The virility of the hihi - or stitchbird - is attracting interest from conservationists and gamblers alike.
An international competition betting on which of 128 male birds has the fastest sperm has raised more than $5300 - money to go towards keeping the species alive.
Punters have been making $10 bets on The Great Hihi Sperm Race - www.hihispermrace.nz - which runs until April 22.
The winner will be the bird that has the fastest sperm, as judged by the University of Otago's Dr Helen Taylor, who studies such things with the help of a mobile sperm lab, a microscope and a video camera.
The competing birds are from four of the seven remaining populations, one at Whanganui's Bushy Park Sanctuary where the species has been re-introduced.