They cheer other birds on to their eggs in order to save themselves the time-consuming brood care: A study in the case of the African cuckoo finch now shows how birds with cuckoo behavior manage to adapt the appearance of their eggs to the respective victim. It also shows that the victims of the brood parasites are by no means helpless and develop strategies to recognize fakes. Thus, the scammers’ genetic strategy has a catch that the cheated 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 alien chick hatches, it pushes the legitimate eggs and chicks out of the nest to be fed alone. This creates great harm for the forced adopters: they lose their own brood and their instinct compels them to raise the hungry imposter. But the victims have not remained helpless. They have co-evolved with their enemies to produce specially colored eggs and have a keen eye for fakes: if an egg looks strange, they will throw it out of the nest. However, cuckoo and co have also adapted to this: the brood parasites cleverly match the coloring and markings of their eggs to those of the respective host.
How can brood parasites lay different eggs?
What is particularly astonishing is that the brood parasitic birds can also imitate the eggs of different host species, whose eggs all look different. Within a parasitic bird species there are female lines that specialize in certain prey. Biologists have long wondered how this works. Specifically: how can a brood parasitic bird species mimic the eggs of several different bird species at the same time? And how do the Forgers pass each skill on to their youngsters without mixing up the color schemes? It was already assumed that these are genetic imprints that are inherited solely through the maternal line. There were already indirect indications of this in the case of the cuckoo. The researchers led by Claire Spottiswoode from the University of Cambridge have now demonstrated this principle in more detail by carrying out a genetic study on cuckoo finches (Anomalospiza imberbis). Their victims are various species of African songbirds, whose differently looking eggs can imitate the brood parasites. To unravel the genetic secret of this ability, the researchers collected DNA samples from 196 cuckoo finches from 141 nests belonging to four bird species.
As the team reports, their genetic studies confirmed that female cuckoo finches inherit their ability to mimic the appearance of their hosts’ eggs from their mothers alone. The researchers were able to show that the genes responsible for this are located on the female W chromosome of the birds. In this way, the different lineages of female cuckoo finches were able to develop specialized egg mimicry for different host species, the scientists explain. 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 the brood parasites. This is because it allows male cuckoo finches to mate with females of different egg colors without possibly messing up the appearance of the eggs. If, on the other hand, they also inherited genetic information for the egg appearance, mixed colors or patterns could arise that the host birds would immediately recognize as foreign.
A disadvantage in the arms race is emerging
However, as further results of the study show, the strategy has a catch: As is becoming apparent, the cuckoo finches could fall behind with their hosts as the arms race progresses. The researchers report that some victims of the brood parasites have evolved the ability to spawn distinct eggs even within their own species. For example, one of the prinia species (Prinia inornata) studied can lay eggs with a blue, white, red, or olive green background and recognize all of these variations as its own. Cuckoo finches have already responded by not only mimicking the eggs of their different host species, but also mimicking some of the characteristic variations seen in the eggs of different females within each host species.
However, as the researchers report, they apparently do not succeed in doing this with the egg color olive green – and this is exactly why it is currently spreading among the host birds. As the team explains, the results of their study show that in the victims of the brood parasites, the color of the eggs is not based on genes that are only inherited from females. Males also contribute to new color mixtures, increasing the potential for the formation of tamper-proof egg signatures. “Cuckoo finches are missing out on an important source of evolutionary innovation, and that could prove problematic in this ongoing arms race. The way they inherit their ability to mimic host eggs has a downside, as it’s likely to make repelling victims more effective by limiting one’s ability to adapt,” Spottiswoode explains. “Accordingly, we could now observe the emergence of egg signatures that cannot be forged, which could force cuckoo finches to switch to other, naïve host species,” says the scientist.
Source: University of Cambridge, professional article: PNAS, doi: 10.1073/pnas.2121752119