This made me think (birds coming in from the countryside and the diseases they may bring)
We've just been through quite a hard winter and i did hear some bird populations have taken quite a hammering (although when i read further, its worldwide populations)...but even so, a hard winter and in the same article (the independant i think) it also stated nature hates a vacuum...surviving birds will often have two of three clutches in the next season, possibly because lower populations mean less competition for food, therefor more possibility for breeding. It all makes semse really.
And on the disease front...Magpies, Wood Pidgeon, Jackdaws, Jays, all traditionally countryside birds that are seen regularly in urban areas now....are they having an effect on traditional urban/town populations ? perhaps...