Filming the wildlife in our skies stretches the skills of any wildlife cameraman. Birds move in three dimensions and constantly change direction so keeping them in the frame and in focus is challenging. In the third episode of this illuminating series, Scots-based cameramen John Aitchison and close friend Mark Smith reveal some of the more unlikely corners of the world where they have filmed birdlife, from a rubbish dump in Dehli to a motorway flyover near Birmingham. The results are mesmerising.