It's all about the calories!
The heavy hitter of high energy cold weather bird food is suet and usually the first item that comes to mind when thinking about what birds eat in the winter. These dense “combination cakes” are a smorgasbord of protein, fiber and mostly fat; all critical nutrients for winter survival. Suet cake options are available with a bit of extra nutrition in the form of added berries, peanuts and sunflowers.
Small seeds, such as millet mixes, are a good compliment scattered on the ground or in open trays. This will give the doves, sparrows, juncos, and others who prefer not to perch on a feeder ample space to feed.
Snowy weather tip: Stomp around and under feeders to keep the snow packed and the seed more easily accessible.
Insects in winter?
Some species, like the golden-crowned kinglet, are not particularly attracted to feeders. They prefer insects. When winter arrives, they spend most of their time foraging. They check under the bark of trees, amongst evergreen leaves, in the ground and in other hidden spaces for overwintering mature and pupae members of the bug world.
As in other seasons, it's important for overall bird health to keep all winter feeders clean, well-stocked and hanging in predator-safe locations.