Niger seed (nyjer seed or thistle seed) is known to be a favourite of finches, due to the shape and structure their beaks, which are suitable for a smaller type of bird seed.
“Excellent quality Niger seed from Vine House Farm. The seed is very fresh, clean and shiny and the goldfinches love it. There's far less waste than with seed I've had from other suppliers. Will buy again.”
“You can never be quite sure what will happen if you add a new food or new feeder. Birds have to find it, like that food and so on. But it went well for us - after a few days we have a flock of goldfinches visiting.”