How costly is clutch formation in the Audouin's Gull Larus audouinii?

During the Audouin's Gull's breeding season at the Ebro Delta in 1993, 24 f resh eggs from eight three-egg clutches (modal clutch-size) were collected at the peak of the laying period. Eggs were processed to obtain formalin-fi xed yolks, which were halved and stained using the potassium dichromate met hod. Digitized images of the yolks were examined to assess the daily rates of yolk deposition. We used these data in combination with egg compositiona l analysis to build a model of energy demands during the formation of an av erage clutch in Audouin's Gull. To show how the different parameters of clu tch formation affect the daily energy investment peak, we performed a simul ation analysis in which the rapid yolk development (RYD) period, the follic le triggering interval (FTI), the laying interval (LI) and the albumen synt hesis period (ASP) were allowed to vary simultaneously. In our sample, the mean RYD period was seven days with a range from six to eight days. There w ere no significant differences in yolk volume among eggs in a clutch, but a lbumen volume was significantly smaller in third eggs. According to our mod el the albumen synthesis of the a-egg coincides with the energy demand peak for clutch formation. This peak represents an increase by ca. 42% in femal e energy requirements. Values obtained from the simulation analysis showed that only the ASP of the a-egg and the RYD durations of the second and thir d follicles produced noticeable reductions in peak energy investment. We pr edict that in gulls, whose laying intervals seem to be kept constant, signi ficant increases of the durations of the RYD periods of second and third eg gs, or even significant reductions of yolk size of these eggs, may operate simultaneously to match the energy demands during clutch formation to the p revailing food conditions.