There are seemingly thousands of birds in existence at any given time in my general geographical area (Puget Sound, WA), yet I’ve never seen a bird dead or dying from natural causes (aside from “feline assistance”). Do birds seek isolated areas to pass on? Given the high numbers of birds in existence, why have I never seen the carcass of an elderly, naturally-expired bird?
My guess is they go to a quiet place and bravely await their fate. I used to have pet cockatiels. Somehow, when they knew they were sick, etc. and it was their time to go, they went to a corner of the cage and just placed their face in the corner. They died standing up with their eyes open.
You could check this and see if your questions are covered.
They go to Birdie Heaven.
I once saw a bird fall from a power line, dead as a nail.
You’ve never seen a dead bird?
I see them all the time (“I see dead birds”). In fact, SO and I were weeding the garden the other day and found one.
I often see them beside roads, under trees, on nature strips everywhere. Dead birds are so passe.
Of course, all these dead birds I see could be related to the paper mill and power plants around here, I guess. 
Don’t you know? Birds are immortal. When they ‘die’, they become the statues of important people in various cities, and one day, they will all be unleashed upon humanity, enslaving us under a new avian order! All hail the pigeons! We mammals will be their servants!
I’m okay. Geez, it’s pretty hard to type with my nose…