BRITISH WARBLERS
the whole behaviour is so simply explained by selection, in that those members that did not construct their nest sufficiently securely would be very liable to lose their offspring, we may conclude that this is the means by which the degree of adaptation has been attained.
The nest is a delicate and beautiful structure composed principally of the seed-heads of the common reed. The foundation is formed of the leaves, or pieces of the outside of the stem, of the dead common reed, intermingled with the thinner stems of various rushes woven together with moss and wool, and the interior of seed-heads and small stems of rushes, the lining being entirely of the former. The external part is much the same as the foundation, wool and moss being interwoven. The manner in which the nest is actually woven to the stems is remarkable, and I much regret never having seen the birds at work "laying the first brick," so to speak, for to myself it is a mystery exactly how the initial step is taken. The most characteristic and at the same time most interesting part about the nest is its great depth. Such a peculiarity cannot but be for some purpose, and the reason often advanced is that the eggs are thereby prevented from rolling out when the reeds are bent. I have watched the nest swaying in a wind, and have even bent the reeds for the purpose of finding out how far it would have to be tilted before this would happen, and the conclusion thus reached was that even if it were half an inch shallower the wind would have to be of very exceptional strength before it could bend the reeds sufficiently to bring about such a result; for it must not be forgotten that the nest is not placed at the top of the reeds, but in the centre, three or four feet from the water. If indeed it be true that there is a danger of the eggs thus rolling out, what must we say as to the young? Herein, at least, lies a possible danger. Watch them when a week or ten days old clinging to the nest, the topmost birds level with or even slightly above the sides of the nest, and then imagine the nest itself no deeper than that of the Whitethroat, and the danger will be at once apparent.
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