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CHAMPAIGN, Ill .– Eye size likely plays a role in the struggle between parasites in avian broods – birds that lay their eggs in the nests of other species – and their hosts, which sometimes detect foreign eggs and eject them or abandon them, report scientists in the journal Letters of Biology.

Brood parasites succeed by offloading parenting work to other species. The young of the hosts suffer from competition with exotic birds. If the host birds do not recognize eggs that are not their own, the brood parasites may produce more offspring than would be possible if they were simply raising their own.

An adult common redstart feeds a parasitic cuckoo chick in its nest.

Photo by Tomas Grim

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Some bird species targeted by brood parasites may recognize a foreign egg. These birds will pierce or grab the egg and eject it, abandon the parasitized nest or – in some cases – embed the parasite’s egg by building a new nest on top of the old one. This allows them to devote their parenting efforts only to their own offspring.

Photo of a yellow warbler

Yellow warblers sometimes abandon their nests when a cowherd lays an egg there. Another strategy is to build a new nest on top of the old one, blocking the parasitic egg.

Photo by Pookie Fugglestein

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The inability of some host birds to recognize foreign eggs in their own nests is somewhat puzzling, said Marc Hauber, Professor of evolution, ecology and behavior at the University of Illinois Urbana-Champaign, who co-led the new research with Ian ausprey, a recent doctorate from the Department of Biology at the University of Florida and Florida Museum of Natural History.

“Birds have a much better overall vision than we humans do. They have four color receivers instead of our three. They can also see in the ultraviolet range, ”Hauber said. “What we didn’t know prior to this study was whether their eyes were adapted to rejecting eggs.”

Photo of a large reed warbler

The great reed warbler, in the photo, sometimes has its nest parasitized by cuckoos.

Photo by Olda Mikulica

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To study the relationship between eye size and brood parasitism, the researchers turned to data collected in the 1970s by Stanley Ritland, a University of Chicago student who measured the eyeballs of more than 4 000 species of birds in museum collections. Ausprey and his colleagues digitized Ritland’s data and explored the implications of eye size on a variety of features. In a recent post to study, for example, he found that birds with larger eyes were more likely to feed on insects or other prey requiring hyperopia, while those with smaller eyes tended to eat nectar or seeds that could be detected up close.

“Having bigger eyes is similar to having a bigger camera lens,” Ausprey said. “By collecting more light, large eyes improve a bird’s visual acuity, its ability to resolve an image in dark conditions and over long distances.”

When analyzing the eye size of different species of birds in relation to their lifestyle as brood parasites, hosts, or non-host birds, Ausprey found that brood parasites had eyes larger than host birds – beyond the difference expected due to their larger body size – and that birds with eyes larger in relation to their overall body mass were less likely to have their nests parasitized. The size of the eyes of host birds was also positively associated with their likelihood of recognizing foreign eggs – unless the eggs looked a lot like the host’s own eggs, the researchers found.

Pictures of, clockwise, from above, common cuckoo, European bee-eater and European robin.

The researchers report that brood parasites, like this common cuckoo, at the top, have larger eyes than the birds they are targeting – beyond the difference expected due to their larger body size. Common cuckoos will parasitize the nests of European robins, lower left, but not those of European bee-eaters, lower right.

Photos of Tomas Grim

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“Non-host birds tend to have larger eyes than hosts,” Hauber said. “One interpretation of this is that parasites go for birds with poor eyesight.”

Hauber collaborated on a recent to study who identified specific brain structures that play a role in the interactions between a parasitic insect and its host. But the new bird research is the first to show how a sensory system like the eyes contributes to the interaction between the parasite and the host, Hauber said.

These findings are a major step towards understanding the evolution of these interactions in birds, Ausprey said.

“Nest parasitism exerts enormous selective pressure on host populations, with major implications for population demography and the persistence of local species,” said Ausprey. “It is incredible that a trait as simple as the size of the eyes can provide a powerful window into the sensory systems involved in the co-evolving arms race between nest parasites and their hosts. “


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