 

 operation, the male bird most probably attacking, 

 and the female employing the ruse. I satisfied 

 myself, however, that the same bird sometimes does 

 both one and the other. How often this is so, and 

 whether there is a tendency on the part of either 

 sex to resort by preference to one or the other 

 method, it might be difficult to find out Yet I 

 cannot help thinking that this is the case, and that 

 a process of differentiation is in course of taking 

 place. The facts are or appeared to me to be 

 these. In the case of the great skua, both sexes 

 almost, but not quite, always attack, and there is no 

 ruse. In that of the Arctic skua both sexes some- 

 times attack, but far more frequently (that, at least, 

 was my own experience) one alone does so, and here 

 a ruse is employed. In the former case we just see 

 occasionally, as an exception, the raw material (the 

 non-attacking of the one bird) that might conceivably 

 be utilised by nature for the elaboration of another 

 form of defence. In the latter we may see this other 

 form being elaborated. 



