The one on the right is doing an act of injury to attract away other big birds who prey on its eggs and chicks. The common enemies of Piping Plovers are seagulls and falcons.

Their activity is on the beach. If not protected, their eggs are endangered due to human activity:

Chick. The pic was taken 25-30 meters away:

Stay away from my baby! The parent Piping Plover is chasing away a Least Sandpiper:

Parent Piping Plover: