Nestling mortality in Australian Pelicans: a model for understanding avian brood reduction
Pelican chick in its nest
How much food does there need to be to make sure that all the chicks in a pelican nest survive?
Hatching asynchrony is when chicks in the same brood hatch at different times, because there is a large time interval between the laying of different eggs. This often means that younger siblings are killed by older siblings if there is not enough food to go round. This project looks at what changes the time interval between eggs being laid, and what effects this has on this chicks.
Male pelican standing over his two chicks, who are engaged in a siblicidal battle
Pelicans are popular and iconic species, with the Australian Pelican being one of the best known Australian birds.
Three of the seven species of pelicans in the world are endangered. The other species have undergone a historical reduction in numbers, and this includes the Australian Pelican.
The Australian Pelican is a facultative brood reducer. This means that the parents leave a gap between laying eggs, so that the chicks hatch at different times. If there isn’t enough food for all chicks the oldest chicks will kill the youngest chicks in the brood.
Lots of species that show this behaviour are endangered, including hornbills, cranes, many birds of prey and parrots. If we can understand the mechanisms resulting in chick mortality, we can also develop methods to increase the number of chicks that survive for conservation purposes.
Australian Pelicans are one of the most recognisable birds in Australia
This project has found the proximate mechanism of brood reduction is hatching asynchrony and siblicidal aggression among nest mates. Hatching asynchrony occurs when incubation starts as soon as the first egg is laid, even though more eggs may be laid later. This has the effect of staggering the time at which the chicks hatch. There are no less than nineteen different ideas about why hatching asynchrony occurs. A bird in which hatching asynchrony occurs tend to lay more eggs than ever survive. This simple observation seems to run against the theory of evolution that underpins the science of biology. Evolutionary theory suggests that birds should lay as many eggs as will survive, no more, no less.
By giving one chick a head start, the adults encourage competition between the chicks. This results in the younger one being killed by its older sibling, unless there is plenty of food to go around. In this way the parents save themselves the effort of collecting food for two chicks, when only one is ever likely to survive. Of course the pelicans can no more predict how good a year will be than we can. So they still lay two eggs, because this year just might be that really good one, when fish are abundant and it will be possible to raise two young.
Food availability appears to be the ultimate determinant of whether brood reduction occurs. If fish stocks are low, adults lay their two eggs up to a week apart. They incubate them from when the first egg is laid, so that they hatch asynchronously, producing an older, larger socially dominant chick and a younger smaller, subordinate chick. Under conditions of food stress the older chick kills the younger chick. If food is abundant adults lay eggs on the same day, chicks hatch synchronously and both survive.
Pelican chicks also differ in quality, depending on their hatch order and sex. Second-hatched male chicks are less likely to survive than second-hatched females, and weigh less if they do survive to leave the nest. First-hatched female-chicks have higher circulating androgen levels and are more aggressive than first-hatched males.
- The Australian Pelican has the largest bill in the bird world, with the largest ever recorded bill being 49 cm long
- Their wingspan can reach 2.5 m, and they can weigh up to 13 kg.
- These Pelicans start breeding when they are two or three years old.
- Find out more about the Pelicans at Adelaide Zoo here