They prefer to live in a warm and damp environment.
Garter snake in house siding.
Lifting up the loose edges allows for some of the snakes to drop out.
If the affected panels of your siding are easy to remove doing so will allow the snakes a quick exit before.
While garter snakes are generally solitary creatures they generally hibernate in large numbers to prevent heat loss and keep their bodies warm.
I had a family of garter snakes living inside my siding exactly same situation as you had when i put the deck on i removed siding to attach the ledger and half a dozen snakes came out of the lower trim piece just like you describe.
Yards and gardens easily become their cozy homes so they can feed themselves on earthworms tadpoles insects mice lizards and other reptiles found in this area.
These are individuals that needed to be relocated.
Gaps between siding and stone veneers brick or any similar material should be repaired to prevent moisture damage and stop snakes from gaining entry.
Through gaps between brick and siding osborne also warns that snakes can get inside by slipping between bricks if mortar is missing and also between bricks and siding on the exterior of your home.
The snakes climb up into the openings and use the space behind the siding.
These may be located at.
That one does not look like any of those so most unlikely it is poisonous.
Visually inspect your siding for openings through which the snakes can find access.