I agree with you Sam, I too have noticed the trends you mention around the Haverfordwest area. I do think willow warblers are in less numbers than previous years, swifts are about the same, blackcaps seem to be better numbers here. I also note house martins and swallows seriously down, only a handful of nests, and also sedge and reed warbler, in areas like the marsh at Fortunes Frolic, are next to none, I think I had 3 singing about a month ago.
And I have also noticed really good numbers of blackbirds, robins, especially song thrush, garden warbler and wrens this year. Could it have been the cold snap or just the easterlies that brought the weather?