one swallow doesn’t make…

In what experts say is the first documented evidence of the species “overwintering” here, a solitary swallow has been monitored from November to the end of February in a village near Truro, Cornwall.

Paul Stancliffe, a spokesman for the British Trust for Ornithology (BTO), called the discovery “incredible”. As further evidence of climate change, volunteers have also recorded “early returns” by many migrants this year, as well as unseasonably early nesting by birds that ordinarily remain here.
The Telegraph (UK), 16 Mar 2008