I have realized that I actually have a lot more in my yard for birds than I thought. Taking a bit of an inventory of my yard and how it fits into each category has been really helpful.
I noticed that when the Pine Siskin were by themselves (or sometimes with one more), it was displaced by every bird who came to the feeder. They were very timid and scattered easily. But once there was 3 or more Siskin at the feeder, they didn't budge.
At one point a Black-capped Chickadee was trying to land at the feeder, where 5 Siskin had planted themselves firmly for a snack. The Chickadee tried several times, even flapping its wings and trying to lunge forward, and the Siskin didn't move, almost even ignored him. Finally, a couple of the Siskin opened their beaks at him and that was enough to scare him off.