The effect of intraspecific egg destruction on the strength of Marsh Wren eggs

Marsh Wrens (Cistothorus palustris) regularly peck and break eggs of conspecifics, presumably to drive conspecifics away and thereby reduce the potential for intraspecific competition. We hypothesized that egg-pecking behavior represents a strong selective force that should favor adaptations, such as structurally stronger eggs, that lower the effect of conspecific attacks on Marsh Wren reproductive success. We tested this hypothesis by investigating whether Marsh Wren eggs are structurally stronger than would be expected for their size. We compared the strength of Marsh Wren eggs with that of eggs of 10 other passerine species and found that Marsh Wren eggs tolerated 2.9 times greater pressure than would be expected for their size. To identify the structural mechanisms responsible for greater strength of Marsh Wren eggs, we conducted two analyses. First, we related variation in the strength of Marsh Wren eggs to variation in their volume, shape, and eggshell thickness. Eggshell thickness was the only significant predictor of the strength, explaining 30% of the variation. Second, we selected 101 passerine species that lay eggs of similar size and compared eggshell thickness and shape of their eggs with those of Marsh Wren eggs. Marsh Wren eggs had significantly thicker eggshells and rounder eggs than eggs of the other species, indicating that both characteristics contributed to their unusual strength. Our results are consistent with the hypothesis that, in Marsh Wrens, conspecific egg destruction has led to the evolution of unusually strong eggs.