Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/63

 It is remarkable that many, deep as the nest is now, are not precipitated headlong into the water; but their long legs and their innate power of clinging, no doubt make their position more secure than it often really appears to be. The nest is one of those beautiful adaptations of Nature which justly excite our wonder and admiration, for on the one hand it must be sufficiently deep to eliminate all possible risk of the young being thrown out, while on the other it must not be too deep to retard the process of incubation. If the depth were too great, there would be a danger of the young in their early stages of growth not being sufficiently covered with the feathers of the parent bird to ensure their bodies being maintained at a proper temperature. How important this must be in the case of the eggs can be readily understood, but it is even more important—and this can be inferred from the behaviour of the parents—to maintain the young for the first few days after they are hatched at a certain temperature. Sleep is to some extent dependent upon warmth, and sleep is a vital necessity for the helpless naked young; therefore if the nest were too deep they would not be properly surrounded with the breast and belly feathers of the parent birds. Even as it is, the female when brooding appears to me to be in a most uncomfortable position. In the construction of the nest she does the greater part of the work, carrying billsful of the seed-heads of the reeds, which she rapidly fixes, and as soon as the outer part of the nest is completed and it begins to take its proper form, she lies in it and shapes the interior with her breast by turning this way and that, and at the same time often uses her bill to assist in arranging the material in the interior. Her efforts to build the nest are not without what appears to be the usual hindrance caused by the exuberant spirits of her mate, for, while she is at work, he often flies at and pursues her, with the result that there is much fluttering amongst or above the tops of the reeds, with an audible clicking of bills as they meet. Five or six days may elapse between the time when the nest is commenced and the laying of the first egg,