Available food is obviously key to the survival of otherwise migratory birds, but it is interesting to ponder on how and why they end up staying:
1. Why don't they go south with rest and get some sunshine and easy food?
2. If they get left behind for various reasons (winds, age etc.), can't they still head off?
3. Do they hang around for a bit and then maybe give up and go south?
4. Is there some consistent proportion of birds that stays and then only some survive?
5. Is it the propensity to stay genetic? Is there a hardier breed of robin that stays?
6. If they stay, then clearly they should have some advantage when spring comes, so is there a built-in driver against migration that is counter-balanced by the hazards of winter?
Migration seems a pretty mysterious subject even now. Any interesting books that might help put my fevered mind to rest?