Winter Feeding

I have an island which I can't get to from November to February. I tried making bulk feeders containing black sunflower seed but without success.

I lost many birds during the 2 coldest recent winters and I wonder has anyone else found a way of leaving large quantities of feed to last long periods. I have only recently come to love the many varieties of birds which inhabit the island. They have all re-established this summer and I am concerned for the coming winter. Thanks. Des L