Is this a fight over territory?

Today I captured this footage. A bird is inside the box and another one enters. They appear to be fighting for a few seconds after which the loser flies out of the box and the winner stays. It's hard to tell who is the winner, the "original" bird or the "intruder"...

Btw: no birds have roosted in the box for the last three nights. Maybe they lost interest in roosting there because of the extremely mild weather. However, there are still frequent visits of different birds during the day.