Why do birds lay eggs of different colors

How does the cuckoo lay different eggs?

Amazing variety: speckled, brownish, blue ... the appearance of cuckoo eggs always corresponds to the clutch of birds that the breeding parasite wants to "cheat". But how does the cuckoo produce this range of different imitations? According to a genetic study, the female animals are characterized by a certain host bird species and inherit the corresponding egg appearance. Male cuckoos, on the other hand, do not have any genetic information that affects the appearance of the shells.

He simply saves himself the annoying brood care: the cuckoo cheers its egg among strange bird parents and leaves them to raise its voracious offspring. But the victims of the sophisticated brood parasite are not completely helpless. They always take a critical look at their clutch: If an egg looks strange, they throw it out of the nest. The cuckoo therefore adapts the coloring and drawing of its eggs to those of the respective host with astonishing accuracy. Biologists have long wondered how this is possible. What is clear is that female cuckoos cannot vary the appearance of their eggs - they are each set to a specific host bird species. A research team led by Frode Fossøy from the Norwegian University of Science and Technology in Trondheim was able to prove that the egg color blue is a genetic condition that is inherited exclusively through the maternal line.

On the trail of blue eggs

For their study, the scientists collected genetic samples of cuckoos and blue eggs from museum collections. Some of the material was over 100 years old. In these samples, they examined features of both the genomic DNA and the genetic make-up in the mitochondria. In addition to the cell nucleus, these energy power plants also carry their own genetic material. It allows specific conclusions to be drawn about the maternal line of inheritance, because the mitochondrial genetic material is only passed on from the mothers to the offspring and changes only very slowly.

Using the mitochondrial DNA, the researchers were able to show that the egg color blue is linked to a specific female line of inheritance. This does not apply to the male cuckoos: "The father has no influence on the color of his daughter's eggs," says Fossoy.

It is probably due to the sex chromosomes

According to them, the effect is likely due to the fact that the genetic information for the appearance of the eggs is on the female sex chromosome. Birds have female W and male Z chromosomes, which correspond to X and Y in mammals. In contrast to mammals, male birds have ZZ pairs of chromosomes, while females have a ZW combination. That means: only Ms. Cuckoo has a W chromosome. So if 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 is extremely useful for the cuckoo: it enables male cuckoos to mate with females of different egg colors without possibly confusing the appearance of the eggs. If, on the other hand, they inherited genetic information for the egg look, mixed colors or patterns could arise that host birds would immediately recognize as foreign. The unusual principle of inheritance is therefore another basis of the cuckoo's sophisticated strategy of fraud.

Source: Norwegian University of Science and Technology, Nature Communications 6: 10272 doi: 10.1038 / ncomms10272.

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5th April 2019

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