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.
There are big prizes, including family passes to three sanctuaries with hihi, cases of wine, T-shirts and native bird cards with images by Whanganui's Merilyn Payne.
Hihi are unique to New Zealand and were widespread in the North Island until the arrival of man. Now they are vulnerable. "They're cheeky little birds with a tonne of personality," Dr Taylor said. "But we'd like them to be a bit more loved by the public."
The money raised will go towards supplementary feeding and providing and maintaining nest boxes.