Because of the various actions which they perform. Thus, for example, we have seen the hooded-crow in Zetland, when feeding on small shell-fish, able to break some of the tenderer kinds by means of its bill, aided, in some cases, by beating them against a stone ; but, as some of the larger shells such as the buckie and the wilk, cannot be broken by such means, it employs another method, by which, in consequence of applying foreign power, it accomplishes its object. Seizing the shell with its claws, it mounts up into the air, and then loosing its hold, causes the shell to fall among stones, (in preference to the sand, the water, or the soil on the ground) that it may be broken, and give easier access to the contained animal. Should the first attempt fail, a second or a third is tried, with this difference, that the crow rises higher in the air, in order to increase the power of the fall, and more effectually remove the barrier to the contained morsel. On such occasions, we have seen a stronger bird remain an apparently inattentive spectator of the process of breaking the shell, but coming to the spot with astonishing keenness when the efforts of its neighbour had been successful, in order to share in the spoil. Pennant mentions similar operations performed by crows on mussels. - Fleming.
Because, as some kind of self-interest and self-defence is, no doubt, their motive, may it not arise from the helplessness of their state in such rigorous seasons; as men crowd together, when under great calamities, they know not why? Perhaps approximation may dispel some degree of cold; and a crowd may make each individual appear safer from the ravages of birds of prey and other dangers. - G. White.
Because of the gentle check which the cold throws upon insensible perpiration. The case is just the same with blackbirds, etc.; and farmers and warreners observe, the first, that their hogs fat more kindly at such times ; and the latter, that their rabbits are never in such good case as in a gentle frost. But when frosts are severe, and of long continuance, the case is soon altered ; for then, a want of food soon overbalances the repletion occasioned by a checked perspiration. I have observed, moreover, that some human constitutions are more inclined to plumpness in winter than in summer. - G. White.
Because of our ignorance of their requisite food. Every one who has made the attempt, well knows the various expedients he has resorted to, of boiled meats, bruised seeds, hard eggs, boiled rice, and twenty other substances that Nature never presents, in order to find a diet that will nourish them; but Mr. Montagu's failure, in being able to raise the young of the curl-bunting, until he discovered that they required grasshoppers, is a sufficient instance of the manifest necessity there is for a peculiar food in one period of the life of birds. - Knapp*
* Journal of a Naturalist, 1829.