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
) 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.

The nesting parasites laid their own clutch after leaving eggs in another female's nest. This improved their chicks' survival rates because the risks from other parasitic birds, nest desertion, and predation decreased over the season.

So why don't all the females do it? Åhlund and Andersson found that from one year to the next, many females switched tactics. The scientists also found some evidence that nesting parasitism occurs during the prime years of a female's reproductive capacity.

"Parasitism in goldeneyes seems to be a conditional strategy," said Åhlund. "If the ecological and social circumstances permit, the most fit females can combine parasitism and normal nesting to increase reproduction markedly. Less fit females—young and inexperienced ones—may use parasitism as a way to produce at least some offspring if they cannot have their own nest."

A Family Affair

Goldeneye hatchlings are relatively easy to take care of once they're born. "Their eyes are open, they're covered in down, they can feed themselves, and they don't require a lot of tending," said Eadie. "The females might not defend against it [parasitism] because the cost of raising an extra chick isn't that high."

And the chicks may be their relatives. Goldeneye females return to their place of birth to nest, so mothers, sisters, and daughters are all likely to nest in the same area.

In research published earlier this year in the journal Trends in Ecology and Evolution, the authors found that female goldeneyes who had been birth nest mates were more likely to pair together in their daily activities, and that the proportion of eggs found in the nests of former nest mates was higher than pure chance would allow.

"Relatedness and kin discrimination may have even wider importance for the evolution of alternative reproductive tactics and animal sociality than previously thought," said Åhlund.

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