12" x 12" is sufficient for a "pop hole." Larger wont hurt, although there is little practical reason to more than double that size. It can also invite certain predators, where present, that might otherwise be deterred by a smaller opening.
Smaller than a foot square, however, leads to crowding, feather scraping and cuts and associated physical harm.
Smaller than a foot square, however, leads to crowding, feather scraping and cuts and associated physical harm.
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