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Understanding how resources are allocated to form eggs is crucial to our better understanding of avian reproductive strategies. However, little is currently known about how egg synthesis in wild birds might be constrained by the availability of specific micronutrients. Here, we investigated the potential role of calcium (Ca) in constraining egg synthesis in the Audouin's Gull (Ichthyaetus audouinii (Payraudeau, 1826)). In particular, we evaluated the relationship between plasma Ca levels (mg/dL) in incubating females (as an indicator of the physiological response of females to increased Ca demand associated with clutch production) and several fitness-related egg traits such as egg size (i.e., egg volume), egg shape, and eggshell thickness from three-egg clutches. Egg size was positively related with incubating female plasma Ca levels, with the slope of this relationship being significantly higher for later-laid eggs. The observational nature of this study and reversed timing precludes causal inferences, but observed relationships supported the constraining role of Ca in egg synthesis and suggested that Ca may also have a role in modulating the intraclutch pattern of egg-size variation typical of this gull species. Key words: Audouin's Gull, avian reproduction, calcium, egg production, egg size, eggshell, Ichthyaetus audouinii. La comprehension de l'affectation de ressources sont allouees pour former des ceufs est essentielle a une meilleure comprehension des strategies de reproduction des oiseaux. Pourtant, l'incidence de la disponibilite de micronutriments sur la synthese des ceufs chez les oiseaux sauvages demeure meconnue. Nous avons examine le role possible du calcium (Ca) dans la synthese des ceufs chez le goeland d'Audouin (Ichthyaetus audouinii (Payraudeau, 1826)). Nous avons plus particulierement evalue le lien entre les concentrations plasmiques en Ca (mg/dL) de femelles couveuses (en tant qu'indicateur de la reponse physiologique de ces femelles a la demande accrue en Ca associee a la production de la couvee) et plusieurs caracteres des ceufs associes a l'aptitude tels que leur taille (c.-a-d. le volume des ceufs), leur forme et l'epaisseur de leur coquille, pour trois couvees. La taille des cufs presentait une correlation positive avec les concentrations en Ca du plasma des femelles couveuses, la pente de cette relation etant significativement plus grande pour les ceufs pondus plus tardivement. Etant donne le caractere empirique de l'etude et sa chronologie inversee, l'etablissement de liens causaux n'est pas possible, mais les relations observees appuient l'hypothese selon laquelle le Ca controle en partie la synthese des ceufs et semblent indiquer qu'il pourrait egalement jouer un role dans la modulation de la variation de la taille des ceufs d'une meme couvee, un aspect typique de cette espece de goeland. [Traduit par la Redaction] Mots-cles: goeland d'Audouin, reproduction des oiseaux, calcium, production d'ceufs, taille des ceufs, coquille d'ceuf, Ichthyaetus audouinii., Introduction Avian females influence offspring quality through genetic contribution, but also through allocation of resources to their eggs, including antibodies (Hammouda et al. 2012), hormones (Rubolini et al. 2006), or [...]

We analyzed offspring sex ratio variation in Mediterranean Cory's Shearwater (Calonectris d. diomedea) during two consecutive breeding seasons in two colonies. We test for differential breeding conditions between years and colonies looking at several breeding parameters and parental condition. We then explored the relationship between offspring sex ratio and parental condition and breeding parameters. This species is sexually dimorphic with males larger and heavier than females: consequently we expected differential parental cost in rearing sexes, or a greater sensitivity of male chicks to adverse conditions, which may lead to biased sex ratios. Chicks were sexed molecularly by the amplification of the CHD genes. Offspring sex ratio did not differ from parity, either at hatching or fledging, regardless of the colony or year. However, parental body condition and breeding parameters such as egg size and breeding success were different between years and colonies. Nevertheless, neither nestling mortality nor body condition at fledging varied between years or colonies, suggesting that male and female chicks were probably not differentially affected by variability in breeding conditions. Key words: body condition, Calonectris diomedea, Cory's Shearwater, egg volume, sex ratio.