Columns

Last week I explored some of the variations of siblicide, or cainism, in golden eagles and other birds, as described by zoologist Douglas Mock in More Than Kin and Less Than Kind: The Evolution of Family Conflict. As that subtitle implies, Mock has also studied less extreme forms of sibling competition, such as begging for parental attention. He’s also interested in the parents’ role in various degrees of domestic mayhem. 

Most parent birds, it seems, don’t try to referee conflicts between their offspring. They just watch the drama unfold. Mock says he expected to see some kind of intervention in his field work on great egrets, at least to see a parent call a timeout by sitting on the combatants. But that rarely happened. 

In some cases, parental favoritism is blatant. Magellanic penguins lay two eggs, starting incubation with the first so that one chick has a head start. Both typically hatch, but the parents feed only the first chick; the second starves to death. A relative, the royal penguin, lays a smaller egg first, then a larger one, whereupon it boots the first egg out of the nest. 

Overt infanticide seems rare among birds, unlike mammals. There’s one well-documented instance of a European black stork touching down at a nest, feeding the chicks, and then picking up the smallest and pitching it over the side, to its death. It’s not certain that the killer was a parent, but the circumstantial evidence is strong. 

Something similar also occurs in family groups of coots. The phenomenon was first described by Gordon W. Gullion, who observed American coots at Jewel Lake in Tilden Regional Park as Starker Leopold’s graduate student at UC Berkeley. (He went on to become a professor at the University of Minnesota and an authority on the ruffed grouse.) 

Coot chicks go through a dramatic transition as their mature. They hatch out as grotesque little creatures, but lose their orange head feathers and bald pates within three weeks. In an article published in The Auk in 1954, Gullion wrote: “Since the parents seem to recognize their brood by the color pattern of the majority, the oldest birds become subject to occasional attacks when they start turning light…Sometimes these parental attacks are quite severe, and it is conceivable that young might occasionally be killed by their parents during this transition period.” 

Thirty years later, William Horsfall reported similar behavior in the common coot of Eurasia, which he called “tousling.” That sounds innocuous enough, but involved the parent grabbing the chick’s head and shaking it vigorously, sometimes with fatal consequences. Tousling appears to be directed at the larger chicks 

in a brood, who would otherwise outcompete younger siblings for food. If not killed outright, the victims sometimes stop begging for parental handouts and waste away. 

Gullion’s Tilden Park observations seem to fit with the idea of parental intervention on behalf of younger chicks, which received support from an experimental field study by Bruce Lyon in British Columbia in the 1990s. In each of 21 coot broods, Lyon gave half the chicks a trim, sniping off their orange feather tips. He found that unclipped chicks were fed more, grew faster, and had better survival rates than their barbered siblings. Hatching order didn’t seem to affect the outcomes. 

Other biologists have speculated that the gaudy plumage of coot chicks may help parents decide where to make their investments. Maybe the color of the bald head somehow indicates the state of the chick’s health. Alternatively, the whole gaudy getup may be a chick’s way of competing for parental attention—a visual adjunct to begging. 

One study compared chick plumages and life histories for the whole rail family, which includes coots. Most species had plain chicks. Ornamentation tended to correlate with territoriality, large broods, aquatic habitats, and complex mating systems (some rallids go in for polyandry.) American and European coots exhibit the first three traits, but they’re monogamous, at least socially. I don’t know if anyone has done the indicated paternity tests. 

There’s another wrinkle, of course (there’s always another wrinkle): female coots, apparently “floaters” without a territory, often lay their eggs in other coots’ nests. Does chick coloration let parents identify their own offspring in a mixed brood? This doesn’t seem to be a factor: vigilant coots usually eject alien eggs or bury them in the floor of the nest, so very few actually hatch. 

You would not expect the lowly mudhen to have such a complicated set of behaviors. (My great-grandfather, a sometime duck hunter, once brought home what family legend remembers as a mudhen and asked my great-grandmother to cook it with the ducks. This was not a success. The presumed coot had to be discarded and the kitchen thoroughly aired out. The hunter never lived it down.)