Variations in measurements among white pelican eggs and their use as a hatch date predictor

Abstract

Incubated bird eggs tend to lose weight linearly, and it is therefore possible to predict hatch dates from egg measurements taken during incubation. From 30 white pelican eggs measured in the laboratory, we found that egg volume (V) was estimated by V = 0.500 LB2 (L = length, B = maximum width). Specific gravity (SG) was then obtained as W/V (W = weight in air). A regression line of SG against days of incubation based on values for naturally incubated eggs was used to predict hatch dates for additional samples of 10 eggs from each of 55 subcolonies on five islands in Manitoba in 1978. Mean SG of 21 freshly laid eggs was 1.1009 ± 0.0033 g∙cm−3. The regression line for naturally incubated eggs was SG = 1.1081 g∙cm−3 – (0.0042 g∙cm−3∙egg−1∙day−1∙days). Actual hatch dates of 17 subcolonies were strongly correlated with predicted dates (r = 0.95). Hatch synchrony was independent of subcolony size. Mean error for subcolony predictions was 0.4 days, but was 2.8 days for individual eggs.Only width and consequently volume differed significantly among the colonies. Within subcolonies, variability in linear dimensions was mainly due to interclutch differences.Observations of vocalizations and pipping of eggs during visits are also useful for predicting hatch dates.