1 921.1 Recently published Ornithological Works. 323 



covers a few square inclies only, so that the isolation of the 

 breeding pair is more theoretical than practical. 



Mr. Howard ascribes the restricted nature of the Guille- 

 mot's territory to the fact that the species, as a whole, 

 would suffer if each male resisted intrusion on its breeding- 

 ledge, owing to the scanty number of available sites, while, 

 on the other hand, the food-supply is practically unlimited. 

 The explanation seems adequate in this case, but is less 

 convincing when we come to consider the difference between 

 the breeding-habits of the Raven and Rook. The former 

 requires not merely a home, but also an estate surrounding 

 it, on which he brooks no rival ; the latter is content to 

 live in a bird-town in the tree-top, from where he sallies 

 forth with his companions to seek his living on communal 

 ground. Here shortage of nesting-sites cannot be urged as 

 the reason for such close association_, nor is there any 

 advantage gained with regard to food-supplies, so mutual 

 protection is assigned as the necessary condition of the 

 Rook's existence. Now it is quite true that many cases are 

 on record of rookeries being raided by Carrion Crows, but 

 no serious resistance seems ever to be made l)y the Rooks, 

 and no combined action taken by the members of the colony 

 in opposition to t!ie raiders. This is the more remarkable, 

 when it is remembered that such species as the Common 

 and Arctic Terns, though far weaker as individuals, when 

 acting in concert, can drive off not only the Hooded or 

 Carrion Crow, but even the Marsh-Harrier. Another in- 

 stance where communal bi*eeding is practically useless for 

 purposes of defence, is that of the Cormorant. The parent 

 birds of one nest will view with absolute indifference the 

 robbery of another nest only a few feet away by Crow or 

 Gull. In these cases the difference in the territorial idea 

 is much more than merely of degree. One might almost 

 say that among birds, even in the same families, there are 

 individualists and socialists, — and we may take the Rook, 

 the Martins, the Terns, and the Guillemot, as examples of 

 the latter class ; while the Warblers, the Falcons, the Pipits, 



