MARSH WARBLER
this will have to be generally evolved in the future, though perhaps not so needlessly strong as is sometimes the case even now. One other feature with regard to the nest deserves mention, and this is the depth of the foundation. Every nest is by no means alike in this respect; for whereas in one case the foundation may be of the normal depth, in another there may be a solid platform of dead grass measuring between two and three inches. So that here again the variation is considerable, and it is difficult to see what useful purpose can be served by the increased labour that is sometimes bestowed upon it.
From four to five eggs appear to be the normal clutch of the species. On the average one egg is laid every twenty-four hours, either during the night or the early hours of the morning. Incubation lasts about twelve days and during this period the excitement of the male becomes appreciably less. He nevertheless shows signs of irritation when his territory is intruded upon by another male of his own or another species, and displays it in the manner already described. His song decreases gradually in strength, being confined in a great measure to the early hours of the morning, and even then it can scarcely be compared to its former glory. Both sexes share in the duties of incubation, and they often relieve one another approximately every half-hour. While one of them is engaged in incubating, the other occupies its time in searching for food, preening its feathers, or pursuing an intruder. The changing of places is done stealthily. The relieving bird does not fly directly to the nest, but approaches it slowly by hopping from stem to stem through the dense undergrowth, so that one is oftentimes made aware of the change solely by the shaking of the various plants as the bird travels amongst them. The one that is being relieved slips off the nest quietly and disappears, leaving scarcely any trace of its departure. How far they are aware of one's presence and how far their conduct is affected thereby is difficult to say, but they are decidedly more timid than the Reed Warbler under similar
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