Where have all the Swallows gone?

Last year, the first swallow on my patch came on the 8th of April, this year it came as late as the 12th; so where are they? Normally, by now the skies would be full of swooping hirundines, but the most i've seen at once was eight. Is it the cold weather, southerly winds or something compleately different? Any insights, I would be greatfull enlightened...