Intra-clutch pattern of albumen delta C-13 and delta N-15 in yellow-legged gulls Larus michahellis: female dietary shift or resource allocation strategy?

摘要

Energy or nutritional constraints associated to female dietary shifts during the clutch production period may play a role in generating intra-clutch egg size variation in yellow-legged gulls Lams michahellis. To explore this possibility, we determined albumen delta C-13 and delta N-15 values in three-egg clutches (modal clutch size) from three different breeding episodes: Ebro Delta 2004 and 2006, and Columbretes Islands 2004. Rather than a shift in females' diet, consistent intra-clutch patterns of variation in egg size and albumen isotopic values (particularly in the case of albumen delta C-13, which values held constant throughout the laying sequence) pointed to an intrinsic mechanism as the most feasible cause for the relatively smaller size of third/last-laid eggs. However, diet "quality" for breeding females seemed to affect infra-clutch egg size variation. In particular, a deficit of specific nutrients for egg formation associated to refuse scraps exploitation (as suggested by depleted albumen isotopic values) likely resulted in the more apparent intra-clutch egg size profile for the Ebro Delta 2004. in the absence of dietary shifts, the observation of consistently higher delta N-15 values for third-albumens suggested a greater contribution of endogenous resources to their synthesis, as conversion of stored reserves into egg proteins results in greater isotopic fractionation, thereby yielding enriched isotopic signatures (particularly for delta N-15 that shows greater isotopic fractionation with respect to that commonly assumed for delta C-13). We point to reabsorbed material derived from the hormonally-mediated regression of the female reproductive system (which is likely the intrinsic mechanisms resulting in the intra-clutch pattern of egg size variation: the hormonal hypothesis) as the most feasible endogenous source of nutrients for the synthesis of last-laid eggs, as optimize reproductive investment and maximize female fitness.