The incidence of twinning in avian species is a phenomenon that has been rarely encountered. A number of domestic species have been shown to produce twins but in very low numbers. In wild populations, only 14 species have been documented producing twin embryos or nestlings. Despite this, it has been postulated that birds are just as likely as any other vertebrate to produce twin offspring. Here we describe the discovery of dizygotic twins in a long-term study of breeding ecology in the White-throated Sparrow (Zonotrichia albicollis). The twin containing egg was 12% heavier than the mean and had a mass greater than 94% of eggs collected. The twin containing egg was wider and longer than the majority of other eggs collected during 2010 but still within the expected range for White-throated Sparrows. Genetic analysis demonstrated that the twin embryos were full siblings but of different morph and sex. This is the first documented case of twinning in our study site out of over 2000 samples over 25 years of study, and likely the first confirmed case of twinning in this species.

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