Summary
In contrast to earlier navigation hypotheses, based as they are on theoretical constructs deduced from our knowledge of the physical world, the notion of olfactory navigation is an unexpected outcome of empirical research. Referring to sceptical articles on the issue in this journal and elsewhere (e.g. Schmidt-Koenig 1985, 1987, 2001, Wiltschko 1996), and in order to fill a gap in a recent review on avian navigation (Wiltschko & Wiltschko 1999), I describe the most instructive experiments providing evidence that birds are able to home by utilizing atmospheric trace gases perceived by the sense of smell. (1) When released in an unfamiliar distant area, homing pigeons with bisected olfactory nerves fly considerable distances, but fail to approach the home site (Fig. 1, 2, 3). Largely analogous treatments in control birds and experimentals make it extremely unlikely that the failures are due to non-olfactory side-effects. (2) Elimination of trace gases from the inhaled air by means of charcoal filters prior to release, combined with nasal anaesthesia upon release, prevents initial homeward orientation, whereas nasal anaesthesia alone (after smelling of natural release-site air) does not (Fig. 5). (3) Pigeons exposed to natural air at one site and released, without access to natural air, at a quite different site, fly in a direction corresponding to homeward from the site of exposure, but not from the current actual position (Fig. 6). (4) Long-term screening from winds in an aviary at home prevents subsequent homeward orientation from distant sites. Deflecting or reversing winds in a home aviary results in accordingly deflected or reversed orientation (Fig. 7). (5) From areas made familiar by previous flights homing is possible also on a non-olfactory basis. This can be explained in terms of the utilisation of visual landscape features.
In as far as related experiments were conducted using reliable methods, the results are unequivocal. On the whole, they can be understood only provided that the birds are able to deduce their position relative to the home site from atmospheric trace gases, and that this ability requires previous opportunity to correlate current wind conditions with simultaneous olfactory conditions at the home site over a lengthy period of time. As an attempt to explain the underlying system, a working hypothesis is presented which postulates that (a) long-range gradients exist in the ratios among several airborne trace substances and that (b) their directions can be derived, at the home site, from changes of ratios in dependence on wind direction. Atmospheric hydrocarbons investigated by means of gas chromatography in an area covering 400 km in diameter did in fact include such postulated ratio gradients (Fig. 8). Their directions were fairly stable even under varying conditions of weather and winds. Correlations among gradient directions and changes of ratios according to wind directions were also found, but the long-term angular relationships have not yet been definitely determined. By means of computer simulations using actually measured atmospheric values as inputs, navigational performances could be created corresponding to those observed in homing pigeons (Fig. 9).
Experiments with swifts and starlings indicate that olfactory navigation methods are applied also by wild-living species (Fig. 10 and Fig. 11). A schematic model (Fig. 12) illustrates how they might be integrated in the process of long-distance migratory orientation. Also, the question is raised whether long-distance foraging flights of albatrosses (Fig. 13) and other oceanic birds might be controlled by olfactory signals involving long-range ratio gradients of atmospheric trace gases (Fig. 14). A few experiments are suggested for testing the potential application of olfactory navigation in natural bird life.
Zusammenfassung
Die 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.
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Wallraff, H.G. Zur olfaktorischen Navigation der Vögel. J Ornithol 144, 1–32 (2003). https://doi.org/10.1007/BF02465514
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DOI: https://doi.org/10.1007/BF02465514