The clock stands outside the aviary in London Zoo. Every half hour the clock 'strikes' and it comes to life putting on quite show as little metal birds appear and disappear, some flying in circles around the clock. When I arrived at the clock there was already a crowd of people waiting for the 'show'. All the movement in the clock is controlled by pneumatics. The clock was built and designed by Tim Hunkin.
The photo is a bit dark but note the green bird (with cable attached) to the left in the photo.