ADRIAN HAILEY* AND P. M. C. DAVIES

ABSTRACT

There was an ontogenetic change in the diet of the water snake Natrix maura, from earthworms and tadpoles in juvenile snakes to fish in adults. This was related to the absolute sizes of these prey types and to the scaling of encounter rates with snake size. Within each prey type relative prey weight RPW was independent of snake size, except for fish taken from drying pools (RPW inversely related to snake size). Overall, RPW increased with snake size (prey taken during normal foraging) or was independent of snake size (including fish from drying pools). Snakes handled fish too large for them to ingest. Multiple captures were common from drying pools, the fish being smaller than when a single fish was taken.

Foraging behaviour of wild N. maura could be described as: exploratory activity and cruising (finding slow-moving or trapped prey during slow movement); sentinel predation (an extreme sit-and-wait strategy for catching fish); active pursuit and undirected 'fishing' (less commonly observed). Sentinel N. maura took up different positions in the water according to their size. Breathing took up 20 per cent of their time; those at the surface spent shorter intervals foraging and breathing than those with deeper perches. Strike rate was once per 7.4 minutes; capture success was low, 2 out of 124 strikes, neither ingested. The relationship between the different types of foraging used by N. maura and other natricine snakes, and their stimulus control, are discussed.