A new study has revealed that parasitic chicks nestled in other birds’ eggs ‘exercises’ while still in the egg to try and kill their ‘foster’ siblings.
London researchers discovered that chicks from parasitic bird species move in the egg more than other species. This is to increase their strength to defeat other chicks after they are born.
Certain species of bird, most famously the common cuckoo, trick other bird species into raising their chicks for them – a behaviour termed ‘brood parasitism’.
The nest becomes unattended for just a second and the parasitic mother drops her eggs into the nest. She then flies away before they are seen.
The unsuspecting adult ‘host’ ends up raising the parasite’s chicks for them, even when the chick kills its foster sibling rivals.
Some birds, called ‘broodparasites’, lay their eggs in the nests and eggs of other species. These other species, called ‘hosts, raise the foreign chicks at great expense to their own chicks. The brood parasite chicks can attack the host chicks when they hatch. Pictured is a larger honeyguide attacking the smaller chicks of its host species, the little-eater. This picture was taken by Claire Spottiswoode, an expert in brood parasitism at the University of Cambridge and co-author of this new study, on location in Africa
Some brood parasite chicks including the cuckoo push the eggs of their host out of the nest one at a while in an impressive display physical exertion.
Stephanie McClelland, a biologist from the Royal Holloway University of London, said that it’s like a newborn baby picking up a bowling ball. It’s crazy.
Others have a dangerously sharp point at the end of their little finger that they use to pulverize the eggs of their host (in case of the lesser honeyguide or greater honeyguide).
For their study, McClelland and colleagues studied movements in 437 eggs from 14 species of birds – hosts, parasites and other bird species as a comparison – from three continents (Europe, America and Africa).
Parasite species included the common cuckoo (Cuculus canorus), the brown-headed cowbird (Molothrus ater) and the lesser honeyguide (Indicator minor).
Among the host species were the great reedwarbler (Acrocephalus Arundinaceus), one of the cuckoos’ preferred hosts, and the common waxbill, Estrilda Astrold.
The academics measured embryo movement (when the birds were still in the egg, unborn) in multiple brood-parasitic and non-parasitic bird species.
They did this by beaming infrared light through the egg and recording when that beam was disrupted – an indicator of movement.
Pictured is a common cuckoo chick in its host species’ nest, a marshwarbler (Acrocephalus Palustris), in Denisovo in Russia. The cuckoo hatches before its host species’ chicks, so it can push these rival eggs out of the nest, eliminating them. This ensures that the adult hosts are devoted to the cuckoo’s young.
Experts studied movements in 437 eggs from 14 species of birds, pictured here as silhouettes. The parasitic species are those in red. There are five species in black that are the hosts of these parasitic species (which parasitic species lays its egg in which species’ nest is denoted by the matching symbols). Four other birds are also shown in black without a symbol. These are non-parasitic birds that were used in this study as a comparison.
The experts concluded that the brood parasite species had more embryo movement than the host birds or other non-parasitic birds.
In their research paper, they state that brood parasites showed significantly higher muscle movement during incubation as compared to non-parasites.
“This suggests that embryo movement may be increased, which could facilitate the development a stronger musculoskeletal system to handle the demanding tasks of young brood parasites.”
Researchers point out that multiple brood parasite species have evolved shorter incubation periods, to make sure they hatch before the host chicks, and stronger eggshells.
Greater strength and stamina are required to hatch from these stronger structures, meaning they’re well prepared for the first task of their time on Earth – murdering rival chicks.
The new study, published in Proceedings of the Royal Society B: Biological Sciences, suggests embryo movement may be overlooked in the evolution of bird behaviours, including brood parasitism.
The common cuckoo provides one of most amazing and famous examples of brood parasitism in the UK.
Nick Davies, a professor of behavioral ecology at the University of Cambridge has previously described the species in the past as “nature’s most notorious cheat”.
A photograph showing an adult bird ‘host’ (a dunock, hedge sparrow, or a sparrow), feeding a huge cuckoo chicken that is more than twice its height. The cuckoo chick (bottom) would have been placed into the adult dunnock’s nest by its cuckoo mother only weeks earlier. When it’s placed craftily in the host’s nest, a young cuckoo hatches after only 12 days and quickly pushes the hosts’ eggs or babies out of the nest. It leaves the nest after 19 day, but the hosts continue feeding it for two weeks more, during which it has grown to be much bigger than them.
The bizarre scene is the result of the cuckoo’s sneaky habit of laying eggs in the nests of other species and leaving the unwitting birds to raise their chicks. A reedwarbler (left) with a large, deceitful cuckoo chicken (right).
Professor Davies is the author of several books on the cuckoo and brood parasitism in birds, including Cuckoo: Cheating by Nature, published in 2016.
Perfectly illustrating his description, images emerged in 2018 of a dunnock, or hedge sparrow, that had been duped into feeding an enormous cuckoo chick.
Professor Davies wrote in the Daily Mail that ‘every summer, thousands will have their eggs tossed overboard from young cuckoos.
“The host parents are then tricked to spend their summer raising a cuckoo, instead of their own brood of chicks.”
More photos of a small reed warbler parent feeding a huge cuckoo chick that emerged last year are both tragic and perversely comic.
Professor Claire Spottiswoode is co-author of the new study and one of Professor Davis’ colleagues at Cambridge. She has captured some incredible photos of brood parasitism on location in Africa.
One photo, which looks like something from a horror movie, shows a greater honeyguide chicken attacking smaller chicks in the host species, a little bee-eater.
Interestingly, brood parastism is a common problem in birds. Bird ‘hosts’ have developed different methods to detect parasite eggs and save themselves from being killed.
A larger, ‘parasitic purple indigobird nestling (right), alongside its two jamesons host chicks, and rivals. Some species of birds secretly place their eggs in nests of other birds so that the mothers can effectively raise their offspring. Some species, such as the cuckoo kill rival birds and the mother’s true offspring by kicking them out the nest.
This has resulted in what Professor Davis calls an evolutionary arms race between the parasite and the host over many decades.
For example, African finch chicks (a parasite species) have evolved tiny details on the insides of their mouths to mimic those of their host species, the grassfinch, a study showed last year.
Hosts are able to detect foreign eggs and either eject them or abandon them. They have also developed the ability to make their eggs unique, covering them with unique patterns or colours to ensure they don’t confuse with foreign eggs.
But over time, the parasite species evolves so their eggs better mimic the host – and so the ‘evolutionary arms race’ continues.
BROOD PARASITISM and THE EUROPEAN CUCKOO
A reedwarbler (right) holding a tasty morsel in its beak.
Some birds thrive by sharing the responsibility of raising their young with other species.
The European Cuckoo, whose distinctive call is immortalised in the sound of the cuckoo clock, is the bird in which this habit has been most thoroughly studied.
Female European Cuckoos only lay their eggs in nests made by other birds.
A cuckoo egg is usually very similar to the eggs of its host (often, one of their eggs is removed by the cuckoo).
The host might recognize the intruding egg, abandon it, or incubate the cuckoo eggs.
The young European Cuckoo hatches shortly after hatching and immediately pushes any solid object it comes across over the nest’s edge using a scoop-like depression in its back.
The foster parents can now devote all their attention to the young cuckoo after their eggs and rightful youth have disappeared.
This is often a difficult task because the cuckoo chick grows larger than its host adults and can no longer care for itself.
The scene of a small foster parent working to keep up with the voracious appetite of an outsized young cuckoo has amazed bird watchers.
Different females from the same European Cuckoo colony often parasitise different host species.
Some cuckoos may be skilled in parasitizing the nests Garden Warblers. Others of the same population might nest in the nests Reed Warblers. Still others may nest in the nests White Wagtails.
Each female’s eggs closely match the host chosen (even though one may have large, densely marked eggs and another may only have smaller, unmarked blue eggs). The mimetic patterns can also be genetically determined.
According to some reports, the different genetic types of females mate randomly with males.
How these gentes are maintained within the cuckoo populations is not fully understood.
Brood parasitism can be found in approximately 1% of bird species, which includes members of diverse groups such as cowbirds, weavers and ducks.
Brood parasitism doesn’t just affect females from one species that lay eggs in the nests and hatching eggs from other species.
Sometimes, females from a variety of species lay eggs in the nests or eggs of other females.
Source: Stanford University