How do they know???

There's been a sudden change in the weather today here on the Heath: the temperature dropped mid-morning and it began to snow! Large flakes which, mercifully, didn't settle. However, as if by (cliched) magic, birds began to tumble out of the sky: Pied Wagtails, House Sparrows, all the regular Pigeons and Doves, Blackbirds and - most unexpectedly - a large flock (15+) of Greenfinches. These fed on the lawn, on the Sycamore buds and on the feeders for a good half an hour, often with Goldfinches. It seemed as if they were feeding up before the threatened period of wintry weather...