Emigration and seasonal migration of the northern viper (Vipera berus) in a chalk grassland reserve

Abstract

- Long-term monitoring of individually recognisable northern vipers (Viperaberus) has provided new evidence concerning emigration and seasonal migration. Northern vipers are well known to migrate from their wintering areas in springtime to forage and/or reproduce and then eventually return to the same wintering areas to hibernate. Less well known is their capacity to emigrate and in this study we suggest that this can be recognised when a viper selects a new wintering area at least 150 m from that used previously. Evidence is provided showing that there are two significant opportunities for emigration. First, neonates may wander considerable distances from their place of birth before selecting their hibernacula. Second, in the following spring some of the emergent juveniles disperse from the neonate-selected wintering areas but lack the homing ability to return to them and are consequently obliged to select new hibernacula. The sub-adults and adults developing from them have sufficient homing abilities to return to the wintering areas they occupied as juveniles. Only 45 % of males and 23 % of females showed signs of emigration and, interestingly, males emigrated over significantly greater distances than females. The direction of viper displacements in springtime varied considerably being influenced by the position of the sun, topography, weather, barriers to movement, and corridors of cover. Irrespective of migration direction, the sub-adults and adults appeared to make accurate returns to their wintering areas. The return of many adult males to these areas was commonly prior to their second moult and close to mid-summer; much earlier than reported previously. This is interpreted as an opportunity to occupy a warmer area to facilitate repopulation of the testes with spermatocytes prior to hibernation and is equivalent to the behaviour of gravid females that remain in the wintering area to promote the development of their embryos. But unlike females the males remain hidden from view until the autumn. The significance of neonates and juveniles as the life stages that emigrate is discussed in relation to viper reintroduction programmes.