Patterns of within-clutch variation in yolk lutein in the Yellow-legged Gull Larus michahellis: the effects of egg laying order and laying date

In gull species, the level of carotenoids in egg yolks is known to decrease along the egg laying sequence. However, the extent of this laying order effect is expected to vary among clutches according to laying date, due to possible differences between early-nesting and late-nesting females with regards to their ability to control the transfer of maternal resources to the eggs. A laying date effect could also result from a seasonal decline in the availability of dietary carotenoids. In this study, we collected data from Yellow-legged Gulls (Larus michahellis) breeding in southern Tunisia to investigate how nesting date interacts with egg-laying order in shaping patterns of within-clutch variation in yolk lutein, the most abundant carotenoid in this species. In particular, we examined (1) whether yolk lutein concentration declined along the laying sequence and (2) whether the extent of the decline differed among clutches according to laying date. Our results show that yolk lutein decreased along the laying sequence at the clutch level. More interestingly, they also show that the decrease in yolk lutein levels along the laying sequence was more pronounced in late clutches than in early clutches, although lutein levels in adult plasma did not vary according to laying date. Overall, these results support the hypothesis that the Yellow-legged Gull employs the brood reduction strategy and that said strategy is more drastically applied in late clutches than in early clutches. The relevance of female age and seasonal variation in chick-rearing conditions for these findings is discussed.