Shell Resistance and Evaporative Water Loss from Bird Eggs: Effects of Wind Speed and Egg Size

Wind speed had a negligible effect on the rate of evaporative water loss from chicken eggs. Mean water loss was 0.132 mg cm⁻² h⁻¹ in still air, and 0.138 mg cm⁻² h⁻¹ at 400 cm s⁻¹. Boundary-layer resistance (rb) decreased from 2.17 s cm⁻¹ in still air, to 0.91 s cm⁻¹ at 100 cm s⁻¹, to 0.54 s cm⁻¹ at 400 cm s⁻¹. The boundary layer had no effect on water loss because shell resistance (ri) was the limiting variable with values of 384.0 s cm⁻¹ at still air, 385.0 s cm⁻¹ at 100 cm s⁻¹, and 374.9 s cm⁻¹ at 400 cm s⁻¹. Shell resistance changed with time. During the first 2 h of our experiment in still air, ri was 294 s cm⁻¹ and gradually increased during hours 2-5 and 5-24 to 366 and 395 s cm⁻¹. This change was less apparent at 100 and 400 cm s⁻¹ wind speeds. Boundary-layer resistance increased as egg size increased. For a finch egg (0.8 g), a goose egg (190 g), and an ostrich egg (1,480 g) rb was 1.63, 2.40, and 5.00 s cm⁻¹, respectively. Boundary-layer effects and wind speed are of little importance in the gas exchange of bird eggs, even the very large eggs of the ostrich and recently extinct elephant bird, Aepyornis.