Hihi are so rare and endangered that even the London Zoological Society has stumped up money to survey their progress at Whanganui's Bushy Park.

Ecologist Kate Richardson, who works for Wildland Consultants, was in the midst of a four-day survey during Conservation Week.

She has a PhD on the elusive little birds, and her job was to find as many as possible within the 100ha forest reserve.

The birds need to be surveyed before their breeding season begins. Richardson will do her best to find out how many there are.

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She'll catch and identify those that were raised in nest boxes and have been banded.

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.

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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.