The mark of Cain

IS NATURE nasty? The behaviour of baby eagles in the nest might lead you to think so. A golden eagle chick has hatched at Glenveagh National Park in County Donegal, the first Irish-born eagle in almost a hundred years.

Its parents were brought to Ireland from Scotland as part of the Golden Eagle Reintroduction Project.

Lorcan O’Toole, who directs the project says that the female nested last year but her eggs failed to hatch. Unusually for eagles, which are monogamous, the pair went their separate ways and she took up with a new partner. They had two babies this year but one of them perished, almost certainly a victim of the notorious ‘Cain and Able’ syndrome, a strange behaviour of young eagles.

A robin or a blackbird will lay an egg each day until her clutch is completed. Only then will she start incubating. A mother needs food to produce eggs but it’s difficult to get enough if she has to sit on eggs for most of the day. By postponing incubation, more eggs can be produced. But there is a downside; the babies develop together and hatch at the same time. This means that the nestlings are all of the same size and equally vociferous in their demands for food. As a result, she tries to feed all of them. In seasons where there is plenty of food this is not a problem but, when food is scarce, there’s not enough to go around and all of the chicks go hungry. Entire broods may die and those which fledge are underweight, ill equipped for the challenges of life.

Birds of prey do things differently and golden eagles are a typical example. An eagle usually lays two eggs although, in about 10% of nests, there are three. Four are recorded occasionally. As soon as the first egg is laid, the female starts to incubate it. This limits her feeding time and she is slow to form the second egg, which will take her three or four days to produce. With this head start, the first chick hatches several days before the second. The bigger chick, being more vocal and aggressive, brow-beats the parents into feeding it first and, only when the big one has had its fill, does the little one get anything. This seems unfair and cruel to human eyes but, as a way of raising young, it is ruthlessly efficient. Should there be a food shortage, as there often is with birds of prey, the older chick gets all of the grub and has the best possible start in life. The younger chick starves and, if it dies, the older one will eat it, ensuring that the resources which went into forming the second egg are not wasted.

But golden eagles have refined the process further. As soon as the young chick hatches, the older one begins pecking at it. Once it’s strong enough to do so, it kills and eats the sibling. In only about one nest in five, does the smaller chick manage to survive. The aggression won’t cease until the babies begin growing feathers; they coexist amicable from then on. There is some evidence that the second chick’s survival depends on food availability. If Cain, the older chick, has enough to eat, he may not bother attacking Able. But when food is scarce, killing and eating the younger sibling before it has a chance to grow and demand food, is an effective way to use the limited resources available. Although a mother eagle won’t intervene to prevent the murder taking place, the second chick has a better chance of surviving the more time she spends at the nest. Of course, she can only remain there if the male brings her plenty of goodies, so food availability, rather then the mother’s presence, may be the crucial factor in saving the youngster.

There are two types of eagle; those which wear trousers and those which don’t. The golden eagle is one of the ‘aquila’ eagles, the ones which have their legs covered by feathers. Many of the aquila species indulge in Cain and Able behaviour. The bare-legged ‘haliaeetus’ eagles, such as the fish-eating American bald eagle and the white-tailed eagle formerly found in Ireland, don’t indulge in such barbarism. Just why the practice should be widespread in one group of eagles and not in the other is a mystery.

But eagles are not the only birds to engage in siblicide. The masked-booby, which lives in the Pacific Ocean, is an elegant relative of our Irish gannet. A booby lays two eggs but when the second chick hatches, the older sibling pushes it out of the nest. The mother makes no attempt to retrieve the rejected chick and it dies of exposure and starvation. An owl’s first egg tends to be bigger than the second and the subsequent eggs are smaller still, ensuring that the earliest hatched chicks are stronger than the later ones. Barn owl chicks will eat the bodies of their dead brothers and sisters but only occasionally do they kill the smaller ones. In some penguin species, the situation is reversed. They lay a small egg first and then a larger one. The chick from the small egg survives only if the one from the big egg fails. The second egg provides insurance against disaster.