Attack strategy of an ambush predator: which attributes of the prey trigger a pit‐viper's strike?

Summary1 Why do larger predators generally consume larger prey items? Endemic pit‐vipers ( Gloydius shedaoensis ) on a small island in north‐eastern China ambush passerine birds, usually from the branches of small trees. Both minimum and maximum prey sizes increase with predator size. 2 To clarify the reasons for this ontogenetic shift, 251 snakes in ambush postures were approached and offered potential prey items (dead birds, or models covered in feathers) to clarify the cues that trigger a foraging strike. 3 The snakes struck at the prey item in 101 of these trials, and this ‘decision’ was influenced by the size, movement and temperature of the prey item. A pit‐viper's body size influenced its prey‐size selectivity, with larger snakes refusing to strike at smaller prey items. Larger snakes also scavenge dead birds too large for smaller snakes to ingest, but do not ignore live birds: even the largest snakes use prey movement and prey temperature as cues to elicit feeding strikes. 4 The ontogenetic shift in prey size thus reflects a combination of processes. The absence of large prey from the diet of small snakes is due to gape‐limitation (these snakes strike and attempt to swallow much larger prey). The absence of small prey items from the diet of larger snakes is due to active refusal to strike at small prey, as well as a behavioural shift to scavenging and to terrestrial rather than arboreal ambush‐sites (and thus, higher rates of encounter with large prey items) by larger pit‐vipers.