Swiftly Gone ....

Three years ago we were regularly seeing 5 or 6 House Martins and a smilar number of Swifts and often on a warm evening a group of 20 diving around over our garden. I have hardly seen any this year, in fact just one loan Swift battling with force 7 winds today. What is happening? Are their Summer nesting sites damaged, disrupted or destroyed? Are they lost to predators or bad weather on route or are their Winter destinations the problem. Who knows? Please let us know, we are missing part of our Summer. It's really sad. :-(