L. TINBERGEN'S HYPOTHESIS OF THE ROLE OF SPECIFIC SEARCH IMAGES

SUMMARY1 The paper opens with an appreciation of L. Tinbergen's findings, that predation by tits on certain caterpillars in summer is less than expected on the basis of chance encounters at very low and very high caterpillar densities, but more than expected at moderate densities; and of Tinbergen's hypothesis that birds only find and accept prey after acquiring the appropriate specific search image, which only happens when the prey exceed a certain threshold density. 2 Larvae of the eucosmid Ernarmonia conicolana (Heyl.) spend the winter concealed just beneath the cuticle of ripening pine cones, where they are preyed upon by tits which leave recognisable traces of their attacks on the cones. Their predation was exceptionally light at very low densities, but increased abruptly with slightly increased densities of conicolana. Contrary to Tinbergen's hypothesis, the birds probably found and sampled the larvae at the very low densities, but rejected them as an uneconomical food. 3 The tits' predation intensified with increasing density of conicolana ; but relaxed somewhat at higher, less frequently occurring, densities. This implies that the birds learn how many larvae to expect in different localities; and slacken their search when the expected number of larvae have been taken from the cones. 4 Some implications of this predation by expectation are discussed. It probably cannot account for the reduced predation on caterpillars at high densities recorded by Tinbergen; but could apply to a wide range of concealed foods at which birds (or other predators) leave recognisable traces of their attacks.