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From an evolutionary point of view, the nest parasitic cuckoo and its servant species are constantly trying to outwit each other. For example, servants can distinguish cuckoo eggs from alder birds based on their color. Now Czech scientists point to studythe bird’s color perception does not depend on the light in the nest.
How do domestic cuckoos see foreign eggs in their nests? How do birds know when foreign eggs are present in their nests? You can tell by the number of eggs, the size of the eggs or their colour, says bird ecologist Marko Mägi in the annual Bird Science News.
Birds can’t count their eggs: at least scientists don’t have a widespread understanding of this. Although cuckoo eggs are usually slightly larger than eggs of ordinary species, size is not decisive. It is known that birds actually prefer eggs that are larger than their own, because egg size predicts that it will lay large and strong young. It is also known that cuckoos, in order to better hide their eggs, lay eggs with patterns similar to those of their host species. However, lighting conditions play an important role in the perception of color and pattern.
Human color perception is permanent. This means that his brain makes corrections based on the lighting conditions so that a person always perceives the color of an object in a relatively similar way. If a person did not have this ability, he would see the world like a camera. For example, a white ball would remain white in a photo taken at noon, but would appear yellow or red at sunset. At the same time, a person would see the ball as white regardless of the lighting conditions because the human brain corrects the color perception.
A lot of research has been done on bird color perception, so it is well known that birds are very good color perceivers in the animal kingdom. However, it is not clear whether the bird brain can also compensate for color vision. If so, this may help to identify foreign eggs.
The evolutionary race between cuckoos and raptors never ends. As a result, the pattern of cuckoo eggs is often very similar to that of servitor eggs. As a result, servitors are also unable to recognize foreign eggs. However, in some cases, cuckoos go too far and their eggs are noticeably too spotted or too dark in color. This makes it easier for servitor birds to recognize the eggs – provided, of course, that the servitor’s color perception allows noticing this difference.
It is well known that birds can identify foreign eggs in nests when the light is still fairly dim but not yet completely dark. The nests are located in different places: some are well-lit, some are more secluded, and some are in dark cavities. Cuckoos can also lay eggs in nests in cavities if the opening of the cavity is large enough.
One of the cuckoo’s victims is the alder bird. In a study area in the Czech Republic, cuckoos parasitized the nests of alder birds that nested in nest boxes. The opening of the nest box was seven centimeters in diameter, so it was relatively easy for the cuckoo to lay its eggs in the nest.
In the nest box, the researchers placed a cuckoo-egg-sized white egg in the alder bird’s nest, which had similar reflective properties to the alder bird’s own eggs. From previous experiments, they knew that alder birds did not always remove the white eggs from their nests, but occasionally did so.
LED lights were also installed in the nest boxes, changing the generally dark background of the nest boxes. In this way, the question of whether the removal of eggs of a foreign species from the nest depends on the light conditions was answered. The light became more brown, blue or white.
It is known that the alder bird can distinguish between its own eggs and foreign eggs during the day. Usually, she will distinguish eggs that are more brown than her own bright blue eggs. Meanwhile, the kingfisher chose the cuckoo eggs, which are bluer than its own eggs – the same trend has been observed in several other bird species.
It was found that pushing the foreign egg out of the nest did not depend on the lighting of the nest cavity: it did so in 9 to 20% of the cases. This suggests that the alder bird always sees colors in the same way. How color vision compensation works physiologically is still unclear. However, it is clear that the bird’s eyes adapt to the light conditions and the background color of the nest.
Despite the relatively small number of birds studied, the results are similar to previous studies based on alder birds, which increases the certainty that this is not a coincidence. Seeing and perceiving color is not the only factor in the evolutionary race, but it is still one of the most important factors, and scientists are still constantly unraveling this mystery.
The study was published in the journal Journal of Vertebrate Biology.
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