Avian Incubation
The widest range of incubation habits is displayed among birds. In warm-blooded species such as bird species generally, body heat from the brooding parent provides the constant temperature, though several groups, notably the Megapodes, instead use heat generated from rotting vegetable material, effectively creating a giant compost heap while Crab Plovers make partial use of heat from the sun. 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 hatching difficult or impossible. As incubation proceeds, an egg will normally become lighter, and the air space within the egg will normally become larger, owing to evaporation from the egg.
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.
The incubation period, the time from the start of uninterrupted incubation to the emergence of the young varies 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. In general smaller birds tend to hatch faster but there are exceptions, and cavity nesting birds tend to have longer incubation periods. It can be an energetically demanding process, with adult albatrosses losing as much as 83 g of body weight a day. Megapode eggs take from 49 to 90 days depending on the mound and ambient temperature. Even in other birds, ambient temperatures can lead to variation in incubation period. Normally, of course, the egg is incubated outside the body. However in one recorded case, the egg incubation occurred entirely within a chicken. The chick hatched inside and emerged from its mother without the shell, leading to internal wounds that killed the mother hen.
Embryo development remains suspended until the onset of incubation. The freshly laid eggs of domestic fowl, ostrich and several other species can be stored for about two weeks when maintained under 5 C. Extended periods of suspension have been observed in some marine birds. 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.
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