Angry Goldfinch

Last night I came upstairs to hear a loud and angry bird sound.  This bird was screaming and it sounded so close I thought it could be in the house.  My canary, Migwe, was enjoying some outdoor time on the deck. I moved slowly and carefully not wanting to scare this angry bird before I got a good look at it.  I slowly moved toward the deck door to see a male goldfinch perched on Migwe's cage, looking down, and screaming at my canary.  I knew it was male because of the bright yellow color.  Goldfinches and Migwe look similar.  Both are yellow and black but Migwe is slightly larger.  And he sings prettier too.  Why was this goldfinch so upset?  Did it perceive a threat to it's territory?  I guess it's possible I misunderstood and maybe I was hearing a friendly greeting.  But I think I can hear the difference between a greeting and an attacking sound.  The goldfinch flew off when it saw me watching but continued to scold from the basswood tree.  Migwe really seems to enjoy the stimulation of being outside.  How do I know this?  When I ask him if he wants to go outside he bends forward and extends his wings in the same way he does after I give him fresh food and water.  Fresh air and stimulation are probably good for him.  Sometimes I play youtube video's of Russian canary singing for him too.  He listens intently so I think it's good for him.