When the winter is going to be bad, we get more fruits on our trees and bushes. The animals fatten up for the winter that way. Nature takes care of them.
If the birds are getting bigger, they need the increase in size for something. It's not just body weight, it's also length. Why would birds need to get longer?
So many questions!
Generally when you see a change like that in a relatively short period of time is would indicate some sort of selection pressure, e.g. longer, heavier birds are able to more efficiently access resources which increases their reproductive success, and the longer, heavier trait is passed along to their offspring.