Question

what is a key insulator in black - throated loons?
1 sheltered, underground lair
2 layer of blubber
3 rich supply of blood to the extremities
4 dry layer of air and feathers
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Explanation:

Black - throated loons (or divers) have a layer of air trapped between their feathers and a dry layer of feathers that acts as an insulator. A sheltered underground lair is not related to their insulation. A layer of blubber is more associated with marine mammals like whales, not loons. A rich supply of blood to extremities is for thermoregulation in a different way, not insulation. So the key insulator is the dry layer of air and feathers.

Answer:

1. dry layer of air and feathers