Why do some parents practice infanticide?

16 June 2023

Yesterday, Karl II at the Karula National Forest Black Stork cam made a difficult decision. It was the first time that he had ever practised brood reduction. In the past, it has been the female that has done this when resources were bleak. Little Benjamin was sacrificed so that the other three might thrive. There is still no promise. The storklets are too young for both parents to go out foraging. Like the ones in the Polish nest today, they could easily become targets for predation by other species. There has been no rain and water and the animals associated – frogs and little fish – needed to keep the storklets alive are in short supply. We can only send our best wishes to Karl II and Kaia and to the folks in Estonia who work so hard for these very rare birds, such as Urmas and his team.

Here is a good article that was translated that appeared on the Looduskalender English Forum today that I think might interest some of you wanting to learn more about his particular avian behaviour.

Why do birds kill their chicks?

“This year, as well as in previous years, the black stork webcam watchers have been witnessing events that appear to be incomprehensible and dreadful. One of the recently hatched fluffy-balls has been tossed to the nest rim and doomed to death, or even eaten up by its parents.

Infanticide as a means of regulating the brood size is not universal, but still quite common phenomenon among birds, particularly the species that are relatively big in size. The reason lies almost always in the limited resources, i.e. the parents do not have enough energy, time and food to nurture and feed all of their chicks.

Does this mean that the bird should have laid a smaller number of eggs? This would have been a very big mistake: what if some of the eggs would not have hatched? It is wise of a bird to lay more eggs, just in case. The reserve egg serves as an insurance in cases when something misfortunate happens during incubation, or when the foetus dies inside the egg. For instance, spotted eagles mainly lay two eggs, but frequently enough the second egg is rotten. Despite this, there will be at least one chick and the breeding season is not wasted.

More eggs means that the bird can adjust the size of the brood at a later date, if necessary. The optimal number of offspring may vary between the breeding seasons, and when it becomes clear that this time all nestlings cannot be fed, their number can be reduced accordingly. If all of the eggs produce a chick and the resources turn out to be plentiful, then there is a chance to raise all nestlings.

Asynchronous hatching, observed in many species, provides an opportunity to adjust the number of nestlings. This means that one of the chicks hatches, begins to beg for food and grow well before its siblings. Other chicks hatch later, one by one. Compared to its siblings, the last hatchling is weak and feeble; it is not that capable of begging for food, and it gets parental care only if the food resources are particularly plentiful. If they are not, the chick dies, mostly of starvation, sometimes with the “help” of its siblings (in some bird species, for instance blue-footed booby, siblicide is practised in all broods). In this way the number of chicks can be adjusted to the prevailing conditions.

“Brood reduction” like this is regularly practised by for instance raptors and seagulls. The common gull always has a clutch of three eggs, but the last egg is smaller and lighter than other eggs. This egg very rarely produces a viable offspring. On the other hand, to have a third egg as a reserve is a reasonable strategy.

A famous English demographer Robert Malthus once wrote: “Through the animal and vegetable kingdoms, Nature has scattered the seeds of life abroad with the most profuse and liberal hand; but has been comparatively sparing in the room and the nourishment necessary to rear them.” There is an enormous difference between the amount of resources that parents are able to invest in their offspring, and the amount of resources that the offspring require.

Ecologists call this parental optimism. There are two hypotheses to explain the parental optimism, i.e. the overproduction of offspring. The first is bet-hedging: do not put all your eggs in one basket or, in other words, do not invest in only a single offspring. The second explanation is a possibility to select the best individuals from the overproduced offspring. Among mammals, for instance, selective abortion of weak foetuses occurs. Both hypotheses can act at a time and lead to overproduction of zygotes in all spheres of living nature.

It is obvious that the siblings alone cannot be made responsible for coping with the consequences of the optimistic overproduction of their parents. Unlike newborn hyena cubs, most animals are not able to cut the throats of their siblings. Therefore, sometimes the parents themselves must make corrections to the number of their too optimistically produced offspring.

Infanticide, killing of offspring by adult individuals of the same species, is a widespread phenomenon in animal kingdom. No doubt, it occurs also among humans. Peter Freuchen, a Danish journalist and traveller who spent years among Greenlandic Inuit in the early 20th century, describes an exemplary case in his memoirs: “A mother had four children to feed, but no help was anticipated. Everyone knew her story – how she hanged three of her children to save them from starving to death. She was often referred to as a beautiful exemplary model of maternal love.”

So let´s be not too hard on the black stork mother who jostled her fourth chick away from the nest bowl. It could have been a decision of vital importance, to grant the survival of the remaining three fluffy-balls.”