House martins

Looking at the postings over the last few days / weeks, there has to be more than a dozen House martins in the county. I`ve only seen 2 in the past month - now we`re into the first week in May! Swallows, Sandmartins, in good numbers, now Swifts over our house, are they usualy here by now or am I missing something? Is there some explanation besides global warming?