REDIRECTED ACTIVITY. Again, usually the result of conflict situations. The bird is afraid to attack intruder so it attacks its mate. Sound familiar?!
INTENTION MOVEMENTS. The bird starts an act but does not complete it--flicks wing, opens gape, snaps bill. All three of these behaviors are often ritualized and incorporated into more elaborate behaviors. For example, mating Herring Gulls preen themselves and/or pull at grass and both behaviors are often seen in aggressive situations too.
Red-necked Grebe with young on back.