A truly interesting, important article entitled "Infrasound and the Avian Navigational Map" (Journal of Experimental Biology, volume 203, pages 1103-1111, 2000) by Dr JT Hagstrum of the US Geological Survey in California, USA, offers tantalising insight into possible causes of major losses of pigeons from four of five races held in diverse locations in 1998. One of these races was from France to Britain, two were from France to Holland, and two were held in the USA. The narrative and Dr Hagstrum's considered comments may help to shed further light on the whole subject of navigation and the multiple cues that pigeons use firstly, as a compass to point them in the right direction, and then secondly, those that they use as a map in order to arrive home. The stories unfold as follows.

In the first case, on Sunday, June 29/97, the Concorde, the world's first Supersonic Transport (SST), lifted off from Paris bound for New York. On that same day, a race involving over 60,000 pigeons to celebrate the centenary of the Royal Pigeon Racing Association (RPRA), started with a release at 06:30 hr at Nantes, France, with birds headed for lofts all over Britain.

The overall result was a disaster, and very few birds ever made it home. Initially the RPRA considered the cause of this massive failure to be poor weather conditions. However, at an average velocity of 40 mph (65 kph), many of the birds released at 06:30 hr should have been approaching or were actually flying across the English Channel by 11:30 hr. The SST from Paris reaches supersonic velocity after crossing the French coast, and in this instance, would have been flying over the Channel between approximately 11:20 to 11:30 hr. The sonic boom from its passing may well have caught these birds as they neared or traversed the Channel.

According to Dr Hagstrum, in flight the SST creates a cone-shaped shock wave, called a Mach cone, that travels with the bow of the aircraft at supersonic speeds, and moves away from the sides of the cone at the speed of sound. Since atmospheric temperatures and sound velocities increase below the SST, the paths of the downward shock waves are refracted upwards with lateral distance from the aircraft. The distance from directly beneath the flight path of the SST to points on either side where the rays just graze the surface of the earth before being deflected upwardly describes the boom carpet where direct shock waves reach the ground. A formula which includes data on the surface temperature, the elevation of the aircraft and the vertical temperature gradient, showed that the boom carpet on the ground that day would have been about 60 miles (about 100 km) wide. Hence, pigeons flying within that boom carpet could have been adversely affected by the sound. Most of the birds that arrived home that day had velocities averaging between 20-30 mph (30-50 kph).

At the surface, the shock wave of the SST lasts 0.23 seconds within the boom carpet, and an overpressure of about 110 Pascals (128 decibels). The loudest sound that human ears can tolerate without temporary or permanent hearing changes have amplitudes of about 28 Pascals (123 decibels). Dr Hagstrum suggests that if indeed infrasonic map cues (infrasonic = below the frequency range of waves normally perceived as sound by the human ear) are of great importance in navigation by pigeons, strong shock waves from the SST could have disrupted the RPRA race through their interference with the known extremely sensitive low-frequency hearing of the pigeon.

In another incident, on Friday, July 31/98, two pigeon races were released in southern France. One of these races involved 42,845 birds released at Bergerac, and the second involved 13,610 birds released at Soustons, with both releases destined for lofts in Holland. Both shipments were released at 13:00 hr for the approximately 665-mile (1110 km) flight from Soustons, and the 578-mile (930 km) flight from Bergerac.

The race from Bergerac was completed successfully by Saturday afternoon with average velocities for some birds exceeding 60 mph (over 100 kph). However, by that same evening, fewer than 10% of the pigeons liberated at Soustons had homed; by the following Wednesday, about 60% of the birds from Soustons were still missing. During these two races, the SST from New York to Paris was nearing the western coast of France and was expected to arrive at 17:45 hr. The SST usually begins to decelerate from Mach 2 to Mach 1 about 40 minutes prior to landing. The intersection of the shock waves from the approaching SST with the flight path of the birds from Soustons was calculated to occur about 200 miles (320 km) north of Soustons. Since the birds liberated at Bergerac had likely flown past this point of intersection, the good results in this race suggested that they were probably unaffected by the shock wave from the SST, and as a result, homed successfully.

In the next incident, on Monday, October 5/98, birds in two races were released in the northeastern USA. One of these races involved over 1200 birds flying from New Market, Virginia to Allentown, Pennsylvania, a distance of approximately 200 miles (320 km), and the second involved 700 birds from Breezewood, Pennsylvania to the area of Philadelphia, a distance of approximately 150 miles (241 km). The birds from New Market were released at 08:00 hr and were expected to home in about 4 hr. However, the first bird didn't arrive home until 19:00 hr, and only 13 birds arrived that day. As it turned out, only 50 of 1200 birds in this race had homed by the next evening. The release of the 700 birds from Breezewood was delayed until 10:00 hr because of bad weather earlier that morning. Although these birds were expected to home in about 3 hr, it was over 8 hr before the first birds arrived. Only 5 birds arrived on Monday, and by the following Saturday, only 300 of the 700 birds had returned.

The SST from Paris usually arrives at JFK airport, New York, at 08:45 hr, but because of mechanical problems in Paris, it was delayed more than 2 1/2 hr that morning. Hence, a shock wave from the sides of the Mach cone of the incoming aircraft about 310 miles (500 km) away, traveling at 680 mph (1100 kph), and quite possibly enhanced by favourable atmospheric conditions at the time, would have passed through both race courses at about 11:10 hr. Given the release times, it seems probable that birds from both races, flying at an estimated average of 40 mph (65 kph), could have reached the point of intersection with the shock wave of the SST at about 11:00 hr.

The RPRA race from Nantes to lofts in Britain was within the carpet boom of the outbound

SST. However the race from Soustons to Holland and the two within the US were not, yet the shock wave from the SST could have travelled sufficiently far to have had a negative impact on each of these races. Such long-distance transmission of sonic booms had been known to occur years earlier when the number of SST flights between Europe and the USA increased markedly. Dr Hagstrum makes the point that channelling of sound waves near the surface comes about by reflection from the surface and deflection from a zone of either higher temperatures (temperature inversion) or of high winds moving in the same direction as the generated sound.

The possibility that atmospheric infrasounds could provide important information for migrating birds was first raised in 1969, since it is known that infrasound can be detected hundreds to thousands of miles/km from its source. Thunderstorms and winds flowing over mountains are known to produce infrasound and could serve as acoustic beacons for birds.

In discussing the entire subject of sound and navigation, Dr Hagstrum points out that pigeons use quite a number of cues to navigate, including some evidence from Italy that their sense of smell might be one of these several cues. These remarkable abilities of pigeons in orientation and subsequent navigation raise questions about all of the sensory cues they and other birds might use either singly or in combination to locate their position in terms of their home destination (map sense), and to select and maintain that direction during the flight (compass sense). Work done in 1953 showed that their usual daytime compass was the sun but that, even on cloudy days, pigeons continued to home normally. These findings indicated that pigeons have an innate ( = inborn or inherent) magnetic sense that they used as directional compass. This knowledge was supported by the fact that night migrants, which use stars as their compass, also use a similar pattern of orientation and calibration to an innate magnetic compass. Dr Hagstrum notes further that their magnetic sense provides an ideal compass for navigation but that the geomagnetic field makes a poor map. As well, he says that pigeons leave the release site in the home direction and often follow courses that are inconsistent with the idea of following gradients in the magnetic field. He further asserts that a magnetic map also can't explain why pigeons have problems orienting over large bodies of water.

On the subject of magnetic sense, in 1979, Dr Charles Walcott and his colleagues at Cornell University in the USA published an article in which they suggested that pigeons used information from the magnetic field of the earth for orientation. In a number of tests on pigeons, his group found naturally magnetic material that was always located on one side of the skull in a small 1 x 2 mm piece of tissue between the dura mater (the tough, opaque tissue covering the brain) and the skull. This tissue was later found to contain nerve fibres from the ophthalmic (= eye) branch of the trigeminal nerve (one of the 12 nerves that originate directly from the brain of the pigeon), plus connective tissue, and was richly supplied with clusters of iron-containing structures that Dr Walcott's group identified as magnetite.

In the view of Dr Hagstrum, an infrasound map seems to be the most feasible explanation for the long-range map of birds, and certainly pigeons, which have extremely sensitive low- frequency hearing. He makes the following points. Infrasound waves can travel thousands of miles in the atmosphere with little waning. Significant levels of infrasound can be radiated from features in the landscape at frequencies within the hearing range of pigeons. Experiments conducted years ago showed that pigeons can hear tones 11 octaves below middle C, at that time the lowest sounds ever heard by a laboratory animal. Atmospheric conditions that affect the cues provided by the infrasonic map could help to explain the historical departure problems of pigeons at certain release sites, such as those known to occur regularly at the Jersey Hill and Castor Hill fire towers in the northeastern USA. In these locations, the departure bearings of birds released at these sites were usually quite abnormal compared with the more normal departure bearings of pigeons released at most other locations used by the researchers at Cornell University.

In other matters relevant to sound, Dr Hagstrum suggests firstly, that monarch butterflies, which migrate many thousands of miles, might use infrasound during their astounding migrations; secondly, that important changes in local signals from infrasound could be the cause of the unusual behaviour recorded in animals prior to major earthquakes; thirdly, that sea turtles and other marine creatures might use infrasound signals in water to navigate with accuracy across huge expanses of open sea.Similarly, satellite tracking of albatrosses has shown that they can locate far-away islands, even in crosswinds, over thousands of kilometers/miles of open ocean.

Dr Hagstrum concludes by indicating that hearing as the map sense of pigeons could explain the disruptions that occurred in the four pigeon races mentioned previously. Luckily, he notes, such disruptions are rare, because the shock waves, the homeward-bound pigeons and often, specific weather conditions that favour long-distance propagation of these disturbing waves, must obviously coincide. This information may be one more clue in solving the complex, mysterious puzzle that is associated with orientation, navigation and migration in many species of birds, including racing pigeons.