Birds have adapted over millions of years to deal with the killer weather upon us now. Let's talk about small birds, your feeder birds.
They don't need warm hats and mittens. They need food.
Shelter at night, which they find on their own in cavities or thick evergreens, is critical. But fuel to keep them warm is foremost. Bird engines run at high rpm, so to speak.
Chickadees and other small birds need to eat and eat and eat.
Small seeds are easier for birds to process. So are seeds without shells. Black oil sunflower is sold in this form.
Golden safflower, another popular seed, is high in protein and fat, and has a thin shell easier for small birds to open. (It also is said to be unattractive to squirrels!)
In colder weather larger seeds are said to be a more efficient source of energy. The chickadees visiting our feeders tend to take a seed and fly to a nearby perch to eat. Flight takes energy. Fewer flights would be less costly.
Meal worms, fresh or frozen, are an excellent source of protein and fat.