The little blue penguins who were cleaned up after being coated in oil by the 2011 Rena oil spill are behaving the same as unaffected birds, according to a new study.
Nearly 400 little blue penguins were captured, cleaned, cared for and released back into a cleaned environment after the spill.
The spill leaked about 350,000 litres of oil into the Bay of Plenty when the container ship ran aground and broke up on the Astrolabe Reef in 2011.
Scientists from the Massey University wildbase oil response team used tracking devices to compare the diving and foraging patterns of eight rehabilitated birds with six unaffected birds.
They also analysed the carbon and nitrogen levels in the birds' feathers which showed they were feeding on similar prey.
The study is the first in the world to compare the diving and foraging patterns of birds after a spill, and lead researcher Louise Chilvers said the results were encouraging.
"They're still performing, you know reproducing and surviving, at the rate of any other little blue throughout the country.
"So what it actually means is it appears that the environment's gone back to normal as well, because they're not working unbelievably hard, they're actually working quite easily to obviously get the right amount of food to survive and reproduce."
Dr Chilvers said the result justified the cost of oil response teams worldwide because it showed that rehabilitation and intervention was effective.
But a trust tasked to protect them said dogs and people were the real threat.
Mauao Area Wildlife Trust chair Julia Graham said the people who helped save the penguin from the oil spill were actually the ones killing it.
"We're dealing with lots of alcohol-related issues. People go out at night, they drink, they sit in front of burrows, they put their rubbish up holes and don't realise they're blocking burrows.
"They're breaking bottles and glass and then you've got people taking their dogs out fishing at night."
The trust wanted to work together with the public to make people more aware of their impact on the penguin, she said.
But something needed to happen, because already this season the nesting penguins on Moturiki island had lost 40 percent of their chicks.
"All the babies on the front half of Moturiki island are dead, thanks to people that park up there all night so their parents can't get to feed their babies.
"So if this happens two or three nights in a row, the weight of the babies gets so low that they end up starving to death and these adults aren't actually able to come up and feed their babies because there is just so many people around."
She said the research from Massey University was reassuring, and meant there were more birds to breed and boost the population.