[stormer]

Climate related phenotypic plasticity is more accurately reflected in changes to the ratio between body volume and surface area - in cold climates reducing the surface area relative to body volume is a method of conserving energy.

[concerned about politics]

in cold climates reducing the surface area relative to body volume is a method of conserving energy.

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!