The counting ability of female coots is part of a sophisticated set of defense mechanisms used to thwart other coots who lay eggs in their neighbors' nests, according to Bruce Lyon, an assistant professor of ecology and evolutionary biology at the University of California, Santa Cruz. Lyon studied hundreds of coot nests in British Columbia during a four-year investigation. His latest findings appear in the April 3 issue of the scientific journal Nature.
"The ability of females to count only their own eggs in a mixture of eggs is a remarkable feat that provides a convincing, rare example of counting in a wild animal," Lyon wrote in the published paper.
A member of the rail family, the American coot (Fulica americana) is a slate-gray bird with a white beak, about the size of a small duck. It inhabits lakes, ponds, and marshes, often in large numbers. Coots are ungainly on land, reluctant to fly, and not very impressive in the water, either. Use of the word coot to mean a stupid person or simpleton reflects prevailing attitudes toward these rather comical birds. But this perception is belied by Lyon's discovery of their impressive cognitive abilities.
"I was shocked. At first, I didn't believe the results," he said.
Lyon originally set out to study how coot parents care for their chicks. But the focus of his research changed when he discovered extremely high levels of "brood parasitism," the practice of laying eggs in other birds' nests. Most studies of brood parasitism have looked at birds like cuckoos, which lay their eggs in other species' nests and thereby avoid the trouble of raising their own chicks. But brood parasitism also occurs within species, and Lyon's research on coots is shedding new light on this little-studied phenomenon.
Lyon and his field assistants tracked the fate of every egg in more than 400 coot nests, checking the nests every day. Because no bird can lay more than one egg per day, parasitism was readily detected. In addition, coot eggs laid by different parents can often be distinguished on the basis of color and speckle pattern.
"The egg pattern is like a fingerprint," Lyon said. "It turns out that in most cases, the parasitism is done by females who have their own nests, and often it's the next-door neighbor, so by comparing the eggs in nearby nests we can tell who's doing it."
Parasitism was rampant, affecting 41 percent of the nests and accounting for 13 percent of all the eggs laid in the study population. Furthermore, the cost of parasitism to the hosts was quite high.
"There's not enough food for all the eggs that get laid, so there is a massive die-off of chicks. Typically about half the chicks in a nest starve to death," Lyon said. "That explains both the cost of parasitism to the host and the benefit to the parasite."
Female coots, Lyon found, are quite good at recognizing and rejecting parasitic eggs. Rejected eggs were buried deep in the nesting material and never hatched. In some cases, parasitic eggs were not rejected outright, but were banished to inferior incubation positions, resulting in delayed hatching and decreased likelihood of survival for the parasitic chicks.
"They may use that strategy to deal with eggs they are less certain of," Lyon said. "It's subtle, but it has an effect on reproductive success, because the ones that hatch later are more likely to perish."
Evidence that coots are able to count their eggs was completely unanticipated. Lyon wanted to look at how the presence of parasitic eggs affected clutch size. Coots are indeterminate layers, meaning that they'll keep laying eggs until an external cue tells them they're done. Speculation about the nature of the cue has long favored the sense of touch--when the female feels like she's sitting on the right number of eggs, the development of new egg follicles stops.
(Follicles that have already started developing continue to develop, so there is a delay between the cue and the cessation of egg laying.)
If the birds use a touch cue, the presence of parasitic eggs should cause a female to lay fewer of her own eggs. But Lyon found that when female coots recognized the parasitic eggs in their nests, eventually rejecting them, they did not reduce their clutch size. In contrast, females that failed to recognize parasitic eggs laid one fewer of their own eggs for each parasitic egg they received.
"Rejection takes a long time, so the clutch-size decision is made while the parasitic eggs are still in the nest," Lyon said. "That means they are not using a touch cue. These birds are looking at their nests and counting only those eggs they recognize as their own to make a clutch-size decision."
Lyon noted that the meaning of "counting" in animals is a highly contentious issue among cognitive psychologists and animal behavior experts. What he means by counting is that the birds are making decisions based on the number of eggs in their nests.
"That's pretty amazing for a 'stupid' bird like a coot," Lyon said. "It's very satisfying to rescue a study animal from a bad rap."