A winter bee is a worker (female) bee that is a bridge from one generation of the queen's offspring to another. Her reward for seeing the queen through this challenging period is the potential to live from early winter to spring. The approach of winter creates so-called fat bees. Fat bees are winter bees - bees much better suited to get through the winter. Winter bees have:
Increased levels of a compound called vitellogenin which helps bees store food reserves in their body. This is less necessary in the summer, when they can freely move to and consume food.
Lower levels of juvenile hormones, since there is less brood to take care in the winter.
Enlarged food glands, to store more food.
Higher level of sugars and fats in their blood
The end result is indeed a fatter bee, but also one better able to tolerate and survive the cold weather