It takes a lot of nutrients to build an egg. One of the big questions among researchers who study the eggs of migratory birds is where those nutrients come from—does the mother make the egg directly out of what she eats during the breeding season, or does she save up nutrients consumed on her wintering grounds? The answer appears to be both for Common Eiders (Somateria mollissima), large, sea-going ducks that breed in the Arctic.
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