Background for this study

Brush-turkeys and other megapodes have evolved a unique reproduction strategy among birds: They neither incubate their eggs nor look after their chicks. Instead, megapodes use external heat sources for incubation, such as geothermal heat, sun heat and/or the heat produced by microbial decomposition of organic matter. Some megapodes deposit the eggs in volcanically-heated burrows in the soil. Others, such as the Australian Brush-turkey, build a mound of organic material, in which the decomposition heat - and sometimes partly the sun - incubate the eggs. After hatching in 40-170 cm depth, the chicks dig themselves out of the burrow or mound and live independently from the very first moment.

Megapodes have developed a number of adaptations to this incubation strategy, the four most profound being:

1. The chicks are extremely precocial (=at an advanced stage of development at hatching).
2. The eggshell is very thin, compared to that of birds of similar size. This is an adaptation to the extreme humidity, the high gas pressures and different gaseous environment in the mound or volcanically heated burrow, which is low in oxygen and high in carbon dioxide.
3. The hatchling is relatively large. This is associated with a long incubation period, which is usually between 50 and 70 days. And it is also associated with larger eggs than in most other birds (10-24 % of the female's body weight).
4. Compared to other galliformes or birds, megapodes can lay many more eggs per breeding season. So far, "clutch" sizes of up to 50 eggs are known.

With 22 species, the megapodes of the Australo-Pacific region only represent a tiny fraction of the approximately 9040 bird species known. Why has only this single family developed such a unique breeding strategy? It seems to have many advantages over the "usual" breeding strategy in birds. First, the parents do not have to invest in raising the young; second, they invest either nothing in incubation where heat is provided by volcanism, or they only have to build and maintain an incubation mound; third but not last, the parents can produce more offspring than most other birds.

The various special features of megapode eggs and chicks mentioned above suggest a strategy that is well adapted. So what are the disadvantages of this approach to incubation? Why do not more birds perform this incubation strategy? The key to understanding more about the evolutionary constraints behind the 'megapode strategy' are the chicks. Only if we know how they cope with not being looked after, how they survive and develop, can we start answering these questions.

However, research into megapode chicks is not only important from this theoretical point of view. It also has an important application for conservation. Nine of the 22 megapode species are listed as vulnerable or endangered. Up to now, chicks could hardly be included in management plans since nothing is known about the ecological factors critical for their survival. Results on, for example, what they eat, where they move, which structures they prefer in their habitat and whether or how they recognize predators or members of their own species will help to develop management plans that also deal with the requirements of megapode chicks.

Article and Photos: Ann Göth; Design: Uwe Vogel
Last updated July 2005