How to Estimate Incubation Stage on "Messy" Eggs

We had a student working along predator management crews that monitored the nests of Sharp-tailed Grouse (top pic) and shorebirds – Willet, Phalaropes, Godwits, Snipe, Killdeer – that are in the uplands with the dabblers.

The eggs of all these birds have considerable pigment, so field candling is not an option to judge incubation stage. However, all bird eggs lose about 30% of their weight (mostly water) during incubation. Accordingly, fresh eggs sink in water and lie flat (as shown) but near-hatch eggs float with almost 10% of the egg above water. Floating eggs to estimate stage is surprisingly accurate.

Killdeer eggs – much pigment, typically in little cover.

Frank