Wild Bird Seed - niger seed
Niger or Nyjer [Guizotia abyssinica] is a small oil-rich seed from an annual herb grown in Ethiopia. It is the favourite seed of the Goldfinch as well as some other seed-eating wild bird species. It is becoming more popular with seed merchants. Once sold as thistle seed due to early marketing of the seed as 'thistle' to take advantage of finches' preference for this plant's seeds.
Goldfinches spend every minute of the day looking for seeds, but once there is Niger seed available, they will gather at your feeder. Different kinds of feeders are sold, the most popular being a mesh feeder with fine mesh which they can cling to while they feed The seed is expensive [and has a high carbon footprint] as it is imported from India or Ethiopia, and then has to be heat sterilized to prevent it from growing if any seed spills, as it would become another invasive foreign species, and we already have many of them due to gardeners importing decorative plants for gardens without a thought of the environmental consequences. As all our birds are indiginous and have evolved here, it makes sense only to feed them seeds from plants that are also indiginous. You could plant some thistles to acheive the same effect, but be careful to limit them to one patch or they will colonise your garden - beware, they can spread ten feet underground before reappearing! The birds should take care of all the thistle seeds, but you would have to cut them back and root out any spread.