@gardencat Extremely, normally. They will fight and cause injuries defending their territories.
If you put a mirror up somewhere, they will fight their own reflections !!
@markp Very interesting. Our Cardinals are territorial too, you don't often see two males the same area and my resident male did get quite irate when I hung a mirror in the garden. The bluejays on the other hand seem to come around in a group of 5 or 6 quite regularly although, to be honest I can't tell a male from female jay so maybe it is one male with his haram.
If you put a mirror up somewhere, they will fight their own reflections !!