This goldfinch has Groucho Marx black eyebrows, and his left eye seems to be squinted closed. He flies in a quick swooping pattern and hovers like a hummingbird just before he lands. My theory is that he doesn't have depth perception. There's a lot of fluttering going on as he comes in for a landing.
Although we put out a handful of seeds on the railing several times a day, we keep about a quarter cup of seeds in the bird feeder for the hours we aren't near the deck. Our goldfinch's favorite position is to grab a few seeds from the bird feeder. Then while he's balancing with one food on the perch and one foot on the feeder, he takes a power nap. His head sags; his body relaxes. I can see him breathing. He wakes up about two seconds later as another bird does a fly-by.
Today, when i came into the kitchen after gardening, the goldfinch knocked at the kitchen window 4 times. He sort of bounced off the window, not knocking himself out like birds do when they run into the window by accident.
I went out to the deck. There were no seeds on the railing and no seeds in the bird feeder.
That disabled goldfinch is no birdbrain. He knew how to get my attention in order to get his supper.