Nuthatch waiting for its young.

This bird has a ring on its leg which shows that it has been rung by a licensed bird-ringer. They do this for scientific reasons; all the data they collect is sent to the British Ornithological Union for analysis. This then gives data on breeding, migration and a whole load of other information..all useful to conservation.
This particular bird had some fledglings in a nearby tree and was calling them down for some food that it had collected - hence its 'expression'.