Ecologists fear the widespread loss of native mistletoe due to drought could leave nectar-feeding birds even more vulnerable.
Charles Sturt University ecology professor David Watson said birds relied on mistletoe nectar, fruit and foliage during drought.
But researchers recently discovered a concerning decline in the shrub while monitoring nectar-feeding birds at 2,000 sites across south-eastern Australia.
"(We) found that during the height of the drought, when it was not just really dry, but critically also quite warm at night, almost all mistletoes died," Professor Watson said.
Professor Watson was a co-author of a report on the decline that was led by Australian National University's Difficult Birds Research Group member Ross Crates.
Birdlife Australia national public affairs manager Sean Dooley said the study's discoveries didn't come as a surprise.
"This research is confirming our worst fears," Mr Dooley said.
He said the critically-endangered regent honeyeater was among the most vulnerable nectar-feeding birds.