Following the tricks of the egg forgers

They cheer other birds for their eggs in order to save themselves from time-consuming brood care: A case study of the African cuckoo now shows how birds with cuckoo-behavior were able to adapt the appearance of their eggs to the victim in question. It also shows that victims of brood parasites are by no means helpless and develop strategies to identify counterfeit products. Thus, the scammers’ genetic strategy has an advantage that scammers can exploit.

Our common cuckoo isn’t the only feathered rascal: around the world, some bird species avoid the cost of parenthood by laying their eggs in other species’ nests. After the space chick hatches, it pushes the legitimate eggs and chicks out of the nest to feed on their own. This causes great harm to forced adopters: they lose their bosom and their instinct forces them to raise a hungry crook. But the victims did not remain helpless. They have evolved with their enemies to produce particularly colorful eggs and have a keen eye for fakes: if the egg looks strange, they will throw it out of the nest. However, the cuckoo and its colleagues have also adapted to this: brood parasites cleverly match the coloration and markings of their eggs to those of the respective host.

How can brood parasites lay different eggs?

What is especially amazing is that the parasitic brood can also mimic the eggs of different species of hosts, the eggs of which all look different. Within the parasitic bird species there are female subspecies that specialize in specific prey. Biologists have long wondered how this works. Specifically: How can a brood of parasitic bird species mimic the eggs of many different bird species at the same time? And how do Forgers pass on each skill to their young ones without mixing up color schemes? It was already assumed that these are genetic fingerprints inherited only through the maternal line. There were already indirect indications of this in the case of the cuckoo. Researchers led by Claire Spottiswood of the University of Cambridge demonstrated this principle in more detail by conducting a genetic study on cuckoo finches (Anomalospiza imberbis). Their victims are different species of African songbirds, the eggs of which can differently mimic brood parasites. To unravel the genetic secret of this ability, the researchers collected DNA samples from 196 cuckoo birds from 141 nests belonging to four bird species.

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As the team reported, their genetic studies confirmed that female cuckoo finches inherit their ability to mimic the appearance of their host’s eggs from their mothers alone. The researchers were able to show that the genes responsible for this are located in the female W chromosome of birds. In this way, different subspecies of female cuckoo birds were able to develop specialized egg mimics for different host species, the scientists explained. The researchers explained that if the genetic information for blue eggs, for example, is located on the W chromosome, it is transmitted unchanged from mother to daughter. According to them, this system makes sense for brood parasites. This is because it allows male cuckoo birds to mate with females of different egg colors without spoiling the appearance of the eggs. On the other hand, if they also inherit genetic information about the appearance of the egg, mixed colors or patterns may appear so that the host birds immediately recognize them as foreign.

A flaw in the arms race is emerging

However, as other study results show, the strategy has a benefit: As it has become clear, cuckoo finches can lag behind their hosts as the arms race progresses. Researchers report that some victims of brood parasites have developed the ability to produce eggs that are distinct even within their own species. For example, one of the studied Prinia species (Prinia inornata) can lay eggs with a blue, white, red, or olive-green background and recognize all of these variations as its own. Coco finches have already responded by not only mimicking the eggs of different host species, but also by mimicking some of the distinct differences seen in the eggs of different females within each host species.

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However, as the researchers report, they clearly haven’t succeeded in doing so with the egg’s olive green color – which is exactly why it is currently so common among host birds. As the team explains, the results of their study show that in victims of brood parasites, egg color does not depend solely on genes inherited from females. Males also contribute new color mixtures, increasing the likelihood of forming untamable egg signatures. “Cuckoo finches are missing an important source of evolutionary innovation, and that could be a problem in this ongoing arms race. The way they inherit their ability to mimic the host’s eggs has a downside, potentially making victims repel more effective by limiting the ability of the individual,” Spottiswoode explains. The scientist says: “According to this, we can now observe the appearance of imprints of eggs that cannot be falsified, which can force the cuckoo finches to switch to other gullible host species.”

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

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