Abstract
The morphology of the reproductive organs of both male and female Reed Buntings (Emberiza schoeniclus) are described in relation to the high level of sperm competition exhibited by the species as a result of extra-pair copulation behavior. In males, the volume of the cloacal protuberance in relation to body weight was 26.7 mm 3 g -1 , which is relatively large for a socially monogamous passerine. The weight of the testes was 23% greater than that expected for a bird of comparable size, and the spermatozoa were among the longest recorded for any avian species (mean length = 291 μm). The morphology of the female tract also showed adaptations associated with a high level of sperm competition: the sperm storage tubules were extremely long (mean length = 865 μm).
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