I have seen some niger seed feeders that seem to be made of metal and have lots of tiny round holes over its whole length, but I wonder how the finches get the seed out as they have such large beaks compared with some other birds, have you had one of these? and do they work?
Yes I agree with Milvus and I suppose the more variety of food on offer the greater variety of birds at your feeders. But, when I bought a Niger feeder specifically for the Goldfinches I found they preferred sunflower hearts rather than the Niger seed. Contrary little beggars. Perhaps when you're really hungry it's easier fill up on a sunflower heart than a tiny Niger seed. But Yes, I have had one and they do work for Goldfinches and Siskins.