|Yawning Dunlin. Crown Point Park, Mission Bay, San Diego, California. February 16, 2015. Greg Gillson.|
However, as you can see above, the Dunlin can bend its upper mandible slightly upward, opposite from its normally slightly down-curved bill angle.
I don't find a lot of information on flexible bills. In North America, other shorebirds such as snipe and woodcocks have notably flexible bills. The bills are able to feel below ground or water for worms and crustaceans that they eat.
The one bird in the world most noted for a flexible bill is the Kiwi, of New Zealand.
Here is the same bird a few moments later showing the typical bill shape.
Dunlin were also known in the past as Red-backed Sandpipers, as they have rusty backs in breeding plumage and most of the belly is covered with a large black patch.
A flock of Dunlin is evidently called a "fling." (Also a "flight" or "trip" of Dunlin.)