Martins build in close proximity to one another, owing probably to shortage of accommodation, and, in their case, the nests have to be so situated as to be sheltered from the wet. If water drips upon them for any length of time, the mud, of which they are composed, crumbles and large pieces fall away, with the result that the eggs or the young are precipitated to the ground. Consequently, not every house or perpendicular cliff will answer the purpose of a breeding station. A few pairs build their nests beneath the eaves close against the walls of my house, and year after year the result is much the same; after every downfall of rain, the water collects into rivulets, trickles down over the eaves, is absorbed by the mud and destroys the nests. Thereupon, the birds set to work and rebuild; but again the nest is destroyed, and again they rebuild, and so on throughout the summer, and only on rare occasions do they succeed in rearing offspring at the proper season. Similar conditions must prevail in many situations; but, clearly, the more binding and plastic the building material, the longer the nest will withstand the action of the dripping water and the greater chance will there be of the young being reared in safety. Observe, therefore, how far-reaching an effect so small a detail as the nature of the mud can have upon the status of the species in any given locality. Where the conditions are favourable, there the birds must congregate to breed, and, like the Guillemot, if each individual exercised dominion over too large an area, the species as a whole would suffer. In all these examples, the fact of different individuals being in such close proximity may afford some protection from enemies both as regards the egg and the offspring, and in so far as there is a mutual advantage such assemblages may be spoken of as communities. A community, however, in the true sense of the word, is a collection of individuals brought together, not primarily as a result of shortage of breeding ground, but in consequence of advantages of communal ownership over individual ownership. A rookery is an example of a true community. Neither shortage of nesting accommodation nor scarcity of food can account for Rooks assembling together to breed; for if the different pairs which go to make up the rookery were to scatter throughout the surrounding neighbourhood, they would, as a rule, find plenty of trees in which to build their nests, and plenty of food.