Tophill Low Nature Reserve has lost five of its six breeding Barn Owls
Local nature reserves have reported dozens of birds being found dead.
At Tophill Low Nature Reserve in East Yorkshire five of the six resident breeding Barn Owls have died.
Wardens have received reports of over 40 Barn Owl bodies being found on local farms.
The impact on other bird breeds is even more severe.
Blacktoft Sands near Goole is home to approximately 300 Bearded Tits. Wardens estimate only 50 survive.
The RSPB reserve is an important national centre for the species. The losses mean that about 7-8% of the UK's Bearded Tit population has died this winter.
Pete Short is a warden at Blacktoft Sands. He said he is devastated by the losses:
"It's absolutely terrible. I've spent the last 12 years of my life trying to get the population of birds up here on site, and we've just lost them almost overnight really."
Experts say that birds can survive the sub-zero temperatures, but the lack of food is causing the deaths.
Barn Owls find it difficult to hunt in the freezing conditions. Other food sources, such as seeds and berries, are frozen and inedible.
The region is an important centre for the Bearded Tit
Richard Hampshire is a warden at Tophill Low. He said the reserve is taking special measures to try and ensure the Barn Owls survive:
"What we're doing at the moment is we're feeding the owls shrews, mice and the like to try and just keep them going through the really bad spell. It's not something that we do regularly, but in order to keep a presence of barn owls on the reserve we are helping them out in this really bad spell."
The fear is that the populations could take years to recover.
In previous harsh winters, such as 1947 and 1962, more than half the barn owl population was wiped out.