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The Significance of a Specific Search Image in the Feeding Behaviour of the Wood-Pigeon

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I. The development of a specific-feeding image in Wood-pigeons (Columba palumbus) was examined by presenting combinations of different seeds on plots demarcated on pastures and leys, where the birds were already feeding on clover leaves. For most trials the baits, tic beans, maple peas, green peas, maize or runner beans, were lightly coated with alpha-chloralose so that after a period of feeding the birds became stupefied and could be collected. Their crop contents represented an unbiassed record of any bait eaten. In some of these, and in other trials, the feeding actions of birds on and off baited areas was assessed by counting the number of pecks or paces made per minute. 2. When offered a choice of tic beans and maple peas at 1.0: 0.9 grains ft2, respectively, 33% of the birds preferentially selected tic beans, 39% favoured maple peas and 28% took either by chance. The heterogeneity of the data invalidated chi-square treatment. A method is, therefore, given for allocating the subjects to these categories by defining the expectation of the binomial distribution that an event will occur a times out of N with a given probability. L. TINBERGEN'S (1960) equation is then used to test a hypothesis that the preference for tic beans and maple peas was the same but maple peas appeared to be slightly preferred. 3. Further trials were performed with maple peas and green peas or green peas and tic beans being offered at approximately equal density and similar results were obtained. However, when maize and tic beans were offered, maize was virtually ignored, although it was accepted by more birds when presented alongside maple peas. Presumably the birds encountered maize but mostly rej ected it as a food obj ect. Nonetheless, captive birds have been trained to eat maize and Rooks Corvus frugilegus readily accepted it during the field experiments. 4. Other experiments involved presenting three or four different seeds simultaneously. A statistical procedure for evaluating whether the birds selected their food or obtained it by chance encounter is given. It is shown that more individuals than expected came to concentrate their attention on particular items ; the items were not eaten according to the birds chance encounters with their food. 5. Various risk ratios sensu L. TINBERGEN were obtained for the different food choices offered. Algebraically the three risk ratios describing the combination of choices available when three seeds (tic beans, maple peas and maize) were tested two at a time should equate. A method of calculating the standard deviation of the observed values and comparing these with expectation is given and by this means TINBERGEN'S equation could be tested. Although algebraically and conceptually sound, it did not provide a full description of the feeding behaviour of the pigeons. 6. The discrepancy noted above (5) is shown to depend on the gregarious behaviour of feeding pigeons, enabling social facilitation in feeding movements to operate. Wood-pigeons copy each other's pecking movements and these become adjusted to the overall density of acceptable baits. In order to exploit a food at low density, if available with other foods at higher density, a pigeon must alter its searching rate. This must lead to fragmentation of the flock, alternatively the bird must compromise and peck at the rate adopted by the other birds, if it is to remain with the flock. This results in the flock tending to favour a particular item, irrespective of the individuals chance encounters with food objects. The biological significance of such a species-characteristic imitative feeding process is discussed and considered to be advantageous in coarse-grained habitats, though this would not be true in a fine-grained habitat. 7. First-winter birds did not differ from adults in the development of specific-search images. Birds which specialised on a particular item had the same mean food intake as individuals which had taken any object they encountered, even though these latter birds had a greater density of food items available. This is considered a consequence of the pecking rate being roughly constant for all flock members. Nonetheless, those birds with the most food items in their crops were invariably specialists and the possible advantages of specialisation are discussed. 8. Direct observation of feeding birds made it possible to define the proportion of the flock which failed to react to the baits available and was not caught. A higher proportion of the flock switched to feeding on the baits with increased density of a particular food object. Different food objects, though in toto making an equivalent density, did not provide such an adequate stimulus and fewer birds responded. The biological implications are discussed.


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Affiliations: 1: Ministry of Agriculture, Fisheries and Food, Infestation Control Laboratory, Tangley Place, Worplesdon, Guildford, Surrey, England


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