Nutritional needs for egg formation in the Shag Phalacrocorax aristotelis

Abstract

Egg production does not impose a major food need in the Shag Phalacrocorax aristotelis because the eggs are small, are formed slowly and are laid at 3‐day intervals. I used dye‐dosing of females, laying time and, after fixing and staining the yolk, daily ring counts to estimate the amounts of protein and energy needed each day to produce a clutch of three eggs. Maximum daily nutrient needs during egg formation were only 1.15 g per day additional protein and 34 kJ per day energy. Yolk formation times of 40 eggs were 13.5 (±1.2 s.d.) days. Based on yolk‐ring counts and laying dates of 15 eggs that contained dye, the lag period between yolk completion and laying was 3.1 ± 0.06 days. The distinct light and dark rings of the stained yolk resulted from differences in the transparency of the yolk spheres. In the dark rings, the spheres were relatively clear, so more depth of stained yolk could be seen than in the light rings, which reflected the incident light.