What is the difference between "by habit" and "out of habit"?

"By habit" means "by dint of habit", "through the force of habit", whereas "out of habit" means "because it is a habit".

• On the other hand , young chickens have lost , wholly by habit , that fear of the dog and cat which no doubt was originally instinctive in them ; (ref.)

• I no longer chose to use Google—I used it completely out of habit. (ref.)