Last summer I sowed a packet of love-in-the-mist so that the goldfinches would have plenty of nigella seeds to eat in the autumn. They did and I'm sure there were more goldfinches in the garden as a result.
They didn't eat all the seeds though as we have had a similar display this year from self-seeded plants. There may come a time when I will regret sowing something that self-seeds so readily, but for now its wonderful to have such a vivid display (for free) that is so popular with one of my favourite garden birds.
If you don't have space for love-in-the-mist, you could try dangling a bird seed sunflower. This is a natural sunflower with a mixed seed back which is an excellent year round source of protein for garden birds. The seed mix includes nigella.