Science
Finding their way
ernard Dixon
Convincing new evidence has just apPeared which virtually solves one of the 11131.0st intriguing puzzles ever tackled by ,si°10gists — the question of how homing ilgeons manage to navigate such great distances, from strange localities, with Incredible accuracy. The first tangible clues to the riddle came in the 1950s, when seVeral investigators established with leasonable certainty that pigeons some"13W use the Sun in determining their flight Paths. Further research has been largely 19conclusive. One possibility is that Pigeons can distinguish differences in the altitude of the Sun at the home loft and the point of release. But this interpretation the one now confirmed by some Lagenious experimentation by Dr A. ,Whiten at Bristol University — has been „.°°gged down in ambiguous research which "as. failed to either confirm the hypothesis °I" Invalidate it. These attempts throw interesting light °n the difficulties which scientists can kencounter in pursuing what may appear to "e a straightforward investigation based 911 a simple hypothesis. One approach tried Itn recent years is to use mirrors to alter ih,e apparent arc of the Sun at the pigeon oft. However, although birds tested in this .,1;411' showed normal orientation after 10-14 "Ys of the treatment, it is possible that eY remembered the real path of the Sun. TL pigeons used were old and may "aye used their memory to foil the eXPerimenters. The same applies to ,exPeriments in which research workers o_ave kept pigeons out of sight of the Sun for several days before releasing them. Again, memory could invalidate the results Fif such experiments — which in any case nave yielded conflicting data. For his attack on the problem, Dr Whiten used a different approach. He placed his pigeons in an apparatus resembling the "skinner box" used to condition rats by rewarding them with food for behaving in a particular way. Whiten's box could be revolved around a vertical axis by remote control and had a key which the pigeon inside could peck to gain access to food when it was made available from a hopper. In use, a screen surrounded the box and rotated with it, obscuring all landmarks from the bird's vision.
Whiten's first step was to try to train his two birds — both experienced homing pigeons — to discriminate, from the Sun's altitude, when they were pointing towards their home loft at Bristol University. Field trials were held at three widely separated sites — Ombersley, 58 miles north of Bristol; Oxford, 60 miles east-north-east; and Bradnich, 60 miles south-west. In each case the pigeon was tested by being orientated towards Bristol (and rewarded with food in the hopper) or in the opposite direction (no reward) to see whether, by the frequency with which it pecked at the key, the bird could distinguish the two directions. Daily sessions consisted of 36 trials, each lasting 30 seconds. The sequence was random and between trials the box rotated slowly either clockwise or counter-clockwise. The result was unambiguous: the birds clearly discriminated between the two directions. They did so only in sunny conditions. When the weather was overcast — and also when the Sun dipped below the screen around the apparatus late in the day — the birds' responses became random. Whiten also tried to train another bird, under overcast conditions, to respond in a single compass direction, but it failed to distinguish that direction when tested. Did the birds discriminate by distinguishing " home " from "non-home," on the basis of the Sun's altitude, or was this rather a case of reversal learning, based on discrimination between north and south? Whiten resolved this by using a periscopetype arrangement of mirrors to change the apparent altitude of the Sun. Tests using this additional apparatus showed that the birds were indeed deceived, responding significantly more frequently in the false home direction than in the true home direction. Finally, Whiten conducted trials in the laboratory, mimicking the Sun by a projector lamp moving in a carriage set to produce any required Sun arc. Again, the results showed that, after a single day's training, the birds could assess the Sun's altitude. 'Naive' birds which had not been trained previously required several weeks of training before they were' able to discriminate.
In his report, published in Nature (vol 237, p. 405), Dr Whiten is careful to concede that these experiments do not establish conclusively that pigeons navigate by using their ability to discriminate the Sun's altitude. He confines himself to arguing that "the very existence of such an ability strongly supports such a conclusion." But his results are very much stronger than those of suggestive experiments in the past. Taking old and new evidence together, the conclusion now seems inescapable.
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