Basically, native birds can be distinguished as follows:
Soft-food eaters (e.g. blackbird, wren, starling, robin) like to eat oatmeal, poppy seeds, bran and sultanas as well as fruit.
Grain-eaters (e.g. sparrow, finch, siskin, bullfinch) like to eat sunflower seeds, hemp seed and chopped nuts.
There are also birds that like both types of food (e.g. woodpecker, tit, nuthatch).
Ready-made bird food mixtures, such as those available at large retail stores, are not recommended for birdhouses. On the one hand they are quite pricey for what they offer, on the other hand they often contain seeds of the allergy-causing ragweed plant.