Avian incubation

The word incubate in the context of birds refers to the development of the chick (embryo) within the egg and the constant temperature required for its development over a specific period. In most species, body heat from the brooding parent provides the constant temperature, though several groups, notably the Megapodes, instead use geothermal heat or the heat generated from rotting vegetable material, effectively creating a giant compost heap. The Namaqua Sandgrouse of the deserts of southern Africa, needing to keep its eggs cool during the heat of the day, stands over them drooping its wings to shade them. The humidity is also critical, and if the air is too dry the egg will lose too much water to the atmosphere, which can make it difficult or impossible for hatching. An egg will normally become lighter and the air space within the egg will normally become larger as incubation precedes owing to evaporation from the egg.

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In the species that incubate, the work is divided differently between the sexes. Possibly the most common pattern is that the female does all the incubation, as in the Coscoroba Swan and the Indian Robin, or most of it, as is typical of falcons. In some species, such as the Whooping Crane, the male and the female take turns incubating the egg. In others, such as the cassowaries, only the male incubates. The male Mountain Plover incubates the female's first clutch, but if she lays a second, she incubates it herself. In Hoatzins, some birds (mostly males) help their parents incubate later broods.

Incubation times range from 11 days (some small passerines and the Black-billed and Yellow-billed Cuckoos) to 85 days (the Wandering Albatross and the Brown Kiwi). In these latter, the incubation is interrupted; the longest uninterrupted period is 64 to 67 days in the Emperor Penguin.^[1]

Some species begin incubation with the first egg, causing the young to hatch at different times; others begin after laying the second egg, so that the third chick will be smaller and more vulnerable to food shortages. Some start to incubate after the last egg of the clutch, causing the young to hatch simultaneously.

Derived meanings

Climate-controlled incubators are utilized in industrial agricultural settings and in neonatal care, especially of human infants. The life expectancy for premature infants has increased dramatically thanks to incubation.

References

• Christopher Perrins (editor), Firefly Encyclopedia of Birds, ISBN 1-55297-777-3