The struggle chick maleo
Maleos are communal nesters and on the largest known site at Bakiriang on the south coast of the north-east peninsula, more than 600 birds nest early in the year with the holes only two or three metres apart. Two hundred of the birds nest on just 1 ha of sand (Watling 1983).
The surface of a sandy beach can become extremely hot, over 50 celcius degree and 80 celcius degree on white and black sand respectively (MacKinnon 1978), yet just a few centimeters below the surface, the temperature is relatively stable at about 36 celcius degree. It seems as if most eggs, on beachs or elsewhere, are laid in positions where the temperature is between 32 celcius degree and 38 celcius degree (MacKinnon 1978, Wiriosoepartho 1980). The depth of the hole might be thought to be critical, and it has been suggested that the bare head of the maleo is efficient at sensing temperature but, in reality, the exact depth and temperature (within certain limits) are not so critical. Instead it seems that the eggs are laid as deep as possible for protection against predators.
Hatching takes about three months and if the chicks survive the one or two day scramble to the surface, for ants are a major predator of chicks in the ground (R. Dekker pers. Comm.), they are able to fly away immediately, already having adult plumage the manner in which they ‘explode’ from the sand and rush away is probably an adaption to avoid the attention of predators (Watling 1983). The great size of the egg is related to the need to produce a chick strong enough to struggle up to the surface (Guillemard 1889).