During early spring cold-blooded animals wake up again after their hibernation. However, the temperatures are not high enough yet to be very active. Snakes, like this Viper, try to find places with a lot of sun and start sunbathing. The heat of the sun gives them enough energy to start hunting again, trying to find their first meal in a few months.
Needless to say Vipers are very vulnerable in this time. Easy to photograph as they can’t move around, but they do really need their sun. There are regularly stories about photographers picking up snakes to place them in a better position, or coming too close, forcing the snake to retreat. This picture was taken with my 300mm with a 2x converter, allowing me to stay at almost 3 meters distance.