See http://www.telegraph.co.uk/news/earth/earthnews/3336271/Climate-change-confuses-migrating-birds.html . This details BTO observations of a single swallow in a Cornish village between December and the following February. As our climate warms more swallows may decide to give the 12000 mile round trip south a miss. It is a chancy thing, as a cold snap could see off all the insect food.
See also http://www.rspb.org.uk/discoverandenjoynature/discoverandlearn/birdguide/name/s/swallow/ .
From a Yahoo group in 2009:
Records of birds hanging on (over winter) include single Swallows at Penzance and Marazion in Cornwall. The latter site was home to four birds until recently, with an earlier cold spell killing three, and the last bird is surviving on insects in a 20m by 4m patch of seaweed washed up against the sea wall. This is remarkably similar to the earlier winter occurrence of four Swallows at an oil refinery in Pembrokeshire, with a single bird surviving until at least 6 January.