I just stumbled upon this:

THE EFFECT OF WINTERING HONEYBEE COLONIES WITH RESTRICTED HIVE ENTRANCES by M. DELIA ALLEN

Wooden entrance blocks were fitted in the hive entrances of one group, restricting the entrance size to 2 in. x 1/4 in. [51 x 6 mm.].

Despite certain indications that the provision of wooden entrance blocks had a detrimental effect on colonies in winter, the effects have not been proved to be different from those expected by chance alone. It can, however, be stated definitely that the restrictions of entrance size by the blocks was not beneficial to the colonies during the mild winter when the test was made.

This result is in accordance with Simpson's conclusion in a review of work on wintering, that in mild winters poor ventilation can be dangerous.

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Comment: I have been using a full width entrance about 3/8" high all year round for years and have had no problem with mice. The standard bottoms here have two sides, and I guess you are supposed to use 3/8 in winter and 3/4 in summer but I stay with the narrow gap. (Plus a 3/4 inch hole in the uppermost story,

PLB

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