It was at least four years ago when I first saw Killdeer on a farm field. Its calling song and the swift move on the ground is unique. It is hard to miss them when they are around. I was thrilled to see at least half dozen of them at Ridge Field Wildlife Refuge soon after.
I presume it’s the mother (male and female look alike) who is always very protective of their babies (or eggs). They would try to mislead you to a different location away from where their eggs really are. If the secret hide-out is unfortunately broken, they would fluff their full display of wings on alert position and ready to fight you off.