Baby birds are starting to hatch at Wellington's Zealandia wildlife sanctuary.

While a cold start to spring has delayed the breeding season for some birds, others, like kaka, already have a few chicks to look after.

Lead ranger conversation Matu Booth said while every species differ, there's been a bit of a delay in breeding this spring, but he pointed out that's not unusual, as every breeding season's different.

"If it's an intensely horrible spring then obviously the breeding season will be cut short ... sometimes we may have a good season, other times we may have a bad season - we always cycle through."

Booth said they won't know how successful the season's been until all the baby birds have grown up and left the nest. Up until then, there's different ways attrition happens.

"Where we give them the tick is actually if a bird manages to fledge from the nest and leave."