Human food, sometimes fed to them by people, could also make them sick.
Lead poisoning was another issue for kea, as they often licked and chewed the nails and flashing on older houses and huts.
Changes to the climate were also expected to affect kea numbers as the warming temperatures could limit their habitat range.
The Bird of the Year awards were announced after a two-week campaign that attracted more than 50,000 votes.
The kea's campaign was boosted by a team of researchers and kea enthusiasts and the Conservation Trust.
Team Kea co-campaigner Laura Young said the team asked everyone they knew to vote kea.
"We lobbied hard to get votes up on the first day, which I think made a big difference."
The team went to great heights - literally - to win.
"One day we climbed to top of Mt Patriarch to get reception and check on the campaign.
"We saw that the Green Party had made an official announcement in support of the kereru so we used what little phone battery we had left to hit back at them with a retaliation video."
Team Kea hoped the Bird of the Year title would raise awareness for kea and all of New Zealand birds facing extinction.
"Everyone needs to see how vulnerable kea are in the wild. We often hear of them hanging out in carparks, being cheeky and stealing things, but don't realise they are in decline. You can't not love them."
Bird of the Year was one of Forest and Bird's most popular annual events. It aimed to raise awareness for New Zealand's unique native birds and the threats they faced by asking people to vote for their favourite species.
This year was the competition's most popular yet. It attracted more than 50,000 votes, up from 20,000 last year. Bird of the Year also attracted worldwide coverage, being featured by BBC, the Guardian and Buzzfeed.
The competition also raised more than $10,000 to help Forest and Bird protect and restore New Zealand's wildlife and wild places.
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Video | NZ Herald