. Audubon . own excavations instead of a< cepting natural cavities. Suchis the habit of tin Woodpeckers and the Kingfishers today. They excavate The Audubon Societies 239 their nesting cavities but they build no nests within for their eggs. Birds thathad been in the habit of burying their eggs, however, and now had to lay themon the surface of the ground where they could be incubated, had other problemsto meet. There were the floods, the cold, wet ground, the numerous terrestrialenemies, all threatening to destroy the eggs. It is easy to imagine, therefore,that those individuals that learne
. Audubon . own excavations instead of a< cepting natural cavities. Suchis the habit of tin Woodpeckers and the Kingfishers today. They excavate The Audubon Societies 239 their nesting cavities but they build no nests within for their eggs. Birds thathad been in the habit of burying their eggs, however, and now had to lay themon the surface of the ground where they could be incubated, had other problemsto meet. There were the floods, the cold, wet ground, the numerous terrestrialenemies, all threatening to destroy the eggs. It is easy to imagine, therefore,that those individuals that learned to raise their nests off of the groundwere the ones that persisted until the habit was formed. The first nests weredoubtless very crude and the beautiful structures with which we are familiarare no doubt the result of a gradual evolution as already indicated. We have stated that nests are ordinarily built by the female birds thoughthe male often makes a pretense at helping. The time required depends a good. A PAIR OF COWBIRDSThey have no home-life, but are parasitic upon other birds deal upon the time at the disposal of the birds, but, with ordinary birds, likeRobins or Blackbirds, it is about six days. Three days are spent on the outsideand a like time on the interior. The same bird, however, if the first nest isdestroyed while the eggs are being laid, might build an entirely new nest in asingle day. A pair of Phcebes, on the other hand, under observation thisspring, began repairing an old nest fully a month before any eggs were the nest is completed the day before the first egg is laid. Incubation does not ordinarily begin until egg-laying is completed, so thatall of the eggs will hatch at about the same time. Otherwise the first youngto hatch would have an unfair advantage over the others in the nest. Occa-sionally one finds Owls or Bitterns beginning to incubate before all of the eggshave been laid, but they are, perhaps, less regular about egg-laying than mo
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