Following the tricks of egg counterfeiters

They cheer other birds on their eggs to spare themselves the time-consuming brood care: A study on the African cuckoo finch now shows how cuckoo-behaved birds are able to adapt their eggs to the appropriate prey. It also shows that the victims of breeding parasites are by no means helpless and develop strategies to spot counterfeit products. So, the cheats’ genetic strategy has a catch that the deceived can take advantage of.

Our common cuckoo is not the only feathered rogue: around the world, some bird species avoid the cost of parentage by laying eggs in the nests of other species. After the alien chick has hatched, it pushes the legal eggs and chicks out of the nest to be fed on their own. This causes great harm to forced adoptions: they lose their own offspring and their instincts force them to raise a hungry trickster. But the victims did not remain helpless. They have evolved together with their enemies to produce eggs with special colors and have an eye for fakes: if the egg looks strange, they will throw it out of the nest. However, the cuckoos and their friends have adapted to it as well: brood parasites cleverly match the color and markings of their eggs to the colors of the host.

How can brood parasites lay different eggs?

It is particularly amazing that parasitic breeding birds can also mimic the eggs of different host species, whose eggs look different. Among the parasitic bird species, there are dam lines that specialize in a particular prey. Biologists have long wondered how this works. Specifically: how can parasitic breeding bird species mimic the eggs of several different bird species simultaneously? And how do Counterfeiters convey each skill to their children without mixing color schemes? It has already been assumed that these are genetic prints that are inherited exclusively through the maternal line. Indirect signs of this were already in the case of the cuckoo. Scientists led by Claire Spottiswoode from the University of Cambridge have now demonstrated this principle in more detail by conducting a genetic study on cuckoo finches (Anomalospiza imberbis). Their victims are various species of African songbirds, whose eggs that look differently can mimic breeding parasites. To discover the genetic secret of this ability, scientists collected DNA samples from 196 cuckoos from 141 nests belonging to four species of birds.

Their genetic testing confirmed that female cuckoos inherit the ability to mimic their host’s eggs only from their mothers, the team said. Scientists have managed to show that the genes responsible for this are on the female W chromosome of birds. The researchers explained that in this way, different families of female cuckoos were able to develop specialized egg mimics for different host species. Because if, for example, the genetic information for blue eggs is on the W chromosome, it is passed on unchanged from mother to daughter, the researchers explain. According to them, this system makes sense for breeding parasites. This is because it allows male cuckoos to mate with females of different egg colors without being able to spoil their appearance. On the other hand, if they also inherited the genetic information about the egg’s appearance, it could result in mixed colors or patterns that the host birds would immediately recognize as foreign.

There is a disadvantage of the arms race

However, as further research results show, the strategy has a catch: as it turns out, cuckoo finches may lag behind their hosts as the arms race progresses. Scientists report that some victims of breeding parasites have evolved the ability to lay eggs even within their own species. For example, one of the prinia species studied (Prinia inornata) may lay eggs against a blue, white, red or olive green background and recognize all of these varieties as their own. Cuckoos have already responded not only by mimicking the eggs of different host species, but also by mimicking some of the characteristic variations observed in the eggs of different females within each host species.

However, the researchers report that they apparently fail to do so for the olive green egg color – which is why it is now spreading among host birds. The team explains that their research shows that in victims of breeding parasites, the color of eggs is not based on genes that are exclusively inherited from females. Males also contribute to the creation of new color mixes, thus increasing the possibility of creating tamper-resistant egg signatures. Cuckoo finches are losing an important source of evolutionary innovation, and this can prove problematic in the ongoing arms race. The way they inherit the ability to mimic the host’s eggs has the disadvantage of making it more effective to deter prey by limiting their ability to adapt, ‘explains Spottiswoode. “Therefore, we could now observe the emergence of egg signatures that cannot be faked, which may force cuckoos to switch to other, naive host species,” says the scientist.

Source: University of Cambridge, journal article: PNAS, doi: 10.1073 / pnas.2121752119

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