It is suggested that it is within the pheasant's natural extinct (presumably from an evolution on the Indian sub-continent) to attack and eat snakes. Certainly the amount of pheasants released each year
Yes, pheasant + climate colder than in India may be too much for snakes. But snakes decline worldwide.
Adders are ectotermic animals. This is monothematic but maybe for example there is inappropriate temperature or high moisture increasing thermal conductivity of the atmosphere. Including hibernation period. However so far only this winter is strangely warm and only in Europe.
Or maybe something is poisoning their prey. Like "Second Generation Anti-coagulant Rodenticides".
Or they don't have place to live because of habitat transformation by people.
Or an alien species eats their eggs.
In the area where I live snakes and lizards are usually very timid. And they have primitive brains, perhaps can't accustom to human intruders. They are probably almost like machines with genetically determined behavior, almost unable to learn. So perhaps fear can disable their normal behavior, because they flee instantly and are not able to understand anything, adapt to new situation.
Or maybe this is the same reason that caused unexplained extinction of dinosaurs, what would suggest, that something is going wrong very seriously !!
Theoretically many factors could be the cause of snakes decline, this needs research in the field.
Buzzard have always coexisted with snakes. This is about
Homo sapiens.