Custer, T. W., Pendleton, G. W., and Roach, R. W., 1992, Determination of hatching
date for eggs of black-crowned night-herons, snowy egrets and great egrets: Journal of
Field Ornithology, v. 63, no. 2, p. 145-154.

Abstract

Floatation of eggs in water and specific gravity of eggs of Black-crowned
Night-Herons (Nycticorax  nycticorax ), Snowy Egrets (Egretta thula ) and Great
Egrets (Casmerodius albus ) were evaluated as methods to  determine date of hatching.
Although specific gravity was a better predictor of hatching date than egg flotation, 
both techniques were imprecise. The regression between specific gravity and the
number of days before hatching  differed among clutches, but not among eggs within
clutches. Specific gravity of eggs predicted hatching date  only to within 3.8 d for
Snowy Egrets, and 4.7 d for Black-crowned Night-Herons and Great Egrets. The mean 
incubation period was 27.3 d for Great Egrets, 23.7 d for Snowy Egrets and 22.8 d for
Black-crowned Night- Herons. For all three species, the A egg (first egg laid) had a
longer incubation period than the B or C egg.

Keywords

eggs, hatching, specific-gravity, Nycticorax-nycticorax, Egretta-thula,
Casmerodius-albus, bird-eggs, aquatic-birds, hatching-date; flotation-