Bluebird egg nestbox check, photo by Sharon from Pixabay
Checking a nestbox

When does incubation begin?

Incubation does not begin until the last egg is laid, so that’s when you usually start counting. There are exceptions – Tree swallows begin incubation on the penultimate (next to last egg) which can result in asynchronous hatching.

Can incubation be affected by weather?

Yes. In some cases, e.g., when weather is cold, start of incubation may apparently be delayed for a week or so. Once they start incubating, eggs should hatch according to the timetable listing in the various bird bios.  Some evidence indicates that really high temperatures may render male bluebirds infertile, in which case the eggs will never hatch.

How do I see if the eggs are developing?

You might try candling one of the eggs to see if there is any development. Candling refers to the process of illuminating the contents of an egg. It’s a relatively simple process. Basically in a dark place, hold up and shine a light a small penlight type flashlight through the egg. Keep the egg in it’s normal upright position and don’t overheat it with the light bulb! Be very careful when you are holding the egg so you don’t accidentally crack it! A few seconds is long enough to see what you need. If it’s totally clear inside, that means no development. If its all or partly dark, or has a “blood ring” there may be some development, but it may have been arrested.



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