Most nesting birds feed their young protein in the form of insects, especially soft and juicy caterpillars. Some birds are strictly insectivorous, and those are unlikely to visit your seed feeders. A great way to see warblers, vireos, and other such birds is to encourage caterpillars in your garden. At your local garden center, ask for native host plants—varieties that butterflies and moths use for egg laying. The eggs will grow into larva, aka caterpillars, aka bird food.