The Goldfinch are mating, so the boys are in their finest, brightest feathers. Birds are very visual, so the brightest, prettiest boys get their chosen ladies. I’m not sure they really choose ladies. I think most of them are happy to get one. Any bird lady.
Some birds do more complicated mating rituals including dancing, crowing, singing, bringing special foods, building a perfect nest. But for the brightly colored birds, it’s all about bright feathers.