A summary 2011
The two prevailing ideas on long distance navigation are based around two ideas:
- Magnetic cues
- Olfactory cues (smell)
It is clear that near home animals, birds and humans build up a map of their neighbourhood with remembered sights smells and landmarks. This is all mediated by the hippocampus.
There is some evidence of birds using the polarised rays of the sun to orientate by.
Above all we must always remember that all navigation is based on using all the available clues, including the direction of the sun. It is difficult.
There quite a number of difficulties with this. Firstly if animals used the magnetic cues, as we do, with a compass, this approach cannot work. If you are a homing pigeon released from somewhere you do not know, you would need a compass, and a map showing where you are released and where home is, and then you can work out the bearing. This does not work.
You can use the dip or inclination of the magnetic field – nearly vertical at the poles and flat at the equator.
Indeed we think that these magnetic field inclinations are the most likely way that animals and birds use the magnetic field. But magnetic fields, inclinations and strengths can vary enormously in different locations and at a local are not the neat bands like the lines of latitude.
There is research that shows that whales tend to strand in places that magnetic fields turn into the land.
It was believed that animals and especially homing pigeons had magnetite beads in their beaks that act as a magnet that showed them the way. Unfortunately this idea was shattered by recent work by David Keays (Institute of Molecular Pathology Vienna) who has sliced up a number of pigeons starting with their beaks to find these magnetite clusters. He found nothing in the beak but magnetite beads were spread all over the body and wings.
Peter Hore of Oxford University has done magnificent work on Cryptochromes which show that it is very likely that the cryptochrome reaction probably formed in the eye can detect magnetism. This is very interesting and maybe the way that magnetism is in fact recorded.
Olfactory Clues (Smell)
The leader in this field is Anna Gagliardo of the Department of Biology Pisa, Italy. She and her colleagues have done very thorough work indeed on this subject.
For instance, they have released Shearwaters about 400 km from their home colony some with magnets, unmanipulated controls and birds made anosmic by washing the olfactory mucosa with zinc sulphate, which destroy specifically olfactory neurons. The ones where the olfactory neurons were destroyed go lost and never made it home. This fit with works done separately by Mouritsen and Bonadonna on albatrosses and other seabirds: magnets never affect homing in these birds.
Gagliardo says that homing pigeons use olfaction for navigation: the supposed mechanism is not that birds (both for pigeons and petrels) just smell their homes. In homing pigeons the mechanism proposed on the basis empirical data is that they learn at their loft area the wind-born odours with the direction of the winds, once at the release site they recognize the local odours so to determine the direction of displacement.
See Background information: here is a pdf of a review of Wallraff’s book “Avian Navigation: pigeon homing as a paradigm.” 2005 Berlin: Springer Verlag.
You will find pieces on the web site that explain that I believe that navigation is like a rope where the individual strands (magnetic and olfactory cues are part of these) become part of a bundle that makes navigation possible. Ingo Schiffner, has proved, mathematically, that homing pigeons use at least four different cues and use different cues at different parts of the journey.
The important thing is that navigation is difficult and you always use all the information that you can get. A vague sense of direction at the beginning of the flight may the best you can get at that time but as you approach home you are looking for more and more positive indicators as they become available.
The point of this web site is to try to try to understand what is happening and put all the different strands in perspective. We will find that different species use different strands in different ways.