68 BIRDS' EGGS. which we can give no satisfactory reason. Thus the Crested Flycatcher's strange custom of using a cast snake-skin in its nesting materials probably originated with the birds in the tropics, where it is still followed by nearly related species of Crested Flycatchers. With them there may be a reason for this habit, but with our bird, living as it does under entirely different conditions, it is doubtless only an inheritance, surviving even when the necessity for it has ceased to exist. Eiglith, change of habit. Some birds are influenced by changes in their surroundings, and alter their nesting habits when it proves to their advantage to do so. Chimney Swifts, who have exchanged hollow trees, in which they were exposed to their natural enemies, for the comparative safety of chimneys, are good examples. But a far better one is given by that prodigy in feathers, the House Sparrow. Is there any available site in which this thoroughly up-to-date bird will not place its nest ? It has taken possession of even the hollow spaces about certain kinds of electric lamps, and has been observed repairing its nest at night by their light ! The Eggs. — Usually, little time is lost between the completion of the nest and the laying of the eggs. The number of eggs composing what oologists term a full set or clutch ranges from one to as many as twenty. At the time of laying, the ovary contains a large number of partly formed eggs, of which, normally, only the required number will become fully developed. But if the nest be rol)l)ed, the stolen egg will frequently be replaced. The long-continued laying of our domestic fowls is an instance of this unnatural stimulation of the ovary. Doubtless the most remarkable recorded case of egg-laying by a wild bird is that of a High-hole or Flicker, who, on being regu- larly robbed, laid seventy-one eggs in seventy -three days ! The eggshell is composed largely of carbonate of lime,
