I have a theory regarding my recent fight in my Swift box after my observations. When the pair arrived last Sunday one was straight in the box with the other spending most the day failing to find its way in(as if its never seen the nest before) now yesterday a third bird enters

the occupied box bearing in mind there are another 20+ boxes on my house it could go in. Which gets me thinking could this be one of the original birds from last year that had returned later to find the other bird paired up with a new bird? I have read Swift are monogamous

however these birds are yet to breed, so I was wondering if this could affect the pair bond strength.