Careful Coots: Do birds count their eggs before they hatch?

A coot may tally the eggs in her nest, a rare example of an animal counting in the wild, suggests a new study.

American coots (Fulica americana) wage covert egg wars among themselves, sneaking into a neighboring nest to deposit an egg for the other family to raise, explains Bruce E. Lyon of the University of California, Santa Cruz. He’s made an unusually detailed study that shows that coots do fight back. They often count up the rightful ones and move or destroy the suspect eggs, Lyon contends in the April 3 Nature.

Biologists have found dozens of bird species that catch interloper eggs from different species, but only a few–some ostriches, weaverbirds, moorhens, and now coots–with defenses against sneaks of their own species.

Lyon and his energetic field assistants monitored coot nests in a marsh in British Columbia every day for four breeding seasons. Stealth egg layers hit 41 percent of the more than 400 nests. A parasitic chick posed a threat because half of all chicks starved, and an intruder among the early hatchlings snagged food that might have kept a later-emerging, rightful resident alive.

Coots remove an egg by rolling it out of their nests or burying it in the nest’s reedy mass. In these ways, the birds ditched only 2 percent of eggs they’d laid themselves–mostly those with cracks–but a third of the eggs from cheating neighbors. “They do recognize parasitic eggs,” Lyon concludes.

Also, he found the first evidence that phony eggs are disproportionately pushed to the undesirable outer rim of the nest, which slows hatching.

The background colors of rejected eggs differed more from rightful eggs than did the shades of eggs that tricked foster parents. The cue therefore is probably visual, Lyon says.

To see if coots can count, Lyon compared the records of birds that eventually accepted parasitic eggs with those of birds that rejected them. Accepting coots tended to lay fewer eggs, roughly one less per interloper egg. At the critical time when the birds’ bodies determined whether to stop producing eggs, both groups of birds faced the same number of parasitic eggs in their nests. Lyon argues that both groups were counting how many eggs they perceived as their own and that the accepting birds were fooled by the counterfeits.

Parasitic-bird specialist Stephen Rothstein of the University of California, Santa Barbara says he’d like to see tests that manipulate the birds’ perceptions, rather than this study’s hands-off observations, before he accepts that coots count.

Another researcher who studies egg sneaks, Malte Andersson of the University of Göteborg in Sweden, is more open to the current findings. He says that many lab experiments have suggested counting ability in captive animals, and the new study provides one of the first examples of animals’ counting in nature.

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