7. Brood reduction

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marika.solo
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7. Brood reduction

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Brood reduction

In birds, parental infanticide is a means of saving energy. By eliminating the individual which has the least chance of survival, the adults aim to increase that of the others. In general, it is the male which kills the smallest of the young and this usually occurs when they are less than a week old.
Nature is very wise.
(! white storks - https://www.birdcenter.org/en/news/news ... za-de-vida)


Essentially, the parents do a cost/benefit analysis of the number of chicks in their nest. Four chicks had already hatched successfully, a large brood to begin with. So, when one late-bloomer crashed the party, it became apparent that there wasn’t going to be enough food to go around. The parents could risk all of the chicks starving from stretched resources, or limit the number of mouths to feed.

According to a 2002 study, paternal infanticide is a rare means of decreasing brood size. Most birds just let the siblings sort it out—a baby bird battle royale at every regurgitated feeding until the weakest one succumbs to starvation. But apparently, stork chicks aren’t aggressive enough, so the parents have to do the dirty work themselves.
(! white storks - https://www.themeateater.com/conservati ... -by-mother)
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Re: Brood reduction

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Brood reduction - parental infanticide reserved

new sources (scientific - author: expert, specialist)
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Re: Brood reduction

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A case of parental infanticide in the black stork Ciconia nigra

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G., Kłosowski & T., Kłosowski & Zielinski, Piotr. (2002). A case of parental infanticide in the black stork Ciconia nigra. Avian Science 2 (1): 59-62.
(https://eounion.wpengine.com/wp-content ... /AS2-1.pdf - p.17 and next)
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Brood Reduction and Parental Infanticide — are the White Stork Ciconia ciconia and the Black Stork C. nigra exceptional?

Brood size in birds is reduced through fatal starvation, siblicide or parental infanticide (killing of own offspring). Both Black and White Storks were observed practising facultative parental infanticide. In the White Stork parents regurgitate large amount of food consisting of many small items on the nest bottom. Chicks pick up food themselves, trying to eat as quickly as possible. No aggression among chicks is observed. As a result monopolisation of food does not occur and elimination of the weakest chick is very ineffective. Sometimes parent storks accelerate brood reduction by killing some of the offspring. Surprisingly, although parental infanticide is a quick and efficient method of brood reduction it is rarely observed, even in species practising it.

Zieliński P. (2002). Brood reduction and parental infanticide — are the White Stork Ciconia ciconia and the Black Stork C. nigra exceptional? Acta Ornithol. 37 (2): 113–119.
(https://bioone.org/accountAjax/Download ... Click=True)
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Re: Brood reduction

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Filial Cannibalism: Why Do Animals Sometimes Kill And Eat Their Own Young?
https://www.scienceabc.com/nature/anima ... young.html

Written by Ashish Last Updated On: 8 Jul 2022
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Tortosa, F. S., & Redondo, T. (1992). Motives for Parental Infanticide in White Storks Ciconia ciconia. Ornis Scandinavica (Scandinavian Journal of Ornithology), 23(2), 185–189. https://doi.org/10.2307/3676447

Abstract:
White Storks Ciconia ciconia parents were observed to kill their smaller chick in9 out of 63 nests observed during a three year study. Infanticidal parents were caring for larger broods and laid larger clutches than non-infanticidal birds. Males killed the chickin 8 out of 9 cases. Victims were born from the last-hatched egg in 4- and 5-egg clutches, they were the lightest in their brood and grew at lower rates than their nest mates during the days preceding their elimination. The last-hatched nestlings in 4-chick broods had lower pre-fledging survival rates than their elder sibs. Potential victims contributed a low fraction to parents' reproductive output, and 4-chick broods were especially costly to raise because parents provisioned them both more frequently and for longer nestling periods. Hence, the presence of an extrachick seems to lower the benefit/cost-ratio to parents rearing a large brood once its elder siblings have hatched successfully. If nestlings do not compete aggressively for food, parents would be selected to eliminate the extrachick themselves.This hypothesis could
provide an explanation for the existence of parental infanticide also in other species.
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Article about bird infanticide, by Tuul Sepp (blog post in Estonian) https://zooloogiablogi.ee/blogi/miks-li ... i-tapavad/
to situation on the nest of Kati and Karl (season 2017), streamed nest in Karula National Park

(google translation - later corrections by our Madli who is Estonian)
Why do birds kill their young?
Tuul Sepp, 03 June 2017

Both this and previous years, black stork nest cam watchers have witnessed seemingly incomprehensible and terrifying events. One of the recently hatched fluffy feather-ball chicks is pushed to the edge of the nest by the parents and destined to die, or even skinned by the parents themselves.

Infanticide to control litter size is not common among birds, but it is still quite common, and it often occurs in larger species. The reason is almost always limited resources - the parents do not have the energy, time or food to feed and care for all hatchlings.

Does this mean that the bird should have laid fewer eggs? It would have been a very big mistake if some of the eggs hadn't hatched. It is probably worth it for the bird to lay more eggs just in case. A spare egg insures in case of accidents during hatching and in case of chicks dying inside the egg. For example, golden eagles usually lay two eggs, but quite often the second egg is rotten. Then a chick will still come from one egg and the year will not go by empty.

More eggs leave the bird with the opportunity to adjust the size of the litter later according to the circumstances. The optimal number of offspring can vary between breeding seasons, and if it turns out that not everyone can be fed this time, their number can always be reduced later. However, if it should happen that all the chicks hatch, and the resources are especially bright this year, it may really be possible to raise all the chicks.

Asynchronous hatching, which occurs in several species, also provides an opportunity to regulate the number of young according to the circumstances. This means that one chick hatches before the others and starts eating and growing. Later, the other cubs also hatch one after the other. The last hatchling is clumsy compared to its big siblings, it can't eat as much food, and it only gets the care of its parents when food is especially plentiful. If it does not divide, this young dies, mostly of starvation, sometimes with the "help" of its siblings (fratricide is a phenomenon that occurs in some bird species, such as the blue-footed tern, in every litter). In this way, the number of offspring can be adjusted to the circumstances.

Such "strain reduction" occurs regularly in birds of prey and seagulls, for example. The tern of the herring gull is always three-egg, but the last egg is smaller and lighter than the others. This son will rarely grow into a viable offspring, but a third egg as a backup is a sound strategy.

The famous English demographer Robert Malthus wrote: "In the animal and plant kingdom, nature has sprinkled the seeds of life with a generous hand. However, he has been rather limited in terms of space and food that would be needed to raise them". There is an extraordinary difference between the amount of resources that parents are able to invest in their offspring and what the offspring need.

Ecologists call this parental optimism. Parental optimism, overproduction of offspring is explained by two hypotheses. The first is bet-hedging - don't put all your eggs in one basket, i.e. don't put all your reproductive effort into one offspring. Another explanation is the ability to select the best from overproduced offspring. In mammals, for example, selective abortion of weaker fetuses occurs. Both hypotheses can in principle work simultaneously and lead to overproduction of zygotes everywhere in wildlife.

Obviously, the responsibility for coping with the consequences of parental optimism, the overproduction of offspring, cannot remain the task of other sons in nature. Most animals are not able to chew through the throat of their nestmate as a newborn, as hyena cubs do. Thus, parents are sometimes forced to adjust the number of offspring obtained in their burst of optimism with reality.

Infanticide, the killing of offspring by adult conspecifics, is a widespread phenomenon in the animal kingdom and has undoubtedly occurred in humans as well. Danish journalist and traveler Peter Freuchen, who lived among the Inuit in Greenland for many years in the early 20th century, describes one example in his memoirs: "The mother had four children who wanted to eat, but there was no help to be expected from anywhere. Everyone knew his story - how he hanged three of his children to protect them from starvation. She herself was often cited as a beautiful example of motherly love as a role model for others."

So let's not be too angry with the mother of the black stork, who nudged her fourth child on the edge of the nest. This decision may have been key to giving the other three puffballs a chance at life.

Urmas in comments:
The article describes a general trend developed during evolution, but not all individuals follow the trend optimally, some have a specific instinct (e.g. elimination of the inferior son) that influences their behaviour more, others less so. Thus, observing different nests of black storks, infanticide may not be manifested in the same way everywhere, but it seems to be the general trend.
Those who observed the behaviour of the parent birds before the removal of the chick, probably noticed that it was not a slouch/dormant behaviour, but the mother bird was watching the chicks intently before the removal and based on that made a decision (to put it human terms). While last year one of the three chicks was clearly smaller than the others, this year the four chicks were quite similar and making a choice was probably not so easy.
At the moment, there is probably enough food for all four chicks, but due to the effect of the dry spring, it will be scarce of food after the frogs spawn, when they mostly have to switch to predominantly fish food. Fish of appropriate size may not be available in July because their reproduction was inhibited due to low water this spring.
But how do black storks know this in advance?
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Re: Brood reduction

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2024

Filial cannibalism (the eating of one's own offspring)

L. S. Cano-Alonso, U. Sellis, E. A. Tamás, B. Kalocsa, B. Janic, D. Pieniak, I. Carbonell, A. Torés & P. Zieliński (2024) Filial cannibalism in the Black Stork (Ciconia nigra), The European Zoological Journal, 91:1, 245-251, DOI: 10.1080/24750263.2024.2314625

Abstract https://www.tandfonline.com/doi/full/10 ... 24.2314625

Filial and sibling cannibalism have never been documented in Black Storks (Ciconia nigra). We gathered information on 34 breeding events in 10 Black Stork nests from Estonia (three nests), Hungary (two nests), Poland (two nests) and Spain (three nests) being monitored with camera surveillance, live streaming webcams or intensive monitoring control of nests.

Overall, we recorded 16 cases of filial infanticide and two cases where the nestlings died by natural causes and were later cannibalized by either their siblings or their parents.

Four nestlings were killed by their parents without any attempt of cannibalism.

In the remaining 12 cases of infanticide (66.7% of the total losses), 8 nestlings were consumed by one of the parents while in 4 cases the parents were not able to swallow the previously killed nestlings.

All victimised nestlings were the youngest, weakest or smallest in their brood.

Eight of 14 cases were identified as being associated with environmental stress or an exceptional matter during the breeding season. In at least five cases, one of the mates was new to the nest.

Females committing filial infanticide swallowed or tried to swallow the chicks in five out of nine episodes where the parent’s sex was known.

Discussion is very interesting too https://www.tandfonline.com/doi/full/10 ... 25#d1e1066
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Re: 7. Brood reduction

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Cannibalism in animals is more common than you think
Published January 5, 2023 By Liz Langley

https://www.nationalgeographic.com/anim ... ertebrates

“Only in recent decades has it been seen as an adaptive strategy for survival and reproduction.
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