Originally Posted by jogresh
My gut feeling "on the ground" (having done a couple of contracts in woodlands in the hills of N Wales), is that they are declining/shrinking in range, particularly woods on the fringes of their ranges. One wood where I've had them previously - Coed Dinorwic, Llanberis - we surveyed 4 times between mid April and late June, and didn't get a single Wood Warbler. It would be tempting to assume that any increase in Argyll corresponds to the decline in SE Britain. I should say, I haven't done any statistical analysis on the data I've generated.
Dinorwic is surely core range, rather than fringes?
One thing I've noticed with Northumbs is that they disappeared earlier from sites with heavy people, and particularly dog-walker pressure; the remaining likely breeding site isn't in their original core area in the county, but is noticeably less infested with dog walkers.
But otherwise the clear trend is for the numbers to be doing best in Scotland, so climate warming may well be important too. Milder winters in particular may allow increased competition for food resources from resident species.