What Decides Clutch Size?

Most birds lay a predictable number of eggs, but why?

This audio story is brought to you by BirdNote, a partner of the National Audubon Society. BirdNote episodes air daily on public radio stations nationwide.


Written by Bob Sundstrom

This is BirdNote.

When nesting, most birds lay a roughly predictable number of eggs. 

Bald Eagles: 2. Bluebirds: 4 to 6. Mallards: 10 to 12. 

Which leads to an obvious question: 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 in a row, while a flicker laid 71 in 73 days before the removal was halted. 

Birds that do this — keep on laying after eggs are removed — are called indeterminate layers. And for them, it seems that the number of eggs is intimately tied to the brood patch. That's the bare skin on a bird’s underside that transfers body warmth to the eggs during incubation. It's the tactile sensation involved in this process that seems to let the layer know she's reached the ideal number of eggs. 

But for some other birds, the scientists’ removing eggs had no effect at all on egg numbers. These are known as determinate layers. And just why these two types of layers exist remains an open question.

For BirdNote, I'm Michael Stein. 



Bird sounds provided by The Macaulay Library of Natural Sounds at the Cornell Lab of Ornithology, Ithaca, New York.

BirdNote’s theme music was composed and played by Nancy Rumbel and John Kessler.

Producer: John Kessler

Executive Producer: Dominic Black

© 2015 Tune In to Nature.org        June 2018  Narrator: Michael Stein

ID#  egg-04-2015-06-09 egg-04

Primary source: Tim Birkhead’s Bird Sense