What good does the behavior do for the survival of the individual?

 Mock, Drummond and Stinson (n.d) define avian siblicide as an act in which the progeny of a bird is killed by its nest sibling(s). The trait may be evolutionary beneficial in a case where the progeny with inferior characteristics is not allowed to grow to maturity and therefore cannot perpetuate its genes while the adaptable characteristics carried on by the good genes is perpetuated by the individual who survives to maturity. Siblicide can be viewed in two dimensions: Obligate siblicide is the unconditional killing of an individual by its sibling. This trait is rare and only a few of the bird species are known to practice it. On the other hand, facultative siblicide only occurs when resources are limited and is much more common in many bird species. For it to occur, there are four common features that are normally observed in the bird’s offspring; they include competition for resources especially food in many cases which the birds are fed by their mother, spatial confinement, hatching asynchrony where one offspring hatches earlier than the other(s) and therefore has an advantage of being bigger and weaponry such as claws or beak with which the aggressing offspring uses to perform the siblicide.

The black eagle (Aquila verreauxi) is one of the earliest bird species in which siblicide was reported. The eagle is a typical example of a species that is known to practice obligate siblicide. The species mostly lives in the western regions of the Middle East as well as in the mountainous terrain of northeastern and southern Africa. The species generally builds its nest on the cliff edges and lays two eggs normally during the period between April and June. The hatching of the eaglets occurs at least three days apart and therefore the older chick is clearly larger than its sibling. Normally, the older eaglet relentlessly attacks the sibling from as early as when the younger sibling is hatched. In one such a case that was documented, the larger eaglet pecked the sibling one thousand five hundred and sixty nine times during a span of only three days. In recorded cases from the nests of the black eagle in which both eaglets hatched, only one sibling was found to exist in two hundred observations. In the majority of cases, it is the senior sibling that fledges (Mock, Drummond & Stinson, n.d.).

The effective siblicide behavior in the black eagle is correlated with the small nest and therefore the junior eaglet cannot escape the nest. This lack of the option to escape from the nest contributes to the death of the victim. The competitive disparity observed on the two eaglets is crucial to the evolution of the aggression behavior. The disparity is created when the parents start to incubate one egg prior to the laying of the second one. Asynchronous hatching in itself is a behavioral adaptation for a secondary adaptation to the size of
the brood to match the available resources. Since only one eaglet survives, the parents produce the additional egg as cover up in case the first eaglet fails to survive. The asynchronous hatching is advantageous to the first eaglet since it can eliminate the younger sibling with much ease unlike a case of synchronized hatching in which the siblings, being the same age, could have fought each other (Mock, Drummond & Stinson, n.d.).

Since the black eagle almost always commits siblicide, the younger sibling rarely survives to maturity. However there are cases in which the older sibling dies early and the younger one therefore survives providing an insurance reproductive value, that is, the younger eaglet’s reproductive value is entirely because of the role it plays as an insurance policy. Siblicide therefore is an adaptation in the sense that the chances of one of the eaglets of the black eagle surviving to maturity are high from which the genes of the parents are carried to the next generation. The two eaglets are competitors for resources and eliminating one of them increases the chances of survival for the other and at the same time promote the likelihood that genes promoting the siblicide behavior shall be represented in the following generation (Mock, Drummond & Stinson, n.d.).

Mock, Drummond and Stinson (n.d) explain that a gene’s fitness is determined more than by its ability to contribute to the reproduction of the individual. The fitness also is based on its influence on the reproduction prospects of close relatives genetically. This is an expanded definition of the success of evolution and is known as inclusive fitness, which is an individual property. The organism’s inclusive fitness measures its reproductive success together with the decremental or incremental influence it imposes on reproductive success of the kin. In the case of the black eagle, the younger eaglet tends to be the victim in most cases. The younger eaglet therefore is marginal in that the reproductive value it holds can be examined in the light of what it subtracts or adds to the success of the sibling. The marginal individual embodies the reproductive value in that it serves as a replacement for the elder sibling incase it dies prematurely. It is therefore an insurance against the death of the senior sibling. The insurance value in this case depends on the chance of the senior sibling dying.

References

Mock, D. W., Drummond, H. and Stinson, C. H. (n.d.). Avian Siblicide: Killing a brother or a sister may be a common adaptive strategy among nestling birds, benefiting both the surviving offspring and the parents. University of Oklahoma

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