Planet Earth Online

Delayed egg laying lets cuckoo chicks hatch early

29 September 2010, by Tamera Jones

Many cuckoo species offload their parental duties, laying their eggs in other birds' nests so that they'll be raised by host parents. As soon as the cuckoo chicks have hatched (and before they can even see), they lift any other eggs they find in the nest onto their backs and then throw them overboard.

Hatching early means that cuckoo chicks can oust other birds' eggs so that they get all the food their foster parents bring home.

But it seems the ability to hold onto eggs for longer before laying isn't necessarily an adaptation allowing cuckoos to be better at getting away with laying their eggs in other birds' nests. Rather, the researchers say the birds' parasitic ways may have evolved as a consequence of this ability.

Back in 1802, a scientist called George Montagu suggested that cuckoo chicks hatch before their hosts' chicks emerge because cuckoos hold onto eggs for longer before they lay. The idea was that incubating eggs inside for longer gives embryos a head start in the development stakes. Other researchers and egg collectors have since anecdotally reported that cuckoo embryos are more developed than in other birds' eggs of the same age.

But until this study, published in Proceedings of the Royal Society B, no-one had actually checked whether or not cuckoos incubated eggs internally for longer.

'People knew that cuckoos lay at 48 hour intervals, whereas most birds lay 24 hours after ovulating. But we didn't know if cuckoos were just having a break or holding onto eggs for longer,' explains Nicola Hemmings from the University of Sheffield and co-author of the study.

Hemmings and colleagues from the universities of Sheffield and Cambridge, and scientists from South Africa, Hungary and Germany had a hunch that cuckoos keep hold of their eggs inside, which ultimately gives their chicks a developmental leg-up.

'A lot of researchers didn't think cuckoos would retain the egg for longer, because they thought a fully formed egg would impair flight. But this isn't the right explanation. Cuckoos have to sneak eggs into other birds' nests, so they have to be nimble. Clearly they can still fly,' says Hemmings.

To find out if longer incubation gives cuckoo and other parasitic bird chicks an advantage, the researchers compared embryo development in various birds' eggs soon after they'd laid them. The birds they studied included the European cuckoo, the African cuckoo, as well as the Zebra finch and domesticated pigeons.

They found that embryos in eggs laid by the European cuckoo, the African cuckoo and the greater honeyguide were more advanced than birds that lay eggs 24 hours after ovulation. And while domesticated pigeons aren't parasitic, they also hold onto their eggs for longer – 40 hours – giving their chicks a developmental head start.

Of the birds they studied, they found that internal incubation is common throughout so-called brood parasitic birds, like cuckoos.

When they hold eggs inside them, birds essentially incubate eggs at their own body temperature of 40°C rather than the 36°C you'd find in the average nest. 'We think the higher temperature might be causing rapid development of the cuckoo embryo so it's ahead of other birds by the time it hatches,' says Hemmings.

To support this idea, the researchers found that incubating a non-parasitic bird's egg at 40°C for an extra 24 hours after laying, rather than the usual 36°C, sped up development of the embryo. 'The Zebra finch embryo was identical in terms of development compared with the cuckoo embryo,' Hemmings says.

And while the researchers don't know why cuckoos have evolved a longer egg retention time, they think this trait may have predisposed them to brood parasitism.

Keywords: Biodiversity, Europe,

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