I would suggest that if you are regularly finding dead birds at your feeding station you should stop feeding altogether for at least a few weeks and completely scrub down the feeders and feeding station.
It's possibly that there is a problem with the sunflower hearts you bought (but if they look OK they more than likely are), but it is just as likely that there is some sort of disease in the local bird population which is being spread via feeders/feeding stations.
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Thanks guys. I think you have given the answers. Have removed feeders and pole, sterilised in domestos and will keep stored for the summer. Also removed and disposed of earth below feeder. The amount of undigested sunflower hearts seemed to confirm that the birds couldn't swallow.
At least our sparrows and blackbirds have survived.