Some birds lay 20 eggs per clutch; others lay just one. Most fall somewhere in between. And yet there’s a little-known species of penguin that lays one and a half.

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Erect-crested penguins are, according to Lloyd Davis of New Zealand’s University of Otago, “the forgotten penguins,” largely because of the inaccessibility of their breeding colonies on the uninhabited Bounty and Antipodes islands 800km off New Zealand.

“No one visits them,” says Davis. “Sure, every now and then, in the past, you’d get shipwrecked sailors or sealers there. But the only people visiting those islands these days are scientists who have gone through the rigorous permitting process, which is basically a fence to keep people out. Virtually no work has ever been done on them and you never see them on documentaries, because no one’s allowed to go there and film.”

For his latest research on the species, published in PLOS One, Davis and his colleagues drew on data collected on a rare visit to the colonies in 1998.

This has revealed that erect-crested penguins employ a highly unusual egg-laying strategy, in which the first of the two eggs they lay is only about half the size of the second. Davis says the difference is more pronounced than in any other bird. The smaller egg is not brooded by the parents and never hatches.

Davis believes that this bizarre situation represents a snapshot in a process of transition between a two-egg and a one-egg strategy. He says that while most penguins raise two chicks, species that nest far from their feeding grounds tend to reduce their clutch to a single egg. Emperor and king penguins, for example, stop laying after the first egg.

For the erect-cresteds, though, it’s not so simple, because for some reason they put all their effort into the second egg rather than the first.

“The problem is that you can’t lay a second egg until you’ve laid a first egg,” says Davis. “So all you can do is reduce the investment in the first egg as much as you can.”

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Main: erect-crested penguins breed on the Bounty and Antipodes islands © DeAgostini/Getty

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