It is, or was, a study which began with no agenda, but like every weather event which suits, has been hijacked to push the climate change agenda.Check out the Wytham wood study, It is run by Oxford University and has been going so long it has involved an awful lot of people, so agendas shouldn't have affected it too much. It is a SSI, very rural and the birds tend to stay local, so not affected by feeding. Well north, I noticed there was a study of their arrival and increase in Finland.
Regarding saving seed, someone did a study of immigrants who had brought veg. seed with them and grown on allotments over here. They had been saving seed and replanting for over ten years, and the researchers compared their plants to seed specially produced for the UK market by seed companies in recent years. The allotment guys far outperformed, it seems that a period of human selection has very definite results. Presumably you save seed from plants that grow well, after a few years you should have plants particularly suited to your local environment.
As to the germination, my guess is that you are more careful about selection and drying. If a commercial operation is looking to harvest and dry 100lbs of beans (say) they are not going to be too fussy so long as they hit a minimum germination rate.
I'm not saying climate doesn't change, on the contrary, it's the norm, but, like I said previously, there are too many variables.
What else has happened since 1947?
2 things which immediately spring to mind are Dutch Elm Disease & Ash Dieback.
So what would the effect be if this had caused a thinning of tree density, allowing more sunlight through, would it be to warm lower layers of the forest, where birds breed?
Rotting wood is exothermic & harbours more insect life than healthy wood.
How much of an effect would an increased food source, available earlier in the year, have?
Also, there is the involvement of David Attenborough & he has been shown to be a climate fraud on a number of occasions, & I would touch anything he had with a bargepole.