Navigation by homing pigeons: updated perspective

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Abstract
During the last decade, some progress has been made in recognizing and separating the principal components determining the homing behaviour of pigeons. This study, an updated continuation of a previous review (WALLRAFF 1990), focuses on new results and improved insight into three constituents that basically characterize pigeon homing. (1) It has been confirmed by continued experimental research that olfactory access to environmental air appears to be a necessary precondition for homefinding from unfamiliar areas everywhere on earth. Empirical research in this context has now also entered the atmosphere. Starting from a theoretical navigation system based on gradients of ratios among three or more atmospheric trace substances, volatile airborne compounds were investigated by means of gas chromatography in a circular area with a diameter of 400 km in Germany. Ratio gradients in a number of hydrocarbons were found which imply spatial information suitable for navigational performances on a level observed in pigeon homing. Angular relationships between variations of compound ratios in space and in dependence on wind direction indicate possibly useful atmospheric preconditions for the development of an “olfactory map”. These interrelations need further investigation and the chemical compounds actually used by pigeons are yet to be identified. (2) Various experiments using olfactory and/or visual deprivation, partly combined with a shifted sun compass, strongly suggest that inside a familiar area pigeons make use of the visual landscape to find the way home. Thus, in a familiar area the home-finding system appears to be redundant in that it can utilize both olfactory and visual environmental signals. Visual orientation by means of topographical features seems to rely on an aerial panoramic view over an extended area rather than on the distinction of small-scale landmarks observed only in a narrow range along previous homing routes. In the past, its possible influence on experimental results has probably often been underestimated. (3) Almost as important as the identification of factors used for homefinding is the recognition of other factors that influence the pigeons' departure directions from the release site. Three such components have been identified which may modify or mask the directional output of the home-navigation system. Initial bearings of pigeons are often (a) polarized towards a loft-specific preferred compass direction (PCD), (b) deflected by attracting or repelling topographical features, and (c) influenced by the compass directions flown in previous homing flights. Under certain circumstances, initial orientation can be disturbed by treatments inducing stress or preventing the opioid-controlled compensation of stress. Such treatments, as for instance transport in darkness or in an oscillating magnetic field, can temporarily abolish the pigeons' motivation to orient homeward, but do not affect their ability ultimately to find the way home. There is no indication that any other kind of information, neither olfactory nor visual, might be used by displaced pigeons to determine their position relative to home. Seemingly conflicting and controversial issues assembled in the literature are discussed in the Appendix.