When nesting, most birds lay a predictable number of eggs. Bald Eagles: 2. Bluebirds: 4 to 6. Mallards: 10 to 12. But how do they determine when they have laid the right number? To find out, scientists experimented by going to nests and repeatedly removing eggs soon after they were laid. Some birds replaced them straight away. For example, a House Sparrow laid 50 eggs in a row, while a flicker laid 71 eggs in 73 days. But for other birds, the scientists’ removal of the eggs had no effect at all.

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