Grass snakes
Couleuvre helvétique (previously Couleuvre à collier)
Natrix helvetica
Couleuvre astreptophore
Natrix astreptophora
They are superb swimmers and can often be observed in lakes, ponds and rivers with their head held straight up out of the water; diving and swimming under water is another strong point being able to hold their breath for a good 20 minutes. They can also be seen from time to time in trees sunning themselves on low branches or in a trunk fork.
and can take place from September/October or even November until March/April under a compost pile or other deep decomposing vegetal matter, a pile of rocks or in a rock fissure.