Siblicide and cainism

The term siblicide is mostly used in the context of bird behaviour. Siblicide occurs when a chick in a brood is killed by its siblings, its brothers and sisters. There are two kinds of siblicide: (1) when the first-hatched chick (“Cain”) always kills the second-hatched chick (“Abel”) – this is called cainism; (2) when the bigger chicks gang up on the tiniest chick and kill it – this happens mostly when food is scarce during the period when the chicks are growing.

The first type of siblicide is technically known as obligate siblicide. This means that it almost always happens, and the clutch size is never more than two eggs. The second type is known as facultative siblicide, and occurs when there is a shortage of resources.

Cainism occurs mainly in raptors. Among southern African raptors, the most famous example is Verreaux’s Eagle. The first egg produces “Cain”, who gets in a few day’s growth before “Abel” hatches,.”Cain”, bigger and stronger, kills “Abel” almost immediately, and this happens regardless of whether there is enough food in the vicinity for the parents to feed both chicks. The second egg is thought to be an insurance policy. If the first egg fails to hatch, the parents don’t have to start incubation all over again.

Facultative siblicide usually occurs when there are many chicks in the nest, and there is a shortage of food during the fledging period. The largest chick is the most demanding, and gets fed first. A hierarchy of size forms, and the smallest and least demanding chick gets fed last, so only gets fed if enough food is brought to the nest for the whole hierarchy. The smallest chick often starves and dies. The formation of the hierarchy can be associated with aggression, with the larger chicks pecking the smaller ones. The end result is that as many chicks fledge in good condition as the parents are able to raise. The alternative is for a all chicks to survive, but all are in equally second-rate condition, and have little chance of surviving the critical period after leaving the nest when they have to start finding their own food.

This form of siblicide is common in owls, which incubate the eggs in the nest from the time the first one is laid. As a result the eggs hatch at intervals of a few days, and a size hierarchy is established from the time of hatching.

There is no evidence that siblicide, or either form, is gender biased.