NAVIGATION
NAVIGATION
When a homing pigeon is released it has to assess how far north, south, east or west it is away from home. A pigeon has sufficient magnetite in its head to make it highly susceptible to changes in LATITUDE, and its own built in biological 24 hour clock helps with it’s LONGITUDE problem. The magnetite has very low magnetic remanence so it is very sensitive to the direction of the magnetic field in both horizontal and vertical planes. Molecular movements within the magnetite are detected as the pigeon circles and are sent by nerves to the brain.
Earth's magnetic field lines vary from our longitudinal meridians in three ways:
- The axis of the magnetic field is at an appreciable angle to the true rotational North/ South axis. (The position and strength of the field are not stable and the field may even turn somersault at some time! At present Earth\’s core is slightly super-rotational relative to the mantle and this is causing a reduction in field strength by nibbling away over the millennia at what is remaining in the semi-permanent magnet solid iron core. If the core becomes sub-rotational soon enough a field reversal could be aborted with the field building up again. The worst condition to worry pigeons would be a very weak transitional field.)
- The magnetic lines dip into the earth at their north and south ends and there is some (constantly shifting) position in the Northern hemisphere where a freely mounted north pointing compass would point vertically downwards instead of to the true North; and vice versa in the Southern hemisphere.
- There are also natural geological deviations in the field lines from the perfect north/south straightness of our man-made meridian lines. Pigeons would be temporarily affected by these aberrations as they would be by mountains; thunderstorms etc. There is no doubt they will be regularly making a re-check during their flight home and adjusting their course accordingly.
A pigeon may not know the magnetic difference between north and south but it recognises the difference between high latitudes and low latitudes (poles and equator) because the field dip-angle changes. To a homing pigeon the dip-angle of the field is the more important factor, not its direction. It appears that a pigeon must remember the dip-angle at the location of its home loft, and experience will then tell it that when its present dip- angle is greater or smaller than that it must have been transported north or south by a proportional distance . (A 15 degree change in dip angle equates on average to a 15 degree change of LATITUDE, (about 1000 miles). Unfortunately for the pigeon this does not apply near the equator where the dip angle can be zero, and in that case if it can\’t distinguish between north and south it will lose its sense of direction. The sun\’s traverse, and the magnetite's residual magnetism (if any), would certainly help.) So far as we know there is no significant difference between a line of force near one meridian and a line of force near any other meridian, and this being the case we have to rule out any magnetic help regarding a pigeon’s LONGITUDE problem.
This leaves us with sunlight (or daylight) in collaboration with the pigeon\’s biological clock. We have to assume that pigeons have, by constantly flying, developed a special sensitivity to daylight periods, even when the sky is overcast. When human beings travel appreciably east or west they get temporarily disorientated by their built- in clock (jet-lag) which disagrees with the timing of dawn and dusk; but this soon passes. With pigeons this disorientation discomfort has turned into a survival skill which has been even more highly developed by selective breeding. The feeling is used to tell them roughly how far east or west they must go to get back to the comfort of their home meridian. Presumably if they were forcibly detained away from home for some length of time they would temporarily lose this sensitivity, and so lose their ability and urge to get back home.
I am highly sceptical of suggestions that pigeons are capable of locating and using the Pole Star.
