The bittern is one of the UK's rarest birds
Staff at Ham Wall and Shapwick nature reserves are worried about bitterns not being able to feed because of the freezing conditions.
The rare birds are struggling to hunt for fish in the reed beds and pools because many of them have frozen over.
Staff are now scattering sprats along the reed bed edges to supplement their diet until the weather improves.
Although no bitterns have yet been spotted eating sprats, wardens say this is helping other birds survive.
Bitterns are one of the UK's rarest birds, and are part of the heron family.
Tony Whitehead, press officer for the RSPB (Royal Society for the Protection of Birds) said: "It's our concern that these birds are suffering because of the frozen conditions we have.
"The water is freezing over and these birds largely eat and rely on fish so they're going to have a lot of difficulty hunting in this weather."
At the moment Ham Wall has got five bitterns and in the breeding season the reserve last year had seven pairs of breeding bitterns which was regarded as a big success by the wildlife charity.
"We're putting sprats on the edges of the reed beds, places where the bitterns would normally breed in the hope the bitterns will come and take advantage.
"Although we haven't seen any bitterns feeding directly on the sprats yet, [we've seen] kingfishers, the water rail, and even grey herons taking advantage.
"We've got a lot of birdwatchers out on the site so we get lots of information from them, so if we see a bittern flying around with a sprat in its mouth we'll know it's from the food we've been putting out."