Before you chase the birds away and seal the opening, make sure the birds haven't laid eggs inside (or they'll become orphans!!). It's spring time now and it's the time that they lay eggs. Later by early summer, those eggs will have hatched and then weeks later those babies will have moved on with parents to other places, at that time then, you can get a wire mesh from HD and cover the opening, then put the vent flaps back.
The draft getting inside from the duct is minimal, besides, a little fresh air from outside will do you good in winter..:-)
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