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Destruction of eggs in nests of a bird’s own species has been reported in many species of birds, including three species of grebes of the family Podicipedidae. We report for the first time four incidents of egg destruction by Western Grebes (Aechmophorus occidentalis) and three by Clark’s Grebes (Aechmophorus clarkii) in mixed breeding colonies at Clear Lake, Lake County, California, from 2014 to 2017. All incidents occurred during the late stage of nesting within a colony. Five incidents of egg destruction occurred at three recently vacated nests in which the previous eggs had been removed by mammalian predators <24 hours earlier; the other two incidents occurred at nests with an unknown history. Egg destruction was perpetrated only by males. The eggs may have been destroyed to usurp nests or to prevent brood parasitism or cuckoldry. Because none of the eggs or adults were marked and no tissue samples were taken, it was impossible to be certain of the relationships among adults and eggs, and whether egg destruction was intraspecific or interspecific. Many species of birds, across a broad spectrum of taxonomic groups, destroy eggs in other birds’ nests or even in their own nests. Some birds routinely destroy eggs of their own species, which is a form of infanticide (Hrdy 1979, Veiga 2000). Others, such as several wrens of the family Troglodytidae (Brewer 2010), routinely destroy the eggs of other species. The circumstances under which egg destruction occurs and its proximate causes are complex, and numerous hypotheses for it have been proposed. These may be classified into broad categories such as consumption, resource competition, and sexual selection (e.g., Hrdy 1979, Spooner et al. 1996, Veiga 2000). Intraspecific egg destruction has been reported in at least three species of grebes (family Podicipedidae). The eggs of the Great Crested Grebe (Podiceps cristatus) are sometimes kicked unintentionally out of a nest (Simmons 1955), but Konter (2008a,b) reported three instances of two eggs intentionally kicked out of a nest and an egg pushed out of a nest during a fight with a conspecific. McAllister (1958) reported finding many Eared Grebe (P. nigricollis) eggs removed from nests and destroyed by a small hole pecked into each egg, which she attributed to conspecifics, but over two years of observation never saw a grebe pecking an egg. Perkins et al. (2005) reported several instances of eggs deliberately removed from nests by incubating Horned Grebes (P. auritus), and Summers et al. (2009) reported a nest destroyed by a conspecific. In this article we provide details representing the first reports of intraspecific and interspecific egg destruction in the Western Grebe (Aechmophorus

Egg Destruction by Males in the Western Grebe and Clark’s Grebe