Are House Wren Troglodytes aedon eggs unusually strong? Test of the predicted effect of intraspecific egg destruction

Abstract

As a result of opposing selective forces, the external strength of avian eggs should be near some size‐specific optimum. However, in certain situations there should be selection on females to lay unusually strong eggs. According to one hypothesis, intraspecific egg destruction should favour increased egg strength as a means of defence against conspecific intruders. This hypothesis predicts that House Wrens Troglodytes aedon, a species well known for its tendency to destroy conspecific clutches, should be under selection for unusually strong eggs. However, the intensity of selection for strong eggs should also be modified by efficacy of nest defence against conspecific intruders in a given species (i.e. efficient nest defence by the breeding pair should weaken selection for unusually strong eggs). The goals of our study were: (1) to establish whether House Wren eggs are stronger than expected for their size; (2) to determine which structural mechanisms are responsible for their unusual strength; and (3) to test a hypothesis that, between wren species, the efficacy of nest defence and the intensity of egg‐destroying behaviour affect the intensity of selection for unusually strong eggs. Our results demonstrated that: (1) House Wren eggs are 1.9 times stronger than expected for their size; (2) their unusual strength is achieved mostly by their unusually thick shells; and (3) eggs of the House Wren (extensive paternal nest defence; male egg‐destroying behaviour suppressed during incubation) are significantly weaker structurally than eggs of the Marsh Wren Cistothorus palustris (reduced paternal nest defence; male egg‐destroying behaviour present throughout incubation). These results are consistent with the hypothesis that the intraspecific egg‐destroying behaviour and the efficacy of nest defence by the breeding adults have played a role in the evolution of strength of House Wren eggs.