I live in N. Dorset and I too have noticed fewer Swallows this year.
We have had them nest every year in our car-port but the very hot summers have caused problems and this year I don’t think any survived from 2 hatchings.
They built their nest high in the roof where it got extremely hot and the chicks fell from the nest, several not surviving; some persevered after we put them back lower down in the roof where there was more ventilation but ultimately I think it’s the extreme heat that is causing the problems.
Maybe they are going further north to nest. We had House-Martins last year (2010) the chicks that I saved after their nest fell off the wall the year before (2009); that was very satisfying but they did not return this year. I feel sure the extreme heat is to blame and they are searching out cooler regions.
Thank you for sending these details. I think of climate change as being more a worldwide problem, I never think that local weather changes could have an impact on birds. But of course it can.