The orientation system of birds - II. Homing and navigation

Citation
R. Wiltschko et W. Wiltschko, The orientation system of birds - II. Homing and navigation, J ORNITHOL, 140(2), 1999, pp. 129-164
Citations number
125
Language
TEDESCO
art.tipo
Review
Categorie Soggetti
Animal Sciences
Journal title
JOURNAL FUR ORNITHOLOGIE
ISSN journal
0021-8375 → ACNP
Volume
140
Issue
2
Year of publication
1999
Pages
129 - 164
Database
ISI
SICI code
0021-8375(199904)140:2<129:TOSOB->2.0.ZU;2-P
Abstract
The analysis of the navigational system of birds is largely based on experi ments with displaced homing pigeons. Kramer's "map-and-compass" model assum es that the home direction is first established as a compass course. This i s confirmed by numerous clock-shift experiments. Consequently, only two typ es 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 youn g birds deprived of magnetic information during displacement were disorient ed, whereas birds receiving the same treatment at the release site were not . This suggests that birds navigate by recording the direction of the outwa rd journey with their magnetic compass, determining the home course by reve rsing it. This strategy of path integration with the help of an external re ference, however, is used only by very young, inexperienced pigeons during an early phase in the development of the navigational system. As soon as bi rds 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 infor mation 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 fro m factors perceived at the release site. The present models on navigation a cknowledge the crucial role of an external reference system by proposing th e navigational "map" to be a directionally oriented mental representation o f the spatial distribution of at least two navigational factors, which are assumed to be environmental gradients. The birds determine their home cours e by comparing the local values of these factors with the remembered home v alues Gradients can be extrapolated beyond the range of direct experience, which explains the birds' ability to home from distant, unfamiliar sites. D eviations from the true home direction, so-called "release site biases", as frequently observed in pigeons and other bird species, may be attributed t o unforeseen irregularities in the distribution of the navigational factors , which cause birds to misinterpret their position. Near home, the navigati onal or "grid map" is supplemented by the "mosaic map", which is supposed t o be a directionally oriented mental representation of the distribution of familiar landmarks. Both "maps" are based on experience. Young birds obtain the relevant inform ation during spontaneous flights hv combining information on the route trav elled with information on the location of prominent landmarks and the direc tion of environmental gradients. In pigeons, the "maps" become functional d uring the;third month of life. They are continuously enlarged and updated a lso in later years. The total size of the navigational "map" appears to dep end on the spatial range of the birds' experience. The "maps" seem to inclu de information on distance; they are notrestricted to homing, but allow fre e movements between arbitrary goals. In several respects, the model of the avian navigational "map" is similar to the concept of the "cognitive map" d iscussed in psychological literature, the main differences being the larger distances involved, the representation of continuous environmental gradien ts instead of separate entities, and the essential role of an external refe rence. The navigational system of birds is thus characterized as utilizing a wide variety of environmental cues. Learning processes, which are based o n a simple; innate mechanism, the magnetic compass, integrate these cues an d form complex, experience-based mechanisms, such as the "maps".