Hunting for cryptic prey : evidence for search image in wild passerine birds

Lawrence, Edward Simon (1984) Hunting for cryptic prey : evidence for search image in wild passerine birds. University of Southampton, Doctoral Thesis.

Abstract

The concept of 'search image' was put forward over 20 years ago by L. Tinbergen (1960) to explain the behaviour of predators when faced with the problem of detecting cryptic prey. The definition of search image tested in this thesis is 'a change in the perceptual ability of a predator to detect cryptic, familiar prey'. Search image is thought to play an important role in two related phenomena : 'switching' and frequency dependent predation ('apostatic selection'). However the link has yet to be proven and search image may only be one of a number of behaviours which can have these effects. The problems caused by the widespread misunderstanding of the term search image and its misuse by ecologists and evolutionists is discussed in detail.The results of two previous studies show unequivocally that birds use search images. But these experiments were restricted to the laboratory environment (and in one case to young, domestic chicks). The aim of this study was to test for the use of a search image by avian (passerine) predators in the wild. The experiments were made as natural as possible by allowing the birds to perform a familiar part of their feeding behaviour in their normal environment. For each experiment I used artificial food (pastry or puffed-rice dyed with food colouring) and presented it to the birds on both matching and non-matching colour backgrounds. I then recorded the feeding behaviour of the birds onto either video and/or audio tapes. I originally intended to examine the factors which might determine how and when wild birds formed search images for their natural prey. In the event it proved necessary to repeat the experiments a number of times simply to confirm whether search images were used at all.Two sorts of experiment were run : some used 'ground-feeding' birds (experiments A to C : mainly blackbirds Turdus merula ) and some used 'barkfeeding' birds (experiments E to G :mainly great tits Parus major). For a number of reasons evidence for search image was obscurred in experiment A. The design was altered slightly, with the result that experiments B and C show that both wild (adult), and captive (juvenile) blackbirds use search images when hunting for cryptic pastry prey. Blackbirds formed a search image during a single feeding session. Longer-term changes were also apparent; they retained their ability to detect cryptic prey from one day to the next. Evidence for the use of a search image by great tits was not as clear. Two important factors emerged from experiment F using captive great tits : (1) certain individuals became expert at detecting cryptic prey whereas others lacked this proficiency : (2) this ability was transient; even experts had relapses. These findings were supported by experiment E using free-flying adult great tits. In conclusion the results support the idea that passerine birds use search images as a normal part of their foraging behaviour.

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