Editor | On 31, May 2018

Darrell Mann

The main attribute of the cuckoo is that it lays its eggs in other birds’ nests in order that other birds raise their offspring for them. We’ve mentioned this story in Issue 147 of the ezine. What we didn’t mention was the fact that, in order to ensure that the unfortunate surrogate parent bird doesn’t throw the cuckoo egg out of their nest, it has to be disguised to look like the eggs that are already there. They need to be the same colour and the same shape. Cuckoo eggs in other words, need to look different depending on the species of the surrogate parent bird.

For roughly a century, researchers have been trying to figure out how different female cuckoos manage to lay such a variety of different egg colors to match different host birds. Now, a group of researchers at the Norwegian University of Science and Technology (NTNU) has come up with an answer to this puzzle, in cooperation with researchers from all over Europe and Asia.

The short answer is that “the female bird decides everything,” says researcher Frode Fossøy.

Fossøy is part of the cuckoo research group at the Department of Biology at NTNU. The results of the group’s work have just been published in Nature Communications.

“We’ve been able to show for the first time that the blue egg color is inherited via the female cuckoo only. The father has no effect on the color of his daughter’s eggs,” says Fossøy.

Researchers have investigated a wide variety of samples from Europe and Asia. They found a clear relation between blue eggs and genetic material that only comes from the mother (mitochondrial DNA), and no relation between egg color and genetic material that comes from both parents (nuclear DNA).

Cuckoos (Cuculus canorus) are parasitic. Female cuckoos lay their eggs in the nests of other bird species, as is well known. The young cuckoo then usually throws the other chicks out of the nest, getting rid of any competition for the parents’ attention.

Potential host birds develop traits to prevent being tricked by the parasites – they get rid of eggs that don’t look like their own. To avoid this, female cuckoos need to lay eggs that look identical to that of the host bird’s, both in color and shape.

This means that there is a sort of evolutionary arms race going on between cuckoos and host birds, where cuckoo eggs come to resemble that of their host birds’ more and more over time.

Different cuckoos can therefore lay eggs of many different colors and patterns. They can be blue, brown, green or grey, and have different combinations of spots and patterns. But each individual female cuckoo can only lay eggs in one color, so different females specialize on different host species. This is part of the reason why egg color is a trait inherited only from the female birds.

Males may make a mess of things

The research group doesn’t have definitive proof that all egg characteristics are inherited from the female bird, just that blue eggs are. Whether other egg colors are also only inherited via females is currently not known.

There has been speculation that cuckoos that lay blue eggs may actually be a different species, but no other characteristics indicate this. The nuclear DNA of the birds that lay blue eggs is the same as any other cuckoo — meaning that the species is the same.

A single male cuckoo can mate with several female birds that lay eggs of different colors. This could make a mess of things. If a male with genes for one color mated with a female with a different egg color, their daughter’s eggs could be an intermediate color that doesn’t match any host, meaning that host birds would be able to spot the difference and get rid of the parasitic egg.

For this reason, there are evolutionary advantages for both the male and female birds that only females carry the genetic trait for egg color. That way, their offspring will have a much greater chance of surviving.

The overall problem the cuckoo has had to evolve in the arms-race is being able to lay the right colour and shape of egg in the right surrogate nest. Here’s what the problem looks like from a Contradiction perspective:

Which seems to match up well with the evolved solution:

• Principle 10/Principle 1 – different females specialise in different surrogate species
• Principle 2 – take out the influence of the (random) male cuckoos

Journal Reference:

Frode Fossøy, Michael D Sorenson, Wei Liang, Torbjørn Ekrem, Arne Moksnes, Anders P Møller, Jarkko Rutila, Eivin Røskaft, Fugo Takasu, Canchao Yang, Bård G Stokke. Ancient origin and maternal inheritance of blue cuckoo eggs. Nature Communications, 2016; 7: 10272 DOI: 10.1038/ncomms10272