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.