I'm talking about the ones that stay in north during winter, not migratory birds. I've noticed that birds like sparrows or tits (yes yes i know funny name) look more rotund in fall, but is that actually fatness or just them fluffing up?
While it may not put on much in the way of body fat, chickadees’ brains gain weight to remember where they hid all their food for winter. This may apply to tits as well since they’re related but I haven’t seen any studies for them specifically.
Just imagine how delighted birds and squirrels would be if introduced to writing and notebooks. We have huge brains yet I can't remember where I left my stuff 5 minutes ago.