I'm not completely sure why the skimmer's bill has evolved with fine and parallel grooves, but I figure they serve a hydrodynamic function for the bird while it's skimming. Though you can't see it from this profile view, the beak is also razor thin, an adaptation that helps it cut knife through the water as the bird fishes.
Nickerson Beach, Nassau County, New York, 8/8/19
Canon 600mm f/4 IS II on EOS 1DX Mark II
1/3200 at f/7.1, ISO 640