Fun Fact: Bees will leave about 10 to 15% of brood comb in the nursery with no eggs. This is deliberate.
One bee chore is as a Heater Bee. These Heater Bees will make themselves hot by warming up their wing muscles. Then they will dunk their body into an empty cell where the babies are incubating.
Next she will rapidly telescope her body up and down for a period of time and then rest when she gets tired.
What she is doing is warming up the was of the cells close by which warms up the babies that are incubating so they don't get cold. The bees maintain a constant temperature in the brood (nursery) area.
Before we learned this, when we would see an empty cell where all the babies are we thought the queen missed laying an egg in that spot. But she didn't goof. She does it on purpose.
This fun fact is from The Buzz about Bees by Jurgen Tautz.