The Tree Swallow usually gives away when it's about to do this. If you observe it closely, it makes some movements (almost like a swallowing or gulping of air but with beak closed...you know like what some do when they have an itchy throat) with its throat area before opening its beak wide open.

Shutter Priority, 1/500, f/5.6, Metering Mode: Spot, EC -1/3, ISO 640 at 200mm. FEC -1 2/3