Global Warming or Chance Happening?

Every year we put out food for the birds and enjoy watching coal tits, blue tits, bull finches, and woodpeckers. As the winter draws to an end other birds appear and have us reaching for our identification guides.
One spring arrival that is easy to identify is the blackbird. However, this year one individual seems to have decided to overwinter here, 500 kms north of its usual winter feeding grounds. So far it has survived which isn't surprising considering how mild our winters have become.
Last summer we saw Goldfinches here for the first time and we live 700 km north of their normal summer habitat.
Is this a sign of things to come?
The cold I wrote of yesterday seems to have relented a little but I'm mostly taking it easy. Hence this blip, taken from the kitchen window.