Forest Preserve District of Cook County, Illinois
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Incubation of Birds' Eggs
Nature Bulletin No. 456-A   May 6, 1972
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

All birds lay eggs. Most reptiles lay eggs. There is one peculiar group of birds, the "mounted builders" found in Australia and neighboring islands, which, like many reptiles, lay their eggs in heaps of soil or decaying vegetation and pay no more attention to them. The young, after hatching, are able to fly almost at once.

All other birds incubate their eggs by supplying heat from their bodies, and give the young devoted care. That heat may be considerable, since the normal temperature of a bird varies from 100 to 112 Fahrenheit according to the species. Ordinarily, incubation does not start until the clutch or "set" is completed. As that time approaches the bird becomes more secretive and stealthy. A bare spot or "brood patch" is developed on the breast by molting. This, specially equipped with blood vessels, becomes suffused with blood. Then the bird becomes "broody". Ducks and geese create this bare spot by pulling down from their breasts so that the eggs can be in direct contact with the skin, and cover the nest with this down whenever they leave it to feed.

Songbirds nearly always lay one egg-per day, usually in the morning, but some of the larger birds -- hawks, owls and geese, for instance -- are more irregular. Once incubation begins the bird seldom leaves the nest, and then not long, because the eggs will not hatch if they get too cold. When both parents are colored alike they usually share equally in sitting on the nest; but when the male has much brighter colors he usually stands guard over it while she feeds or else he brings food to her. In the phalaropes -- a group similar to the sandpipers -- the female is more brightly colored than the male and he not only incubates the eggs but also takes care of the young. Among such big flightless birds as the ostriches, cassowaries and emeus, it is said that the male tends to all or most of the domestic chores.

The length of the incubation period varies from 10 days, with the cowbird, to 80 days with the royal albatross and from 70 to 80 days with the emeu. Sparrows require from 11 to 13 days; thrushes, including the robin, 13 or 14; domestic chickens 21; ducks, depending on the size, from 21 to 30; geese, from 30 to 35; and 50 to 60 days for ostriches. It seems to depend upon the size of the egg, the kind of young and, perhaps, on the body temperature of the parent.

Most ground-nesting birds have relatively large eggs and a long incubation period. Their young -- when hatched -- are wide awake, covered with down and, soon after drying off, able to follow their parents around and feed themselves. These are the precocial species. Most birds that nest in trees or holes, including the songbirds, are altricial species: they have small eggs, a short incubation period, and the young are blind, naked or nearly so, and helpless when hatched. During incubation the eggs have to be turned once or twice a day so that they will be heated evenly and the membranes of the embryo will not adhere to the shell. Some birds, like a hen, do this with the bill; others with their feet. When hatching, the egg is "pipped" from the inside and, to do this, the young bird has a hard sharp "egg tooth" on its soft upper bill. Later, this disappears.

It's interesting to watch a mother bird return to her nest, carefully straddle the eggs, arrange them, wriggle around until she gets them all next to her "brood patch", and then contentedly huddle down. There she will squat, motionless, until perhaps the next day.

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