We have seen this happen a couple of times now at DeKorte Park: a Forster's Tern perches on a Tree Swallow box, then gets agitated when a Tree Swallow tries to fly in to feed the young. 

This scene went on for several minutes late last week, until the tern decided to fly away.

(This happened in Shorebird Pool, at a bit of a distance even for a 400mmm lens.)