1921.1 Recently published Ornithological Works. 323 
covers a few square inches 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 lie 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 by the Rooks, 
and no combined action taken by the members of the colony 
in opposition to the 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 breeding 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, 
