Why crested penguins lay mismatched eggs

Extreme penguin egg favoritism could be quirk of migration

macaroni penguins

MISMATCHED OFFSPRING When macaroni penguins convene at a breeding site (shown), females lay some of the most size-mismatched eggs known among birds.

G. Crossin

In crested penguin families, moms heavily favor offspring No. 2 from the start, and a new analysis proposes why. The six or seven species of crested (Eudyptes) penguins practice the most extreme egg favoritism known among birds, says Glenn Crossin of Dalhousie University in Halifax, Canada.

Females that lay two eggs produce a runty first egg weighing 18 to 57 percent less than the second, with some of the greatest mismatches among erect-crested and macaroni penguins. Some Eudyptes species don’t even incubate the first egg; royal penguins occasionally push it out of the nest entirely.

Biologists have proposed benefits for the unusual behavior: A sacrificial first egg might mark a claim to a nesting spot or improve chances of one chick surviving predators. But those ideas haven’t held up, Crossin says. He and Tony Williams of Simon Fraser University in Burnaby, Canada, propose in the Oct. 12 Proceedings of the Royal Society B that egg favoritism is just a downside of an open-water, migratory lifestyle

two penguin eggs
SIBLING DISPARITY The newer (cleaner) second egg in a macaroni penguin nest outsizes the first-laid one. It’s not unusual for eggs in a bird’s clutch to differ by maybe 5 percent, but Eudyptes penguins are extreme. Stacey Adlard

Among the 16 penguin species that lay two eggs, only the Eudyptes species evolved what’s called a pelagic life, spending their nonbreeding season mostly at sea and migrating, in some cases considerable distances, to breeding sites.

Female crested penguins tend to lay their first eggs soon after arriving at a breeding site, meaning that the egg must have started its roughly 16-day development while mom was migrating. The biology of long swims, now encoded genetically, interferes with producing a full-sized egg. A puny first egg might just be a sign that mom is trying to do two things at once, Crossin says. 

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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