Nature's insulation is fat and fur/feathers but there is a limit to how much that can help. It adds weight and bulk - hummingbirds have to hover their own weigh

Nature's insulation is fat and fur/feathers but there is a limit to how much that can help. It adds weight and bulk - hummingbirds have to hover their own weight so adding more fat is like making bigger rocket: you need more fuel to lift your fuel.