Here's a photo that grabbed our attention. An American Coot attacks a young chick and prepares to kill it. Bruce Lyon, professor of ecology and evolutionary biology at the University of California Santa Cruz, shot the photo during the course of a research project that found that the coot's reproductive life is full of deception and violence.
Coots have evolved remarkable cognitive abilities that allow them to thwart other coots that lay eggs in their neighbors' nests. In 2003, Lyon showed that coots can count their own eggs and reject ones laid in their nests by other coots.
His latest findings, published this week in Nature, show that coot parents can tell the difference between their own chicks and any impostors that manage to hatch in their nest, and they will violently reject most impostor chicks.
Lyon's photo is stunning proof of an adult coot's reaction to finding a chick that is not its own.
Lyon and UCSC graduate student Dai Shizuka, lead author of the new paper, monitored nests on several wetlands near Williams Lake, British Columbia, from 1987 to 1990 and from 2005 to 2008.
Their research is particularly striking because so many birds seem to be unable to recognize the chicks of species such as cowbirds and cuckoos, which always lay their eggs in the nests of other birds. The behavior is called brood parasitism, and its success has posed a longstanding challenge to evolutionary theorists.
"When you see a little songbird struggling to feed an enormous cowbird chick, you have to wonder why it can't recognize the parasitic chick when it is so obvious to us," says Lyon. "The coot study shows that chick recognition can evolve, even when the chicks are the same species and all look the same to us."
The researchers found that coots learn to recognize their own chicks by using the chicks that hatch first as a template to which other chicks are compared. This learning mechanism may explain why it is so hard for chick recognition to evolve among the hosts of cowbirds and cuckoos, says Shizuka.
"Cuckoo and cowbird chicks tend to hatch before the host chicks, so their hosts can't use hatching order as a cue for chick recognition," he says. "As long as recognition has to be learned, you run the risk of learning incorrectly, and that could be the bottleneck. It's not that coots are exceptionally smart. They just have reliable information that allows them to do what we expect all hosts 'should' be doing to defend themselves against parasitism."
In coots, brood parasitism seems to be an optional component of a reproductive strategy based on laying large numbers of eggs. Depositing a few eggs in a neighbor's nest is just another way to increase the number of potential offspring.
Photo at left: an adult coot watches over a large brood.
The chances of survival in a neighbor's nest may be slim, but coots habitually lay more eggs than are likely to survive, Lyon says. Only in the best of years is there enough food for all of the chicks; in a typical year, about half of the chicks in each brood starve to death, he says. If a parasitic chick survives, another chick in the brood must die, which explains why coots have evolved such strong defenses against parasitism.
"We actually set out to study how coots bring their brood size into alignment with the availability of food, and what role hatching order plays in the culling process. But we kept seeing anecdotal evidence in the field that something else was going on," Lyon says. "With the parasitic chicks, they don't just let them starve. They attack them with a viciousness we hadn't seen before."
Adults actively sought impostor chicks from a distance to peck them vigorously and attempted to drown them, pecked them while brooding on the nest, and prevented them from access to the nest to be brooded, the researchers wrote.
Lyon and Shizuka also want to know what cues parents use to recognize their chicks. The possibilities include smell, vocal calls, and visual cues such as plumage. "Those are all plausible hypotheses, but we don't know yet," Shizuka says. --Matt Mendenhall, Associate Editor, with thanks to Tim Stephens, UC Santa Cruz
Photos by Bruce Lyon, UC Santa Cruz
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