THE EFFECT OF INTRASPECIFIC EGG DESTRUCTION ON THE STRENGTH OF MARSH WREN EGGS

Marsh Wrens (Cistothorus palustris) regularly peck and break eggs of c onspecifics, 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 effec t of conspecific attacks on Marsh Wren reproductive success. We tested this hypothesis by investigating whether Marsh Wren eggs are structur ally stronger than would be expected for their size. We compared the s trength of Marsh Wren eggs with that of eggs of 10 other passerine spe cies and found that Marsh Wren eggs tolerated 2.9 times greater pressu re than would be expected for their size. To identify the structural m echanisms responsible for greater strength of Marsh Wren eggs, we cond ucted two analyses. First, we related variation in the strength of Mar sh Wren eggs to variation in their volume, shape, and eggshell thickne ss. Eggshell thickness was the only significant predictor of the stren gth, explaining 30% of the variation. Second, we selected 101 passerin e species that lay eggs of similar size and compared eggshell thicknes s and shape of their eggs with those of Marsh Wren eggs. Marsh Wren eg gs had significantly thicker eggshells and rounder eggs than eggs of t he 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 evol ution of unusually strong eggs.