Sit-And-Wait Predators and Cryptic Prey: A Field Study with Wild Birds

Abstract

(1) I recorded the ability of ten colour ringed robins Erithacus rubecula and nine great tits Parus major to detect immobile, cryptic prey in the wild. Artificial prey were presented at high density on horizontal backgrounds at ground level (experiment 1). Another type of artificial prey was presented at low density above ground on bark (experiment 2). The prey were either 'cryptic' or 'conspicuous'. (2) In experiment 1, five robins simply fed faster on the conspicuous food-there was no evidence that the robins acquired 'search images' during repeated feeding bouts on cryptic prey. (3) Only two great tits and a robin out of the seventeen birds that started experiment 2 learnt how to detect cryptic 'Puffa-puffa' rice prey. The juvenile robin used a 'sit-and-wait' style of foraging which allowed it to sight over half the cryptic prey as quickly as it took to scan for conspicuous prey. However, the remaining cryptic prey were preceded by abnormally long 'search times'; the robin also made unsuccessful visits to the background which were followed by pecks to the bark instead of prey. (4) The two great tits also showed an intermittent ability to detect the cryptic prey; if they failed to sight the prey from long distance (which occurred in over three-quarters of their visits), they resorted to an energy and time consuming 'continuous' foraging style on the bark. (5) Thus the ability to detect cryptic prey from vantage points without relying on movement as a cue, was restricted to only a few birds (of those filmed). Without extensive experience these birds could not learn to deal efficiently with cryptic prey.