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Abstract1. The pecking behaviour of pigeons is described from a frame-by-frame analysis of high speed films and X-ray motion pictures. 2. Each pecking scene has four discrete steps. These steps run from the fixation of the head above the seed to (1) the grasp of the seed by the beak tips to (2) the catching of the seed at the rictus level to (3) the positioning of the seed along the caudal palate to (4) its arrival in the rostral oesophagus. The bird is able to stop the sequence at the very beginning of each step. For example by a refusal to continue after the final fixation, by dropping the seed after the grasp, by ejecting it after the rictus catch when the seed is positioned on the lingual base, and probably also by an ejection following the positioning along the caudal palate. If necessary, an adjustment of the system takes place, prior to each step. Such an adjustment positions the structural elements and/or the seed in the correct mechanical arrangement for the initiation of the next step in the sequence. These adjustments are the preliminary approach at the final fixation, stationing at the grasp, repetition of transport through the mouth to the rictus level, and repetition of the transport type used in the pharynx. The bird has the possibility to adapt each step to either the position and/or the size of the seed. The final approach (step 1) can be a scooping, a straight or a swinging approach of the head depending upon the seed's position, while simultaneously the type of beak opening is adapted to the seed's position and the gape size to the seed's size. Transport through the mouth (step 2) is for a small seed a slide-and-glue mechanism by which the seed is adhered to the tongue and is carried to the rictus level. Usually large seeds are transported by the catch-and-throw mechanism. Intermediate types also occur. For small seeds transport to the caudal palate (step 3) is also a slide-and-glue mechanism, in which the lingual base serves as the adhering element. When large seeds are transported a head jerk is added to this mechanism. Transport into the oesophagus (step 4) for small seeds is a scraping mechanism of the ventral pharyngeal valves which are erected when they are in front of the seed prior to their retraction. An extra laryngeal transporting cycle and head jerk occur when large seeds are swallowed. 3. The slide-and-glue mechanism is extended by prediction of position and structure of glands deduced from the mechanical requirements of the mechanism. After comparison of the deduced glands with a microscopic and scanning electron microscopic analysis of the mouth and pharynx, the position and the structure of the gl. mandibularis anterior, the gl. mandibularis posterior, the gl. lingualis superior, the gl. lingualis inferior and the gl. palatina posterior externa were found to correspond with the prediction. 4. The existence of a cerebral comparator-selector mechanism was assumed to describe decision making processes during the adjustment of the pecking system at the start of each step. This is most clearly shown during stationing, which is a repositioning of the seed after the grasp. The registered position of the seed is compared with a pre-set cerebral template and after the comparison a selector recruits either mechanical units for a positive output (a head jerk and a gape cycle) transporting the seed somewhat caudad, or a negative output (a gape cycle and a lingual cycle) transporting the seed rostrad. 5. The close relationship between the particular positioning of the sensory units and the necessarily coupled recruitment of a set of mechanical units is analysed. For example, during final fixation the visual information must be gathered for the complete composition of the final appraoch. 6. From the stereotyped appearance of parts of the pecking behaviour it is shown that pecking better viewed as a variable sequence of fixed action patterns rather than just one such a pattern, by handling coupling of mechanical units as a constraint resulting from mechanical construction, mechanical operation, positioning of sense organs, availability of neuronal circuits and necessity to learn optimal combination of available mechanical units. Further, it is shown that the shift of the pigeon's food preference to larger sized seeds after trigeminal deafferentation can be explained as a preference for a catch-and-throw mechanism. Finally, it is shown that a partial refinement of an optimal foraging strategy is found even at the lower levels of organization of pecking.

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