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Unfaithful offspring get head start

A blue tit incumbating eggs in the nest

Female tit birds typically mate with rival males before their social partner (Source: Anna Harts)


The finding by Dutch and Australian researchers casts doubt over the long-held idea that female birds use promiscuity as a way of giving their chicks genetic advantages.

Instead, Dr Michael Magrath, of the University of Melbourne's Department of Zoology, says the chicks are stronger, larger and more likely to survive simply because they are laid and hatch earlier.

Magrath says this means they begin feeding earlier than their half-siblings, who may hatch as much as five days later.

For the study, published online today in Current Biology, Magrath and colleagues, numbered 1732 eggs from 190 blue tit bird nests in the order in which they were laid across two breeding seasons.

Head start

They placed the eggs in an incubator and noted the order in which they hatched, before returning them to their original nest.

"Remarkably, almost 75% of the offspring from these 'extra-pair' matings were produced in the first half of the clutch," he says.

On average these extra-pair chicks were hatched 10 hours before their half-siblings. This gave them time to gain an initial size advantage because they start feeding immediately.

Magrath says it has been known for about 15 years that female birds, who commonly form monogamous social pairings, mate with other males.

He says the reasons for this behaviour have puzzled researchers, because unlike males, females don't produce more offspring through extramarital mating.

But studies had earlier showed the offspring of these so-called extra-pair matings were bigger, had better immune response and were more likely to survive.

This led researchers to conclude these matings provided a genetic advantage.

"We don't rule out the idea [extra-pair mating] is to gain a genetic advantage," he says. "But we have shown what's been considered good evidence isn't that strong."

Magrath says when they controlled the results for hatching order, they found the size difference "virtually disappears".

Straying from the nest

He says their study raises other questions such as why there is a connection between laying order and paternity, and what motivates the female to seek out other males.

Magrath says the laying pattern may be related to the timing of copulation with extra-pair matings happening earlier in the breeding season.

In an ongoing study, Magrath and colleagues added model eggs to nests before the female had begun laying and found these females were less likely to mate outside their social pairing.

This would be because once the female starts laying she is guarded more closely by her partner and does not have the opportunity to wander.

Tags: animal-behaviour, birds