Well, if it was big and black and round, then it was most likely a carpenter/wood bee. The female bores a 1/2" round hole usually upwards in almost any type of lumber, then tunnels up to around 8 egg chambers along the inside of the wood. Since the chambers are sealed after each one, her babies will dig their way out and leave thin elongated holes in the side of the lumber.
Apparently, the carpenter bees return to their hatching site the following year. Repeated annual nesting by the exponentially increasing number of carpenter bees could potentially significantly weaken the structure.
Fortunately, Carpenter bees are territorial and defend their nest area from other carpenter bees.
They try to intimidate humans that approach too close by hovering at your eye level or just above your head. If hovering is insufficient and they feel their nest is threatened, they head butt you on the forehead, so they like to hover right in front of your forehead.
I've had to take extreme measures when they made nests in garden and picnic benches, kids playground, etc. critical structures, but for the most part, they are harmless. I occasionally harass them by spraying them with water if I am watering, but I'm unfazed by their close proximity and have had them fly up between my arms while leaning on the patio table, etc.
If you can manage a direct hit by blowing air at them, it's amusing to see them break off and fly away.
Because they are dedicated about chasing off any other flying creature from their nest area, they will prevent paper wasps from building a nest in the vicinity, and I have seen them chase off monarch butterflies and even humming birds.
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