If I understand correctly, the bird feeder is on the deck and scattered seed falls through the deck to the ground underneath. If that's the case, the only solution I can think of is to move the feeder so it's in an open space, so fallen seed is out in the open. Mice have lots of predators and generally avoid being out in the open like that. Other than that, you could try surrounding the area around and under the deck with a metal mesh like hardware cloth, burying it 6" into the soil.
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