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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Intra-clutch pattern of albumen delta(13)C and delta(15)N in yellow-legged gulls Larus michahellis: female dietary shift or resource allocation strategy?
Auteurs :	Francisco Ramirez, Auteur ; Rauel Ramos, Auteur ; Josep Lluis Carrasco, Auteur ; Carola Sanpera, Auteur ; Lluis Jover, Auteur ; Xavier Ruiz, Auteur
Type de document :	Article
Dans :	Journal of Avian Biology (42(3) MAY 2011)
Article en page(s) :	239-246
Langues:	Anglais
Résumé :	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(13)C and delta(15)N 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(13)C, 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(15)N 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(15)N that shows greater isotopic fractionation with respect to that commonly assumed for delta(13)C). 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.

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