After being informed by Tineke and Serge that their wild birds seem to prefer a straight diet of sunflower seeds (rather than the mixture of many seed varieties sold in supermarkets), I adopted that solution for my bird house.
Inside, there's an ample stock of sunflower seeds, while balls of fat and seeds hang on nails around the perimeter of the structure. For the birds, it has become a popular supply store. Funnily, visitors often tend to wait their turn to enter the store. When a rapid flyover detects the presence of an unidentified bird inside the store, a new visitor often perches in the branches of the little tree seen in the lower lefthand corner of the photo. The store has front and rear doors. So, as soon as a bird leaves from one door, another bird enters through the other door.
Early last night, in the darkness, an unexpected intruder raided the birds' store, consuming two balls of fat and seeds. Fortunately, I was able to identify the raider as she was strolling back towards her warm wicker basket in the kitchen, grasping a third ball in her snout, in anticipation of a late-evening supper.
It remains a mystery whether Sophia actually ate the seeds along with the tasty (unidentified) fat, or whether she spat them out, one by one. In any case, my rose pergola has also become a popular meeting-place for birds. So, that would be an ideal dog-proof place to suspend a new stock of balls.