Within-clutch trade-offs between the number and quality of eggs: Experimental manipulations in gulls

We experimentally increased the number of eggs laid by Lesser Black-backed Gulls and examined the relationship between egg number and egg quality. Egg quality was measured in terms of egg composition and the probability that an egg would give rise to a fledged chick. In measuring the latter, we removed the potentially confounding effects (1) between parental quality and equality and (2) between increased egg production and parental condition, using a cross-fostering protocol in which eggs were reared by control parents. Lesser Black-backed Gulls were capable of producing, on average, almost three times the normal clutch of three eggs. Although egg mass did not fall below that of the last laid egg in normal clutches, as the clutch extended beyond three, experimental eggs contained relatively less lipid and relatively more water. Overall, the percentage of eggs that gave rise to hedged chicks (when reared singly by a foster parent) declined significantly with position in the experimental laying sequence; about two-thirds of the eggs laid at the beginning of the laying sequence gave rise to fledged chicks, whereas only about one-third of the last laid eggs did so. This was not due to any seasonal change in the capacity of foster parents to rear the chicks, and neither hatching nor fledging success of the eggs was related to their fresh mass. Thus, as more eggs are laid, important changes in egg composition occur that have in themselves, substantial effects on offspring survival. This study provides clear evidence of a trade-off between egg number and egg quality.