What do Collared Doves eat?

I've got a couple of Collared Doves nesting on an old satellite dish in the garden (I'm live streaming them at https://youtube.com/channel/UCuJayX2M9EkT_BnA6ZEHd-w/live) and I've tried putting out sunflower hearts and mixed seed in the ground feeder for them but they won't go near it.  Plenty of regular pigeons and wood pigeons have been eating the sunflower hearts but not the doves.  They can see the feeder, they watch the other pigeons eating the seed but never go down to have a look.

Does anyone feed them and, if so, what is it that they prefer to eat?