Booth said the birds were always looking for new food sources.
"But for a species to suddenly go from nectar and insect eating to apparently eating fungi, that's a bit of a strange one. One explanation may be that there were some insects on the fungi."
The hihi, or stitchbird, is one of New Zealand's rarest birds because of its carefree, friendly nature and propensity to nest in tree holes, making it an easy target for rats and other predators.
Zealandia conservation manager Raewyn Empson said hihi usually ate nectar, fruit and insects and consuming mushrooms was unheard of.
It could not be confirmed that the birds' condition had been caused by the fungi, nor what sort of mushrooms they were.
"We've never heard of hihi eating mushrooms before, but that's not to say that they don't," she said.
"We do know that tui get drunk on the nectar of flax flowers and have been affected by rhododendron flowers, so it's not unusual for animals to have effects from eating something."
There was no way to know whether animals deliberately consumed mind-altering substances.
"We can't get into their heads so we don't really know what's going on," she said.
The hihi was wiped out from New Zealand's mainland by 1885 and at one time the only surviving population was located on Little Barrier Island.
After extensive conservation efforts, the birds have been reintroduced to the Karori reserve, Tiritiri Matangi and Kapiti Island.
Even if their penchant for hallucinogenic substances cannot be proved, hihi are undoubtedly quirky characters famous for being the only bird known to sometimes mate face to face.
Their Maori name translates to "rays of sun", the story being that the demigod Maui threw the bird into a fire after it refused to fetch him water, resulting in the male's yellow breast plumage.
"They're a delightful little bird," Booth said. "They are special on lots of levels - they're special because they're endangered but also because they're quite unusual."
