Zealandia could be home to New Zealand's first transgender korimako, dubbed the ''butch bellbird''.

Staff at Karori's Zealandia eco-sanctuary use the nickname because the young bellbird has DNA tested as female but acts like a male, and has a mix of each sex's plumage.

Victoria University moult expert Ben Bell was intrigued to hear about the bird's plumage.

"It could be due to a hormonal imbalance or it could be a reaction to shock or an incomplete moult - given the appearance and behaviour, any of those would be unusual though."

It is the first species Zealandia staff have seen showing the unusual gender mix.

About the size of a sparrow, bellbirds are dark olive-green with red eyes.

Both sexes look similar except the male is tinted blue around its beak and eyes, while the female has a white stripe extending back from the bill.

Zealandia conservation officer Erin Jeneway was the first to notice the ''butch bellbird.''

"There's something we can't pin down. We haven't seen anything like this before," she said.

Ms Jeneway refers to the bird as "her" while coworker Matu Booth uses "him" - both hope to find out who's right if the bird survives and breeds.

When the bird was a chick, a feather sample was sent to Massey University for genetic testing, which showed the bird was female.

Now 18 months old, it has the white cheek stripe of a female on one side, but the dark body plumage of a male.

When feeding, the honey eater doesn't flit between flowers like a female, but moves more deliberately, primed to defend attractive food resources.

The bird's calls have also been mixed - it makes both male calls and the lively "chup chup" of the female, but these are much louder and more frequent than is usual for females.

However, the mixed song is less unusual than the plumage and behaviour.

A key note in New Zealand's dawn chorus, bellbirds are known for their sonorous chiming call, which is like a tui but more tonally pure.

The ''butch bellbird'' had been seen frequently around Zealandia's northwest scarp but often forages in unprotected habitat beyond the fence so is at risk from mammalian pests.

Contact Matt Stewart

Weather, science and environment reporter

Email: matt.stewart@dompost.co.nz

Twitter: @smatape