Pigeons and some other birds with a homing sense literally have to follow their noses. A West German research team reports it has proved beyond doubt that smell is the primary navigational sense for such birds.
Ornithologists earlier found that homing birds can use the sun and Earth's magnetic field as compasses to guide their flight. But, as any hiker knows, you need a map as well as a compass to find your way about. What birds use as a ''map'' has been a mystery.
Hans Georg Wallraff and his colleagues at the Max Planck Institute of Ethology at Seewiesen believe such a map may be based on odors, according to an MPI report. Wallraff suggests that trace elements in the air may have systematic changes of concentration that act as a kind of grid pattern for birds that can sense them. These elements, which have yet to be discovered, may be widely dispersed over Europe, Wallraff says.
Floriano Papi of Italy's Pisa University had discovered 12 years ago that pigeons depend on their noses to find their way home. Since then, the Seewiesen team, working sometimes with Pisa scientists, has carried out many olfactory navigation experiments with pigeons. These, the MPI report says, ''have proved once and for all that homing pigeons mainly follow their nose to get their bearings over long distance.''
Pigeons temporarily deprived of their sense of smell often had a hard time finding their home lofts. The greater the distance, the more severe this handicap became. On the other hand, the scientists have shown that normal pigeons use scents for orientation over distances at least as great as 400 miles from home.
What is more, the Wallraff team says it has shown the sense of smell takes precedence over the solar and magnetic compasses. The MPI report notes that ''. . . birds can do without unrestricted perception of magnetic fields . . . but not without their sense of smell.''
Thus, if their findings are confirmed by other investigators, the MPI ornithologists would seem to have settled the issue of whether, and to what extent, homing birds depend on smell.
The possibility of there being some kind of odor map that extends semipermanently over thousands of square miles is another matter. Wallraff himself has reservations about this, saying the notion ''overstrains'' his imagination. Yet it is the only idea he has found that makes any sense at all of his results.
The odor map would be based on a few distinctive, persistent, and highly diluted substances. Their concentrations would change gradually from place to place. The contours of increasing and decreasing concentration of these substances would be a guide for birds that could detect them. Together with their solar and magnetic compass sense, this would give birds a powerful navigational tool.
Whether or not this particular concept of an odor map turns out to be true, it is now obvious that a sense of smell is significantly more important to many birds than had been realized even a decade ago. Meanwhile, and until scientists find out just what it is that birds sense in the air, one can only wonder what role industrial pollution may play. Poisoned soil in Kenya
Coffee export is a mainstay for Ken-ya's economy. But the African nation's heavy use of fungicides on coffee plantations may seriously damage this resource.
These copper-based chemicals have so poisoned the soils of coffee plantations that no other crops can be grown there. Even coffee trees are beginning to show copper damage. Nick Lepp of Britain's Liverpool Polytechnic and Nick Dickinson of Kenyatta University College in Nairobi have found copper concentrations in plantation soils to be 20 times that of untreated areas.
As reported in New Scientist, copper does not concentrate in coffee beans themselves, so no health hazard exists. But the scientists' findings do indicate trouble for Kenya's agriculture. It is another instance where unwise use of pesticides has caused serious environmental damage. Deep-sea air pollution
Although it's no novelty to find humanity's dirt in the open ocean, it still seems astounding to find fly ash deep beneath the mid-Atlantic. Yet sediment traps 3,200 meters (3,520 yards) beneath the surface, and 1,000 meters above the ocean floor, have captured these polluting particles in the Sargasso Sea southeast of Bermuda.
The microscopic spheres are the kind of particles produced by coal-fueled power plants. Bermuda has no such units. So the polluters are likely to be in North America.
W.G. Deuser of Woods Hole Oceanographic Institution in Massachusetts and K. Emeis, V. Ittekkot, and E.T. Degens of the Geological-Paleontological Institute in Hamburg have maintained the sediment traps for five years. Describing their findings in Nature recently, they pointed out that the particles seem to find their way from the surface to deep water fairly quickly. Apparently they are gobbled down by tiny animals. Then they sink rapidly as part of the animals' waste products and other organic debris. This means that the distribution of the fly ash reflects its travels with the wind rather than ocean currents.
The four scientists note that the particles are useful as indicators of deep-sea industrial pollution. Also, since they have been produced for only about a century, their presence is a rough time marker for scientists studying seabed sediments.
Oceanographers have long known that sediments often contain exotic materials such as meteoritic dust. Now they have to take account of man-made, as well as cosmic, sources of strange particles deep beneath the ocean.