So, lemme explain.
While looking over details for a story I'm working on, I noticed in the McFly house there is a bird cage in the background with a bird inside. Said bird exists in both timelines and appeared to be a yellow canary. I kind of latched onto the idea that the bird for the most part is a family pet but is closest with Marty. Hence, we get Kirk here.
Now, diving way into personal head canon and story telling: Kirk is the family bird and they have had him since Marty was about 8 years old. While mostly the family bird, he's more so Marty's bird since Marty spent the most time with him and learned to take care of him. Kirk is Marty's little song bird and Kirk often chirps along to the music Marty plays. Marty adores him and missed him like crazy the two weeks he was gone time traveling. (He made sure to spend quality time with Kirk after he got back.) Kirk is generally a pretty chill bird and gets along with all the McFlys. Content to rest on shoulders, heads or just be nearby. He used to be a bit of a menace when they first got him but he chilled out over time.
And that's about all I got. Pretty simple and just another fun layer to the McFly family. Also he was named Kirk for... well pretty obvious nerdy reasons haha.
Anyways, hope you enjoy Marty and his little friend.