•Jorge et al. (2009, 2010) doubt homing pigeons use odours to deduce position.•They claim odours activate usage of unknown nonolfactory map cues.•They suggest meaningless artificial odours can ...activate navigation.•I argue that their hypothesis is inconsistent and does not explain their results.
The conclusion that pigeons and other birds can find their way home from unfamiliar areas by means of olfactory signals is well based on a variety of experiments and supporting investigations of the ...chemical atmosphere. Here I argue that alternative concepts proposing other sources of geopositional information are disproved by experimental findings or, at least, are not experimentally supported and hardly realistic.
Many birds are able to find their previously established home site after passive displacement to distant localities far from any familiar landmarks. It is evident that they determine their position ...relative to home neither by path integration nor by deducing it from location-specific solar, stellar, magnetic or infrasonic signals. They need, however, unobstructed olfactory contact with the local atmosphere, and they must have experienced natural winds at the home site over at least several weeks. Further, they require the sun or the geomagnetic field as a directional compass reference. The empirical findings proving and characterizing olfactory navigation by birds (mainly homing pigeons) have been reported elsewhere and are briefly itemized here. It is further shown that trace gases are dispersed in the ground-level atmosphere with some regularity, including differently oriented gradients of ratios among certain hydrocarbons. A preliminary navigation model is based on such long-range ratio gradients and on their relation to wind direction. Within a familiar area around home, pigeons rely not only on olfactory inputs but also on the visual landscape for home-finding. The olfaction-based mechanism has been linked to the piriform cortex, the vision-based mechanism to the hippocampal formation. Adequacy of the term 'map' is discussed with respect to either mechanism. Remaining gaps in our knowledge and challenges for future research are addressed.
An earlier developed model, simulating pigeon homing based on fictitious gradients of atmospheric odours, was applied to actually observed spatial distributions of volatile hydrocarbons. The model ...calculations demonstrate that sufficient information on a bird's current position with respect to home can be derived from the ratios among three or more chemical compounds which gradually vary over distances of several hundreds of kilometres, differently in different directions. Flight directions computed by model birds from such observed ratios are roughly but not perfectly homeward-oriented from most positions within the investigated radius of 200 km around home. Performances of model birds are at least as good as those of real pigeons in the field. According to calculations using atmospheric data collected under different wind directions, the birds might, but possibly need not, take the current weather conditions into account when evaluating olfactory signals. It is necessary, however, that the birds acquire, during their long-term stay at the home site, some knowledge of the directions of relevant gradients. Homing experiments with pigeons as well as measurements of atmospheric trace substances are consistent with the hypothesis that this knowledge is gained by correlating wind directions with specific changes of ratios among a number of compounds. This assumed process requires further elucidation.
Numerous experiments with homing pigeons and other birds strongly suggest that birds displaced to unfamiliar remote areas are able to determine their position relative to home by deducing relevant ...information from atmospheric trace gases perceived by olfaction. These findings induced the hypothesis that ratios between several airborne compounds show roughly monotonic spatial gradients, differently in different directions, over distances of some hundreds of kilometres. To test this hypothesis, 192 air samples were collected, successively in 3 summers, at 96 sites regularly distributed over an area covering a radius of 200 km around Würzburg, Germany. Statistical analysis of the gas chromatographic measurements on these samples revealed that such gradients in the ratios between a number of omnipresent hydrocarbons do in fact exist. The gradients are noisy, but not beyond the range that is compatible with the homing behaviour of pigeons which is noisy as well. The directions of the gradients are remarkably robust against changes of weather, especially of winds. Winds, however, shift the levels of ratios in the whole area without dramatically changing the directional relationships. A systematic angular correlation between variations in space and variations caused by winds could theoretically be utilized by birds for navigational purposes. Our analysis dealt mainly with the most abundant anthropogenic hydrocarbons, which are the best‐suited tracers to detect spatio‐temporal distribution patterns. It is very likely that equivalent patterns exist in naturally emitted volatile compounds as well, given that they are subject to similar variability in the distribution of sources and sinks and similar transport patterns.
ZusammenfassungDie Schlussfolgerung, dass Vögel auf geruchlicher Basis mit Hilfe atmosphärischer Spurenstoffe aus unbekannten Gebieten zu ihrem Heimatort zurück finden, ergibt sich nicht, wie frühere ...Hypothesen über. das Heimfindevermögen, aus einem theoretischen Ansatz, sondern aus einer Reihe von Experimentalbefunden. (1) In entfernte fremde Regionen verfrachtete Brieftauben fliegen nur dann heimwärts, wenn sie riechen können; Tauben mit durchtrennten Geruchsnerven fliegen zwar oft weite Strecken, nähern sich aber nicht der Heimat. Weitgehend analoge Behandlungen der Versuchs- und Kontrollvögel machen es sehr unwahrscheinlich, dass das Versagen der Ersteren auf geruchsunabhängigen Nebenwirkungen beruht. (2) Die Entfernung von Spurengasen aus der Atemluft durch Aktivkohlefilter vor der Auflassung, kombiniert mit nasaler Lokalnarkose während des Abflugs, verhindert heimgerichtete Abflüge, während die Lokalnarkose allein (nach dem Riechen ungefilterter Luft am Auflassort) das nicht tut. (3) Tauben, die an einem Ort der natürlichen Umgebungsluft exponiert, aber dann ohne Zugang zur natürlichen Luft an einem entgegengesetzt gelegenen Ort aufgelassen werden, fliegen so ab, als wären sie am olfaktorischen Expositionsort und nicht am tatsächlichen Auflassungsort. (4) Langfristiges Abschirmen des Windes in der Heimatvoliere bewirkt völliges Versagen der Heimorientierung, Umlenken oder Umkehren des Windes bewirkt voraussagbare Ablenkung oder Umkehrung der Abflugrichtungen am Auflassort. (5) Aus durch frühere Flüge bekannten Gebieten ist auch nicht-olfaktorisches Heimfinden möglich. Es ist durch Nutzung visueller Landschaftskenntnis erklärbar.Soweit entsprechende Versuche methodisch einwandfrei durchgeführt wurden, sind die Resultate widerspruchsfrei. In ihrer Gesamtheit sind sie nur dann verständlich, wenn man folgert, dass die Vögel Spurengase der Atmosphäre verwerten, aus denen sie ihre Position relativ zum Heimatort ableiten können, vorausgesetzt, dass sie dort über längere Zeit die jeweilige Wind-Situation mit der gleichzeitigen Geruchs-Situation korrelieren konnten. Zur Erklärung des zugrunde liegenden Systems dient eine Arbeitshypothese, die postuliert, dass es (a) weiträumige Gradienten in den Proportionsverhältnissen zwischen verschiedenen Spurensubstanzen gibt und dass (b) deren Richtungen sich am Heimatort aus der Änderung der Proportionen in Abhängigkeit von der Windrichtung ermitteln lassen. Gaschromatische Untersuchungen atmosphärischer Kohlenwasserstoffe in einem Areal von 400 km Durchmesser haben gezeigt, dass das erste Postulat grundsätzlich erfüllt ist und dass die Richtungen räumlicher Proportionsgradienten unter verschiedenen Wetter und Windverhältnissen relativ stabil erhalten bleiben. Korrelationen im Sinne des zweiten Postulats sind ebenfalls erwiesen, doch sind die langfristigen Richtungsbeziehungen noch nicht hinreichend geklärt. In Computersimulationen mit gemessenen Atmosphärenwerten als Eingangsgrößen konnten Navigationsleistungen erzeugt werden, die denen von Brieftauben entsprechen.Versuche mit Mauerseglern und Staren weisen darauf hin, dass die olfaktorische Navigation eine auch unter Wildvögeln verbreitete Fähigkeit ist. Die Vermutung liegt nahe, dass sie beim Vogelzug zum Auffinden des engeren Brut- bzw. Überwinterungsareals dient. Es sollte geprüft werden, ob sie auch bei den weiträumigen Exkursionen von Albatrossen und anderen Hochseevögeln eine Rolle spielt.
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.
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