After flying into a window, young kereru Bert couldn't eat unaided for a month.

Uninjured but extremely skinny, the juvenile bird was brought in to the safe hands of Animal Rescue and Rehabilitation Centre Wildlife Trust on January 5 in a very poor condition.

Baby Bert.

Poor, meaning it weighed a tiny 220g. Bert should be nearing the average adult weight of 650g, says ARRC co-ordinator Larissa Ansorge.

It's not known whether the juvenile is a boy or a girl, and Larissa says they look the same.

'We don't usually name them, but when I first took him home to our aviary and I told my brother we have a new bird and he thought I said Bert – so that kind of just stuck,” laughs Larissa.

To fatten Bert up, they fed the bird a range of fruit and vegetables. 'Banana, fig, cooked pumpkin, sweetcorn, blueberries – and some native tree berries that we had ripe at home since that's what they would eat in the wild,” says Larissa.

'It took us a month of crop feeding at first, and then hand feeding – now he's progressed to being able to eat enough food by himself to put on weight.”

Bert now weighs about 450g and should put on another 50g to 100g before release on Waitangi Day.

Larissa advises people who find a bird that's flown into a window are advised to put it in a box or somewhere quiet for a couple of hours, as it may be stunned and can be released.

'But if the bird won't fly away after having some time to recover, then they can bring it into us [ARRC] for further care,” she adds.