Many species have been seen practicing infanticide and cannibalism. But birds of prey, also known as raptors, are something of an exception. They have only rarely been seen doing these things, and when they have been spotted the circumstances were usually exceptional. For instance, parents have killed their own offspring to feed siblings, or chicks that were already dead have been eaten by parents or siblings.
But one bird of prey has taken things a bit further. In two separate studies, researchers have caught Eleonora's falcons on camera breaking both taboos. These birds, it seems, are willing to prey on their neighbours' offspring.
One study reports an incident of cannibalism that took place in September 2013 on Akrotiri Peninsula in Cyprus. It was published in the Journal of Raptor Research in 2016.
A nestling is easy prey because it has little strength to fight back, and it can't fly to escape
Eleonora's falcons generally eat winged insects, but during the breeding season they mainly feed on small birds instead. It is therefore not surprising to see adult falcons dismembering birds to feed to their chicks.
But when the chicks in one nest were about three weeks old, their mother was seen holding a one-week-old chick that appeared to be from the same species. The mother then killed the helpless chick and fed it to her young.
"We were certainly not expecting to see this," says Alexander Kirschel of the University of Cyprus in Nicosia, who made the observations with his colleague, Thomas Hadjikyriakou. "We were expecting to see adults bring small migrant prey to the nest to feed their young, but not nestlings of their own species."
Just before the incident, the adult male falcon had been standing guard in the nest. Kirschel and Hadjikyriakou suspect that the adult female, who was out searching for food, stole the chick from a neighbouring nest.
There seems to be a small window in the morning, when both parents are away
"An Eleonora's falcon may not usually have the opportunity to steal a nestling from another nest, because the parents will be protecting their young," says Kirschel. "But given the opportunity, when parents may have left the nest for a short time, a nestling is easy prey because it has little strength to fight back, and it can't fly to escape."
That is in line with the findings of a second study, which documents three incidents of baby bird robbery, all while the nests were unguarded by the parents. These events took place in the early mornings of September 2014 and 2015 on the tiny Greek island of Anidro, and are also described in the Journal of Raptor Research.
"There seems to be a small window in the morning, when both parents are away looking for food for their dependent nestlings," says Ronny Steen of the Norwegian University of Life Sciences in Ås.
In one case, Steen was surprised to discover "a stranger at the entrance of the nest just after sunrise, namely a neighbouring female".
Non-parental infanticide is generally very rare
He saw the intruder arriving at a nest just before 7am and attacking three chicks. She visited the nest four times in total, but her last visit was cut short when the mother of the chicks returned and attacked. Only one chick survived.
The same nest was attacked the following year. The unwanted visitor left with the nest's one and only chick in its talons. Its parents kept returning to the empty nest anyway, bringing food for their lost chick.
Two days later, an intruder attacked two chicks at a neighbouring nest. These chicks, however, were older and fought back with their talons. One managed to escape. Its sibling was not so lucky
"Non-parental infanticide is generally very rare, and has never been directly documented in Eleonora's falcons before," says Steen.
Although he cannot be sure, Steen says that there might be just one intruder, raiding different nests in consecutive years. He also suspects that this intruder stole the chicks to feed its own young – which would fit with the observations that Kirschel and Hadjikyriakou made.
"This study provides evidence of an adult stealing a nestling from a nest, whereas ours shows what the adult then does with the nestling — takes it to its own nest, kills it, and feeds it to its young," says Kirschel.
Kirschel and Hadjikyriakou say the nest where cannibalism took place was the most successful they tracked in their three-year study. It produced three fledglings each year. They suspect that the cannibalism might have contributed to the success.
In other species, such as herring gulls, it seems that only some individuals carry out infanticide and cannibalism. "So it might be something that specific individuals learn, rather than [this being] a generic behaviour in Eleonora's falcons," says Hadjikyriakou.
"We could talk about this as a strategy; a strategy that some individuals adopt," says Steen. "However, this strategy will never be too frequent as this is how natural selection works."
The new studies show what happens when food resources are limited or when breeding densities are high. Infanticide and cannibalism in birds may be more common when they breed in colonies.
Although I cannot be certain, I find it more likely that the female adopts an infanticide behaviour
"Migrating birds have seasonal and other temporal fluctuations, mainly governed by weather conditions, so there are periods or even whole seasons that food supply is low," says Hadjikyriakou. "This puts stress on the breeding population and might trigger individuals to look for alternative sources of food, such as nestlings in nearby nests."
The prey is usually brought to the nest by the male and given to the female to feed the chicks. In all cases, however, it seems that the female was the culprit.
"Although I cannot be certain, I find it more likely that the female adopts an infanticide behaviour, as she is more attached to the nesting ground and she might have a better overview of presence or absence of neighbours," says Steen.
It seems Eleonora's falcons may be quite flexible in their behaviours. In another study, which was described in New Scientist in 2016, researchers found that Moroccan Eleonora's falcons seem to cripple and imprison live birds, apparently storing them for later consumption.
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