Siskins, also called the Eurasian or Common Siskins, are happy to eat among flocks of other birds.
Despite their smaller size, they will hold their ground and shoo off much larger birds including the Chaffinch, above right (partly out of picture).
For comparison, above left shows the slightly larger Goldfinch and much larger Greenfinch at the same feeder.
Siskin with Robin
More Siskins with Goldfinches