Some Ducks Let Young Be Raised by Relatives

Hillary Mayell
for National Geographic News
December 10, 2001

Is there such a thing as foster care among ducks? Two Swedish researchers studying the reproductive strategies of female goldeneye ducks think there may be.

Female goldeneyes (Bucephala clangula) lay eggs in their own nests and in the nests of other female goldeneyes. This behavior, known as conspecific parasitism, is relatively common among waterfowls. Researchers have been intrigued by the practice because, from a cost-benefit perspective, it is not clear why a female would hatch and raise young other than her own.

Over the years, many hypotheses have been proposed. Among the explanations are that goldeneyes nest in dark tree cavities and therefore can't differentiate between their own eggs and another's, that it's a reproductive error or the result of a shortage of nesting sites.

Now Matti Åhlund and Malte Andersson, evolutionary biologists at Gothenburg University in Sweden, have found that female goldeneyes recognize their nest mates and often lay eggs in the nests of close relatives.

The finding raises the possibility that among goldeneyes, brood parasitism is more of a co-parenting activity based on social interactions and recognition of kin than an easy way to increase reproductive capacity.

"To put this in a historical context, in the 1960s these behaviors were considered aberrant behaviors, as if the females were confused and didn't know what they were doing—like they were coming home drunk or something when they laid eggs in another bird's nest," said John Eadie, a wildlife biologist at the University of California, Davis, who also studies parasitism.

"Now we're finding that these are conditional strategies that may be based on relatedness, environmental factors, availability of nests, age, and genetic quality of the female," he said.

Eadie noted that Åhlund and Andersson have pioneered some of the techniques that made these latest findings possible.

Reproductive Strategies

Åhlund and Andersson studied the egg-laying habits of goldeneyes at Lake Mjörn in southwest Sweden. They found three distinct and equally common reproductive strategies among female goldeneyes: Non-parasitic females laid eggs in only their own nests; purely parasitic females laid eggs only in other birds' nests; and nesting parasites laid eggs in their own nests and the nests of others.

The nesting parasites had the greatest success in reproduction. Some were able to double their reproductive output, the authors write in the December 6 issue of the journal Nature.

In this study, nesting parasites laid an average of 12.3 eggs, non-parasites laid 7.9, and pure parasites laid an average of 5.8. The survival rates for parasitic chicks was the same as that of host chicks.

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