At nightfall, all the field bees have returned for the evening.
If the temperature of the inside of the hive is at it's maximum,
the returning bees will "beard" on the outside of the hive,
because if they all went inside, it would become too hot.
(their presence inside the hive would raise the temperature)
They will sometimes "beard" all night in the summer months.
Many beekeepers use screen bottom boards and openings
at the hives top to let a good flow of air through the hive's
interior, but even then, the daytime temperatures heat the
contents of the cells and it takes all night to cool down.
See Micheal Bush's site for recommendations on ventilating
a top bar hive.
Sorry for typing so slow, there was no responses when I started