Abstract
Aechmophorus grebes are socially monogamous and copulate only on floating nests. Motion-activated cameras were used to study the copulation behavior of Aechmophorus grebes at Clear Lake, Lake County, California, USA, during 2015–2016. The sex of both partners was determined in 226 of 435 copulations; of these, 8.0% were female mounting male (reverse mountings), including 6.4% by Western Grebe (A. occidentalis; n = 171), 0% by Clark's Grebe (A. clarkii; n = 31), and 42.9% by mixed pairs (n = 14). One copulation comprised two Western Grebes mounting a third individual on a nest; the sexes could not be determined, but most likely consisted of two males mounting a female. Another copulation comprised a male-male mounting by Western Grebes. These records provide the first documentation of simultaneous extra-pair copulation and male-male mounting in the Western Grebe.
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