Others, raised in natural forest nests, will need to be banded. The aim is to band every one of the birds.
Last year's breeding season had periods of drought, which was tough on them.
There are only a few thousand hihi in the world, and all are at seven New Zealand sites.
They are rarer than brown kiwi, and lots of information on them is collected and shared every year.
Richardson's survey is funded by Forest and Bird and the Zoological Society of London.
The London connection happened because Hihi Conservation Charitable Trust co-chair John Ewen is also a member of the London group.
Massey University conservation biologist Doug Armstrong and his students also survey the birds, and Whanganui scientist Peter Frost has monitored them since they were introduced in 2013.
Bushy Park manager Mandy Brooke is grateful to all of them, and also to Allan Anderson who donated new feeders, to the sanctuary's many volunteers and to Pak'n Save supermarket, which supplies sugar for the birds' sugarwater.