Human presence triggers a motion sensor which starts the song of the Skylark and the physical decent of the iron replica suspended, mid air, by a magnet until the song fades giving way to the rev of a tractor.
The graph on the rusty back board shows the decline in Skylark numbers in the UK since 1967.
Wild life is in drastic decline and heading toward extinction. In the case of the Skylark it is down to habitat loss due to more intensive farming practises in the UK.
You can see this working : http://richardpaton.com/lark-descending/