Navigation by homing pigeons: updated perspective

During the last decade, some progress has been made in recognizing and sepa rating 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 b asically 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 hom e-finding from unfamiliar areas everywhere on earth. Empirical research in this context has now also entered the atmosphere. Starting from a theoretic al navigation system based on gradients of ratios among three or more atmos pheric trace substances, volatile airborne compounds were investigated by m eans 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 obser ved in pigeon homing. Angular relationships between variations of compound ratios in space and in dependence on wind direction indicate possibly usefu l atmospheric preconditions for the development of an "olfactory map". Thes e interrelations need further investigation and the chemical compounds actu ally used by pigeons are yet to be identified. (2) Various experiments using olfactory and/or visual deprivation, partly c ombined 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 orie ntation by means of topographical features seems to rely on an aerial panor amic view over an extended area rather than on the distinction of small-sca le landmarks observed only in a narrow range along previous homing routes. In the past, its possible influence on experimental results has probably of ten been underestimated. (3) Almost as important as the identification of factors used for home-find ing is the recognition, of other factors;that influence the pigeons' depart ure directions from the release site. Three such components have been ident ified which may modify or mask the directional output of the home-navigatio n system. Initial bearings of pigeons are often (a) polarized towards a lof t-specific preferred compass direction (PCD), (b) deflected by attracting o r repelling topographical features, and (c) influenced by the compass direc tions flown in previous homing flights. Under certain circumstances, initia l orientation can be disturbed by treatments inducing stress or preventing the opioid-controlled compensation of stress. Such treatments, as for insta nce transport in darkness or in an oscillating magnetic field, can temporar ily abolish the pigeons' motivation to orient homeward, but do not affect t heir ability ultimately to find the way home. There is no indication that any other kind of information, neither olfactor y nor visual, might be used by displaced pigeons to determine their positio n relative to home. Seemingly conflicting and controversial issues assemble d in the literature are discussed in the Appendix.