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The avian neck is a complex, kinematically redundant system that controls the position and orientation of the head. The kinematic redundancy is resolved by movement principles, which result in characteristic movement patterns. General neck movement patterns are compared between Ratites, Fowl and Waterfowl in order to nd a relationship with anatomical differences and to identify the biological role(s) to which neck movement is adapted. Kinematic analyses show that Fowl move their vertebrae according to a minimal rotation principle that maximizes rotation ef ciency. The resulting movement pattern shows rotations in some joints, while keeping the other vertebrae as straight bars. Waterfowl show a pattern of successive, rather than simultaneous rotations of vertebrae, limited to the rostral part of the neck. A third movement pattern is found in Ratites, which show successive rotations of the vertebrae in the middle region of the neck. The ratite-pattern is related to large vertical head trajectories, and is occasionaly also found in the Chicken. However, due to large body movements in the Chicken, head trajectories are relatively much shorter than in the Rhea. A kinematic neck model based on the minimal rotation principle only produces the Waterfowl pattern if a constraint on the movement of the caudal vertebrae is introduced. We conclude that a fundamental change occurred in the movement pattern of the Waterfowl neck, which is energetically advantageous and an adaptation to aquatic feeding.

Within the Anseriformes, the Anatinae (ducks) shows a wide variation in diet and feeding mechanisms, in contrast to the Anserinae (geese and swans). While grazing is common in the Anserinae, only few species within the Anatinae use terrestrial grazing as their main feeding mechanism (e.g., wigeons). This may be explained by conflicting functional demands of grazing and filter-feeding on the trophic system. In this study, the feeding performance, feeding mechanisms and oropharyngeal anatomy is compared between geese, wigeon and a general filter-feeder/pecker, the mallard (Anas platyrhynchos). The results show that the functional demands of filter-feeding and grazing are conflicting: filter-feeding requires a bald palatal surface and under-tongue transport for optimal functioning of the lingual cushion as a piston, whereas the transport mechanism of grazing requires large maxillary spines and over-tongue transport to retain grass during tongue protraction. The oropharyngeal anatomy of the wigeon shows a compromise in the small size of the maxillary spines that enable a sliding mechanism for the transport of a limited amount of grass. Filter-feeding is sometimes considered as a key adaptation that led to radiation in the anseriforms (Olson and Feduccia, 1980; Lack, 1974). We suggest, as an alternative hypothesis, that feeding on water plants may have led to the evolution of ridge-like structures in the bills, a sliding mandibular joint and the use of a water flow through the oropharynx (tongue pro- and retractions) for food transport in early anseriforms (cf. geese). A selection pressure on filter-feeding resulted in a large increase in efficiency of this system by the introduction of under-tongue transport of food and water (repatterning of bill and tongue movements) that enables the simultaneous intake and transport of a suspension of food particles (cf. Anatinae, a.o. Aythya and Anas). Terrestrial grazing later evolved by the development of maxillary spines, and in the case of the wigeon, a secondary change from the under tongue transport mechanism to over tongue transport for the grazing and pecking mechanisms only.

Development of head neck motion patterns is studied in drinking chickens to examine (1) general motion principles, (2) ontogenetic changes in these patterns, and (3) whether pattern changes are due to scaling effects during growth. Behavioral patterns are analyzed by high speed filming, radiography, and calculation of rotation patterns for each joint during all movement patterns. Flexibility and variability are great, but representative kinematic patterns are selected for immersion, upstroke, and tip-up phases. Five principles were found that control cervical motion. Two principles maximize rotation efficiency: the geometric and lever arm principles. Two trajectory compensating principles occur; one controls compensation for overflexion, and the other corrects curved into straight trajectories of head motion. One principle occurs that minimizes rotation force if large forces tend to develop in one joint. This principle results in a characteristic cervical motion pattern ("bike chain" pattern). There are three developmental periods: (1) hatchlings (2) chickens 1 to 4 weeks old (1-4W), and (3) older than 4 weeks. Each period is characterized by different kinematic patterns. In 1-4W chicks, the rotation force is minimized. In older stages, the cervical joints rotate according to geometric and lever arm principles. The totally different motion pattern in hatchlings results from a different behavioral reaction to water and the influence of large centrifugal forces. Transitions in cervical motion patterns are connected to effects of scaling, primarily changes in head and body weights. Changes in motion patterns are not related to changes in anatomical characters such as flexion extremes and relative length of each vertebra since these are similar in all stages.

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