A "whole generation" of endangered bats could have have been wiped out by Jersey's heatwave, an ecologist warned.
Piers Sangan said two "maternal roosts" of grey long-eared bats were found dead after June's heatwave.
Based on their age and the timing, Mr Sangan said it was a "working theory" that extreme weather was to blame.
It's hoped the island's population will "recuperate" but future climate change could have greater impact, he added.
Mr Sangan added: "We could be looking at up to 100% of this year's babies being potentially lost. Basically a generation died from the heatwave."
The wildlife ecologist, who runs the Jersey Grey Long-eared Project, added: "While the adults were able to abandon the roost and move to somewhere slightly cooler, the juveniles could not make that transition and were lost.
"It is a significant mortality event for the species this year."
Mr Sangan, whose company Sangan Island Conservation carries out ecological surveys, said another cause, such as chemical poisoning, was unlikely given the distance between the roosts.
Had the heatwave been a week earlier or later, the bats may have survived, he said.
This is because they would either have been small enough for their mothers to fly them to safety, or old enough that they could have flown themselves,.
He said the long-term impact was unknown, but there would be adults missing "a number of years down the line".
Mr Sangan and his team run the Jersey Grey Long-eared Project alongside the business because they often encounter the bats in their commercial work.
The four-year project is aimed at finding as much information as possible on Jersey's population.
The grey long-eared bat, a type of "micro bat" that can fit in the palm of a hand, is one of the island's 18 species of bat.
Mr Sangan said it was "under-recorded" on the island but listed as "critically endangered" in the UK, where there are only a thousand left.
The species, which has adapted to co-exist among human settlements, is also in decline in Europe.
Because this type of bat is "very quiet", it has been missed off other survey projects which use acoustics to record numbers, Mr Sangan said.