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| Dr Ann Goeth and Associate Professor Darryl Jones are the authors of Mound-Builders, a book that looks into the incubation mounds built by the Australian species of the megapodes, such as the Australian Brush Turkey.
Megapodes are the only birds in the world who do not use body heat to incubate their eggs. They are found only in the South Pacific region, on islands as far east as Tonga and as far west as the Nicobar Islands (near India).
Australia has three species of megapodes: the Australian Brush Turkey, the Mallee Fowl (see cover photo left) and the Orange-footed Scrub Fowl. These belong to the mound building group of megapodes since they build large incubation mounds of leaf litter. Inside these mounds, the eggs are incubated by the heat produced by decomposition of the leaf litter, and the chicks hatch up to a metre or so inside the mound.
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| Once built, the males, using a temperature sensor located in their palate, regulate the temperature inside the mounds on a daily basis. If the mound is too hot it is opened up a little. If it is too cold more material is piled up on top. The eggs themselves are not tended, even during hatching.
Right: Ann in hide
Below: Darryl Jones with Brush Turkey chick
Photos from Ann Goeth
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| The eggs are very large compared to other birds, weighing between 12% and 20% of the female's weight. The very large yolk provides enough protein food for the developing embryo during its long incubation. The size and weight of the egg are important; because when the chicks hatch they must be very well developed and able to fend for themselves. There is no input from either parent as they go about their (relatively) solitary lives.
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The eggshells are also much thinner than in most other bird eggs, allowing air trapped in the mound to enter the egg and supply enough oxygen for the developing chick. The incubation period is very long, some 49 days for a Brush Turkey.
The following figures illustrate the chick's first 40 hours after hatching, showing how it slowly digs its way out of the mound.
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a: chick uses its legs to break out of the thin egg shell
b: the chick rests in a cavity it has dug and preens its feathers
c: with eyes closed the chick slowly starts to dig upwards
d: frequent rests sitting on its long legs on trampled down loose soil
e: when near the surface the chick begins to tunnel out very actively
f: as soon as the surface is reached the chick quickly runs from the mound to find cover in the surrounding bush.
Drawings by Ann Goeth.
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Unlike other birds, the chicks break the shell of the egg with their large legs and feet (other birds use their egg tooth). They then rest for quite a while, since hatching requires a lot of energy. While it takes two days for a chick to emerge from the mound, it is only in the last couple of hours that it vigorously digs its way out.
The Mallee Fowl, the most endangered of the Australian megapodes and the only one to inhabit arid areas, uses similar methods, but builds the mounds from sand with a chamber inside lined with whatever leaf litter is available.
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The Australian megapodes are indeed unique in many ways and Ann and Darryl's book explores many of their fascinating adaptations. Mound-Builders is published by CSIRO Publishing, and retail price is $40.
Text: V.B. December 2008
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