We know that birds migrate. But why do birds migrate?
They migrate for food.
I always assumed, it was for warmth: When winter comes, a bird that wants to be warmer will simply migrate to warmer weather.
But this confuses correlation with causation.
The cold weather causes the migration, but the relocation has nothing to do with warmth-seeking.
Birds just go to where the food is.
Formally: A does not cause B. A causes C. And B and C are correlated.
Birds migrate for food, not for warmth as such. That is why, when so many people all over the country started feeding stations, southern birds like the mockingbird easily extended their ranges north. Some of our local birds go south, like the female robin; other birds, like the coot, consider this south.
-Annie Dillard, Pilgrim At Tinker Creek