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For coots, hatching order is crucial ID

When birds sneak eggs into others' nest, mom and dad can learn to find their own

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4:27pm, December 16, 2009
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Calling someone an old coot may be a compliment to their ingenuity. American coots possess cognitive skill enabling them to discern their own chicks from impostors. The wetland birds use hatching order to identify their offspring, researchers report online December 16 in Nature.

The discovery documents another score for host birds in their battle with cheaters that lay eggs in the hosts’ nests, and helps to elucidate the evolutionary pressures that can lead to learning.

“Coots are not the first bird you would think of as showing sophisticated levels of discrimination,” says Stephen Rothstein of the University of California, Santa Barbara, who was not involved in the work. “Coots are regarded as one of the dopiest birds — among bird watchers they aren’t necessarily one of the most desirable birds to see.”

Brood parasitism, where birds parasitize other birds’ nests by slipping their eggs in on the sly, is costly for the host birds — in the case of coots, a host’s chick dies for each parasitic chick that lives.  Some species of host birds learn to recognize their own eggs, but in some cases,  parasitic birds precisely match the host’s egg color and patterning, gaining an evolutionary advantage. Although egg recognition by host birds is common, chick recognition by host parents has been reported only a handful of times. This discrepancy has puzzled researchers.

In some birds, “even though they can distinguish very fine detail with eggs, they are happy to feed an enormous chick who looks nothing like their offspring,” says evolutionary biologist Rebecca Kilner of the University of Cambridge in England.

So if evolution has led to sophisticated recognition with eggs, why hasn’t it with chicks? One theory argues that learning can be dangerous. There’s a chance that parents will identify the wrong bird — even a different species — as their own. This mix-up may mean rejecting, sometimes violently, their own children.

“The idea is bite the bullet,” says Daizaburo Shizuka, coauthor of the study with Bruce Lyon, both of the University of California, Santa Cruz. “Forgo recognition and feed what is in your nest.”

The new work suggests that coots can evolve the ability to recognize their chicks if a reliable method presents itself. Long-term field studies by Lyon and Shizuka, spying on coots near Williams Lake in British Columbia, indicated that coots use the “here first” rule — whoever hatches first is most likely to be the host’s own chick.

Coots lay about nine eggs per clutch, which hatch over three to 10 days. Typically the first three chicks that emerge are native to the nest. Coots will also parasitize other coots’ nests, probably as a fallback strategy when they have lost their own nest, Rothstein speculates.

To test how the host birds decided whom to feed versus whom to evict, Lyon and Shizuka transplanted hatchlings among nests. When parasitic chicks arrived at a nest before native chicks, coot parents discriminated against their own young. And when researchers manipulated the nests so a native and foreign chick appeared first together, native and parasite chicks fared equally well.

The work is an important contribution, says Kilner. Brood parasites often get off scot-free if they live beyond the egg stage, although Japanese researchers recently reported that the large-billed gerygone recognizes and forcibly removes the parasitic little bronze-cuckoo chicks from its nest.

Now everyone is wondering how adult coots recognize their individual chicks, researchers say. Coot chicks have notoriously crazy plumage which may be variable enough that parents use it as a signature; vocalizations may also play a role parents’ ability to recognize their young.

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