Incubation periodThe length of time during which incubation takes place, usually expressed in days. Birds may delay the start of incubation until the penultimate or the last egg is laid, thus ensuring that the eggs in a clutch hatch more or less together. In cases where incubation begins with the first egg asynchronous hatching results.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver