She has snatched up a newly emerged mayfly before it could fully escape its former watery world and take flight.

I believe the 2 dark forms at the waterline on the rock are the left over exoskeletons of mayflies. Perhaps those managed to avoid capture.

She is also experiencing her post-nuptial molt, a late summer shedding of all her feathers that prepares her for the long migration south.

Location: Glacier National Park, MT