Polygynandry,face-to-face copulation and sperm competition in the Hihi Notiomystis cincta (Aves: Meliphagidae)

The Hihi or Stitchbird Notiomystis cincta breeding system is highly variable and includes monogamy, polyandry, polygyny and polygynandry. Males have large testes (4.2% of body mass), very large numbers (1460 × 10⁶) of sperm in their seminal glomera and an unusually enlarged cloacal protuberance. These features are also found in other species with highly variable mating systems where males are under intense sperm competition. Hihi copulate in two different positions: face to face and, more conventionally, with the male on the female's back. Face-to-face copulation is unique among birds and appears to be a form of forced copulation. The presence of enlarged cloacas in both sexes could aid the transfer of sperm. Both male and female Hihi appear to benefit from a mixed reproductive strategy where a female Hihi can solicit copulations from males other than her partner and male Hihi can perform extra-pair copulations both with willing females or by forced copulation.