Reptiles may lay eggs or give birth to live young, depending on how they have evolved to cope with the British climate. Egg-layers such as the Grass Snake and Sand Lizard choose a warm place for their eggs (rotting vegetation or sandy banks respectively). Unlike birds’ eggs, reptile eggs do not need turning (indeed it is harmful), so the females can leave their clutch and resume their normal lives. Those which carry their young inside them until they are born, like the Adder or Slow-worm, have to bask out through the summer to develop their young, acting as mobile incubators.
Egg-layers have one advantage - the females do not have to bask out for long periods and therefore expose themselves to predation when they are heavy with young. On the other hand, they risk losing their whole clutch if the summer is too cool to allow the embryos to develop or if the laying site is disturbed.