The orientation system of birds — II. Homing and navigation | Das Orientierungssystem der Vögel II. Heimfinden und Navigation

The analysis of the navigational system of birds is largely based on experiments with displaced homing pigeons. Kramer's “map-and-compass” model assumes that the home direction is first established as a compass course. This is confirmed by numerous clock-shift experiments. Consequently, only two types of navigational strategy appear possible: (1) the use of route-specific information based on an external reference or (2) the use of site-specific information whose directional relationship is familiar to the birds.The use of route-specific information is indicated by the finding that young birds deprived of magnetic information during displacement were disoriented, whereas birds receiving the same treatment at the release site were not. This suggests that birds navigate by recording the direction of the outward journey with their magnetic compass, determining the home course by reversing it. This strategy of path integration with the help of an external reference, however, is used only by very young, inexperienced pigeons during an early phase in the development of the navigational system. As soon as birds become more experienced and are able to use site-specific information, they give up route-specific information in favor of the former. The reasons for this change in strategy lie in the fact that using site-specific information enables birds to redetermine their home course as often as necessary, thus allowing the correction of initial mistakes.Site-specific information means that birds can derive their home course from factors perceived at the release site. The present models on navigation acknowledge the crucial role of an external reference system by proposing the navigational “map” to be a directionally oriented mental representation of the spatial distribution of at least two navigational factors, which are assumed to be environmental gradients. The birds determine their home course by comparing the local values of these factors with the remembered home values. Gradients can be extrapolated beyond the range of direct experience, which explains the birds' ability to home from distant, unfamiliar sites. Deviations from the true home direction, so-called “release site biases”, as frequently observed in pigeons and other bird species, may be attributed to unforeseen irregularities in the distribution of the navigational factors, which cause birds to misinterpret their position. Near home, the navigational or “grid map” is supplemented by the “mosaic map”, which is supposed to be a directionally oriented mental representation of the distribution of familiar landmarks.Both “maps” are based on experience. Young birds obtain the relevant information during spontaneous flights by combining information on the route travelled with information on the location of prominent landmarks and the direction of environmental gradients. In pigeons, the “maps” become functional during the third month of life. They are continuously enlarged and updated also in later years. The total size of the navigational “map” appears to depend on the spatial range of the birds' experience. The “maps” seem to include information on distance; they are not restricted to homing, but allow free movements between arbitrary goals. In several respects, the model of the avian navigational “map” is similar to the concept of the “cognitive map” discussed in psychological literature, the main differences being the larger distances involved, the representation of continuous environmental gradients instead of separate entities, and the essential role of an external reference.The navigational system of birds is thus characterized as utilizing a wide variety of environmental cues. Learning processes, which are based on a simple, innate mechanism, the magnetic compass, integrate these cues and form complex, experience-based mechanisms, such as the “maps”.