Hi Anthony and welcome to WAB,
Sorry to hear of the fatalities in your area of fledgeling greenfinches, I take my peanut feeders down about now to try and stop parent birds feeding said peanuts to the baby birds, it may not be anything to do with that, it's possibly like you say , the weather but probably not that , if you can bag one of the dead baby birds , and take it to your local wildlife trust, most have contact with a biological research centre who would examine for cause of death , take note of how many dead birds there is and whether they are adults or young,Its possible it could be a virous or something ,
Keep us posted ,
Duncan
