While visiting a non-Maryland/DC location this weekend I observed a juv. Common Tern behaving in an defensive? aggressive? posture that surprised me.

It was on a sand bar among Forsters and Common Terns, Short-billed Dowitchers, Greater Yellow Legs and Sanderlings.
When ever a Sanderling would get too close, this juv. Common Tern would spread its wings, lower its head and open its bill in what appears to be a territorial posture.  After it lowered its head it would move forward a bit.

I've seen this with breeding gulls, but it was really unexpected to see from a juv. tern.

Any thoughts on the behavior?

Denise Ryan
Washington, DC