African honeyguide birds lay their eggs in the underground nests of other bird species.
Scientists from the University of Cambridge, UK, used night-vision cameras to film how the chicks interacted with the host brood.
Their recordings show for the first time that honeyguide chicks eliminate their competition in brutal attacks.
Found across Africa, honeyguide birds often parasitise the nests of bee-eater birds underground in abandoned tunnels made by aardvarks.
Previous studies had identified that their chicks have specially adapted needle-sharp beaks.
The maimed bodies of host chicks had also been discovered in nests where honeyguide chicks were raised.
But scientists had largely been in the dark with regard to the chicks' underground behaviour.
"We buried infrared video cameras within the hosts' underground nests to see what happened," said Dr Claire Spottiswoode, who led the research, published in the journal Biology Letters.
The footage revealed the honeyguide chicks grasping, biting and shaking their nestmates to death.
"While the apparent violence with which young honeyguides attacked their newly hatched foster siblings was quite shocking at first sight, it shows the power of evolution to shape amazing adaptations in parasites," said Dr Spottiswoode.
Exploitative behaviour
Despite being blind, featherless and in total darkness, the honeyguide chicks did not struggle to overpower the bee-eater chicks due to their size advantage.
When the attacks took place, the honeyguide chicks were up to three times the weight of bee-eater chicks.
"The honeyguide mother ensures her chick hatches first by internally incubating the egg for an extra day before laying it, so it has a head start in development compared to the host," Dr Spottiswoode explained.
The parasitic parent also ensured odds in favour of her young by puncturing the resident eggs when laying her own.
Host parents meanwhile were unaware of the violence in their nests, with researchers even recording one attempting to feed the parasitic chicks while they tried to attack the other young.
After a month of care, the honeyguide chicks emerged from the burrow, no longer sporting their killer beak-hooks which had grown out.
The exploitative behaviour of honeyguide parents can be compared to that of cuckoos, but evolved separately in the unrelated species.
"This behaviour is exactly analogous to that of young cuckoos, which hoist host eggs or chicks onto their backs and tip them over the rim of the nest," said Dr Spottiswoode.
"But because honeyguide hosts breed in tree holes or underground burrows, honeyguides can't eject host chicks and have instead evolved this highly effective killing behaviour to make sure that they alone monopolise the nest.
"Each time brood parasitism has evolved we see specialised adaptations, which are no less astonishing for being sometimes rather gruesome."
Honeyguide Facts
http://www.bbc.co.uk/nature/14802180
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