Question male bluebirds fighting each other

Lidia Daley
 

Hi just curious.. Haven't seen it before.. Any thoughts. I have a nesting couple in my backyard..I noticed the eggs laid eight days ago.. Female is incubating. Male defending nest.. But this afternoon another male show up and they fought to the ground.. It was pretty aggressive. Finally one chased after the other.. The female has been acting very agitated past day or so. Just wondering if this is common and if anyone has come across this. Usually it's the sparrows every year.. Thanks for you time.  Lidia

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