Blue Tits usually lay between 8-12 eggs, but occasionally as many as 16. One of the largest broods of any bird. So there are a lot of open beaks waiting to be filled.
As you can see, keeping all these youngsters fed takes quite a toll on the adults.
@gardencat I am not surprised, apparently as well as laying the eggs, she also builds the nest single handedly, and the male doesn't always bring her enough food while she is incubating them, so she has to leave them now and again to get her own food.
Then there is the little job of feeding them all. I think the male helps with that.
Then there is the little job of feeding them all. I think the male helps with that.