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 femalesyoung and inexperienced onesmay 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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