Red-legged partridge, Alectoris rufa, single bird on post, Warwickshire, May 2018. Olympus Em1 Mk2. 300mm f4 plus 1.4 extender. 1/400th at F5.6. 1600 iso.
When Red-legged partridges start to call they do not open their bills at all. The throat feathers vibrate a little, which is the only sign they are calling, but after a few minutes they become more intense and the bill does open as the volume of the calls increases. Right at the end they stretch their necks and the whole body vibrates with the effort.