Fast little birdies

My parents have a birdhouse that is currently occupied.
The father and mother bird fly on and off to find big green worms and caterpillars to keep their screeming offspring happy. They must be exhausted by now.

They were very fast and they knew I tried to capture them with my camera. (not so much luck there as you can see).
Everytime I had my camera switched on they took little detours so I wouldn't notice them coming. When it was switched off they flew right to the entrance.

I wonder how they knew...