Brood parasites target host birds with small eyes

Having big eyes may not only have helped the wolf to see Little Red Riding Hood better. A recent study also suggests that having larger eyes makes some bird species less likely to be selected as hosts by birds that are brood parasites.

Birds that lay their eggs in the nests of other species relieve themselves of the burden of raising their own chicks. By recruiting the services of numerous hosts, brood parasites such as cuckoos can end up with a higher reproductive success than if they simply raised their own chicks.

Some host species can recognize an imposter’s egg and will pierce it or turf it out of the nest. But if the foreign egg is not detected, the host parents will end up devoting large amounts of energy to feeding and caring for a chick that does not carry their genes.

Mark Hauber is a professor of Evolution, Ecology and Behavior at the University of Illinois Urbana-Champaign, who co-led the new research with Ian Ausprey, a recent doctoral graduate from the University of Florida. 

“The failure of some host birds to recognize foreign eggs in their own nests is somewhat perplexing,” said Professor Hauber. “Birds have much better vision overall than we do as humans. They have four color receptors instead of our three. They also can see into the ultraviolet range. What we did not know before this study was whether their eyes are adapted to egg rejection.”

To investigate the relationship between eye size and nest parasitism, the researchers used data originally collected in the 1970s by Stanley Ritland, a student at the University of Chicago. Ritland measured the eyeballs of more than 4,000 species of birds in museum collections. Ausprey and colleagues digitized this data and have previously used it to study the impacts of eye size on different traits. 

The results, published in the journal Biology Letters, show that bird species that are brood parasites have larger eyes than bird species that are hosts. This difference was present even after allowing for the larger body sizes of brood parasites. In addition, the researchers found that birds with larger eyes relative to their overall body mass were less likely to have their nests parasitized. 

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

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

In addition, within the range of host birds, those with larger eyes were more likely to recognize a foreign egg in their nest, unless the egg closely resembled the hosts’ own eggs.

This research is the first to show how eyes and other sensory systems contribute to the relationship between parasite and host species in birds. The study is a major step forward in understanding the evolution of such interactions between birds.  

“Nest parasitism exerts enormous selective pressure on host populations, with major implications for population demography and local species persistence,” said Ausprey. “It’s incredible that such a simple trait as eye size can provide a powerful window into the sensory systems that mediate the coevolutionary arms race between nest parasites and their hosts.”

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By Alison Bosman, Earth.com Staff Writer