It is a distinctive and critically endangered subspecies of the yellow-tufted honeyeater, that exists in the wild only as a tiny relict population in the Australian state of Victoria, in the Yellingbo Nature Conservation Reserve.
Schodde and Mason[3] affirm its subspecific status but suggest that there is intergradation across eastern Victoria and south-eastern New South Wales between it and the nominate subspecies L. m. melanops.
However genetic research, conducted on behalf of Victoria's helmeted honeyeater recovery team by Hayes,[4] does not support Schodde and Mason's subspecific arrangement, but confirms the distinctiveness of cassidix both as a taxon and the limits of its current geographic range to the Yellingbo area.
[6][7] Historically, helmeted honeyeaters were patchily distributed in the mid-Yarra and Western Port catchments of central southern Victoria, in the South Eastern Highlands IBRA bioregion.
[8] The wild population of the helmeted honeyeater is now restricted to a five km length of remnant 69 bushland along two streams in the Yellingbo Nature Conservation Reserve near Yellingbo, about 50 km east of central Melbourne, with a small colony of birds bred in captivity established near Tonimbuk in the Bunyip State Park within the historic range of the subspecies.
Key habitat elements include the presence of decorticating (peeling) bark, closely spaced eucalypt stems and dense undergrowth.
The nest is cup-shaped and placed in the outer branchlets of a tree or shrub; it is made of grass and bark, bound with cobwebs, decorated with spider egg-sacs, and lined with soft material.
Females may reside temporarily near nectar flows, or near other honeyeater neighbourhoods before returning to their natal colony and mating at the beginning of the next breeding season.
They spend much time gleaning lerps from foliage, invertebrates from behind decorticating bark, and making repeated visits to places where manna is weeping from damaged eucalypt and melaleuca branches.
Particular threats are habitat degradation through die-off and the lack of regeneration of the mountain swamp gum community, because of siltation and waterlogging, or by disease and weed invasion.