Why do some birds raise the chicks of other birds, to the detriment of their own procreation? This is not magnanimous adoption, this is parasitism. One wonders how a host bird could fail to recognize the foreign egg, and later, how it fails to notice that it is devotedly placing worms and other food into the gaping maw of an alien species.
One famed example is the common cuckoo of Europe which lays its eggs in strange nests and then flies merrily off. This practice of spawning and then leaving another species to invest in raising the kids, known as brood parasitism, exists in several types of birds, at least one species of fish and some insects.
Sometimes, like in the case of the European cuckoo, the interloper fledgling instinctively terminates its interspecies nestmates by throwing other eggs out of the nest. Sometimes the host catches on to the alien egg and destroys it. Sometimes the two species of young coexist, sharing the nest and the food. There are a range of parasitic behaviors and responses, yet one burning question is common to them all: How is it that the hosts don’t notice the alien eggs?
It may be a function of the size of their eyes, a new study in the journal Biology Letters suggests.
Clearing the nest of alien eggs spares the hosts the cost of bringing up the foreign chicks. The risk is that they may fail to recognize or damage their own eggs in the process.
A 2018 study looked at parasitic North American cowbirds laying their eggs in robins’ nests. Cowbirds lay spotted beige eggs. Robins lay blue-green eggs. The researchers concluded that egg color was a factor, after the experimental robins rejected four-fifths of beige-colored fake eggs, but kept the blue-green fake eggs. The team concluded that if the cowbird were to evolve to lay blue-green eggs, the robin would be flummoxed.
Another study in 2019 checked the possibility that egg shape was a factor, and did find growing sensitivity as shapes became increasingly aberrant.
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But the new study finds an interesting correlation between host obliviousness and eye size.
Co-evolutionary arms race
As mentioned, some bird species can recognize foreign eggs and will destroy them, or, vexingly, bury the foreign egg by building a new nest on top of it, the team elaborates. Nobody said nature is kind. On the other hand, some birds don’t notice they have a strange egg – different in size, color and/or shape, which research co-leader Prof. Mark Hauber of the University of Illinois Urbana-Champaign calls “somewhat perplexing.”
It isn’t that they are blind. Generally, birds can see better than we can: “They have four color receptors instead of our three. They also can see into the ultraviolet range,” Hauber says.
The work was based on a project done in the 1970s by a student at the University of Chicago named Stanley Ritland who measured the eyeballs of more than 4,000 species of dead birds in museum collections. As one does.
Hauber, study co-leader Ian Ausprey and their colleagues digitized the data and searched for correlations. They found that large-eyed birds were more likely to be far-sighted hunters eating insects and other prey. Birds with small eyes tended to eat non-ambulatory nectar, or seeds.
“Having larger eyes is similar to having a bigger camera lens,” Ausprey explains. “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.”
All that’s well and good, but here’s the real kicker: When analyzing eye size in relation to brood parasitism, Ausprey found that the parasite birds tended to have larger eyes than the host birds, beyond the difference relating to body size.
Also, the bigger the eyes in the host, the likelier it was to recognize that it had an alien intruder in its nest. Unless the eggs mimicked its own. Then it was game over. Talk about what came first, the chicken or the egg.
Hauber suggests a possible interpretation in which the parasitic birds evolved to pick on birds with weaker eyesight.
It makes evolutionary sense. If you are an indiscriminate parasitic bird and lay your eggs in an eagle’s eyrie, the eagle, with its big eyes, is likely to notice. Your next generation does not live long and prosper, or fly away to lay eggs of its own in someone else’s nest.
These findings are a major step toward understanding how parasite/host interactions evolved in birds, Ausprey said. “Nest parasitism exerts enormous selective pressure on host populations, with major implications for population demography and local species persistence,” he added.
Asked to elaborate on the selective pressure, Hauber and Ausprey said: “Many brood parasites eliminate all host fitness from the parasitized broods: honeyguides kill their nestmates while common cuckoo chicks toss out host eggs. Even cowbirds can eliminate 80% of host chicks from nests of Eastern Phoebes.”
He found it “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 parasite and host. “It may seem intuitive, but the link with the avian visual system has never been tested on such a large scale as ours,” he explained, adding, “And, the data support our predictions.”
The host may even continue feeding the cuckoo after it has outgrown it – a tiny parent feeding a great hulking chick. Not recognizing an egg is one thing, but not recognizing that there’s something off about the giant child?
“Unlike eggs, altricial chicks have an ever-changing appearance--one day they are naked and blind, the next day their eyes are open and covered in down,” Hauber and Ausprey explain. “Also, the healthiest chick is often the largest – and when it’s a cuckoo’s or cowbird’s chick, it would be hard to discriminate against it because the potential error (of mistakenly rejecting one’s own healthiest chick) is super high.”