I bought a sack of black sunflower hearts to restock the garden bird feeders instead of that wasteful mixed seed most of which the small birds discard and the Woodpigeons and our labrador hoover up. It seems the Great Tit (Parus major) and the Blue Tit (Cyanistes caeruleus) are quite partial to these. They need some degree of patience to eat them given that the tasty seed is contained within an edible husk. This deters some less patient birds. I’ve not seen Starling (Sturnus vulgaris) tucking into these seeds on the feeders, for instance. They much prefer to snaffle suet balls and mixed dehusked seed mix.
Eating such seeds comes with the risk of greater exposure to predator threat while the bird cracks the seed open. The Great Tits seem to quickly duck in and out, grabbing a seed and then darting back into the foliage to presumably break it open and eat it. A female Greenfinch (Chloris chloris) didn’t seem perturbed, standing her ground (well, her perch) while she opened it to eat.