This study focuses on possible sources of egg size variation in the northern lapwing Vanellus vanellus, a polygynous shorebirdwith more variable male than female reproductive success. From the hypothesis that female birds allocate more resources into male than female eggs to improve their sons’ abilities to compete for mates, male eggs were predicted to be larger than female eggs, and the proportion of sonswas predicted to increasewith mean egg volume of clutches. However, no size differences between male and female eggs were found. Corresponding with earlier reports in the species, egg size was positively correlated with female body condition, but no relationship between offspring sex ratios and intra-clutch mean egg volumes was detected. These results suggest that a higher variation in reproductive success among males than among females is not sufficient for sexually dimorphic eggs to evolve in birds. Moreover, previous studies have suggested that shorebird eggs are subject to stabilizing selection for equal size within clutches. In contrast, this study demonstrates a consistent variation in egg size, with a significant increase in egg breadth between first and second eggs, and decreases in both breadth and volume between the third and the final fourth egg. The volume of last-laid eggs was on average 2.3% smaller than the mean size of the preceding three eggs. Among the shorebirds studied so far, this consistent pattern of intra-clutch egg size variation is unique to lapwings.
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