The swans are thought to be in Estonia at the moment
Hundreds of swans that usually come to Gloucestershire to escape the cold winter in Russia have so far failed to turn up.
Scientists at the Wildfowl and Wetlands Trust (WWT) in Slimbridge say the Bewick swans are at least a week late.
Research officer Julia Newth said the very mild autumn had affected the migration patterns.
"They need a good north easterly wind behind them - a nice cold snap to bring them to the UK shores," she added.
The swans are thought to be in Estonia at the moment, with a few having reached the Netherlands.