ENHS Nature Trails Jan 99 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

There's no place like home, especially for a homing pigeon

One of the more amazing behaviors in the animal kingdom is the ability of some species to navigate correctly over long distances. All of us here in the Pacific Northwest are familiar with the return of mature salmon to their home streams after years of life in the ocean and the twice-annual migrations of the grey whale. However, it is less well known that this feat of navigational prowess is repeated in many forms by many species. For example, each year Brazilian green turtles, a species of sea turtles, swim 1200 miles across the open ocean to lay their eggs on the beaches of Ascension Island. Monarch butterflies migrate southwards 1500+ miles each winter to specific overwintering grounds in Mexico and then return to their starting sites the following spring to breed. Some species of albatrosses periodically cease their oceanic wanderlust and journey across the entire Antarctic ice cap to return to their birth site to mate. Even these feats pale when compared to the distance record holder, the arctic tern, which circumnavigates the globe from the Arctic to the Antarctic and back again each year.

Navigational skills are also exhibited on a smaller scale by many other species. Some Eastern US salamanders travel 100s of miles-- albeit very slowly--in the spring to specific breeding grounds. Social insects (bees, wasps, ants) are well known for their ability to journey miles from the hive and return successfully in their never-ending quest for food. Superb navigational skills are even exhibited by the domestic cat and dog, many of whom have an uncanny ability to return home after traversing 10 or more miles of previously unseen territory (the movie "The Incredible Journey" is based on a true story).

The homing pigeon provides an outstanding example of animal navigation. Homing pigeons released at unfamiliar locations will in most cases return to their home roost. This intriguing and repeatable behavior spawned the world-wide sport of pigeon racing. In the US, pigeon races can be as long as 1100 miles with the winners -- those arriving home the fastest -- reaping 5 figure financial rewards for their owners. This uncanny ability to return home from unfamiliar release sites begs two questions: what sensory cues are used by the pigeons to identify the direction of home? and how do they maintain that direction once en route? These two issues are quite distinct and have been studied by numerous researchers around the world with surprising and often contradictory results.

To return home successfully after release from a new location, a homing pigeon must first determine the direction its must fly to get home. The initial direction of flight after release, known as the vanishing direction, is fairly variable. Although pigeons do not all fly off in the same direction, the vast majority go off in the general direction of home. Yet even the errant pigeons, which begin their journey going the wrong way, nearly always return home. Because humans generally accomplish this task using a map, this ability to correctly identify the direction of the final destination in animals has been termed the "map" component of navigation, despite good evidence that animals do not have a visual "map" of their world. Pigeons instead rely on other means to find their way home.

Pigeons appear to use multiple cues to identify their direction of home. Vision is obviously important, yet pigeons whose eyes have been covered with frosted lenses to deprive them from seeing landmarks, still orient towards home. Some even return home safely. The same is true for deafened pigeons, so normal hearing is not essential either. One cue that does affect homing is altering the internal biological clock of the pigeon. This is accomplished by raising the pigeon in an artificial environment with a 24 hr light-dark cycle which is either advanced or delayed compared to normal time. Pigeons whose clock has been advanced by 6 hrs alter their vanishing direction by 90E. A 90E shift in the opposite direction is observed if the clock is delayed by 6 hrs. Sadly, many of these birds never return home. Intriguingly, time-shifted birds orient properly and return home if released under overcast skies.

The time-shift observations lead to the hypothesis by William Keeton, a famous Cornell biologist, that pigeons must not need to see the sun to orient home and that they therefore must use another cue besides the sun. He tested this hypothesis by strapping tiny magnets on the backs of pigeons before releasing them. He found that the magnets interfered with homing but only when skies were overcast. When sunny, birds with magnets were perfectly capable of returning home. Other researchers followed up this study by attaching miniature magnetic coils to the heads of pigeons. These coils, which when activated generate a magnetic field of the strength of the Earth's field, also altered the vanishing direction of these birds but again, only when the sun was not visible. If the sun re-appeared, the bird with active coils on its head immediately veered off its course and returned home. These experiments suggest that both the sun and the earth's magnetic field are used by homing pigeons to identify the direction of their home loft.

But other cues are used as well. Anosmic birds--birds whose sense of smell has been eliminated--consistently have trouble finding home when released from unfamiliar sites. Whether the nasal passages are blocked or the cells that actually detect smells are destroyed or the entire nasal region is rendered inoperative by a local anesthetic, the results are the same: birds don't arrive home after release from an unfamiliar site. Yet birds without a sense of smell find home perfectly well if released from a familiar site, raising the possibility that birds use smell in the absence of familiar visual landmarks. But how are smells interpreted? The answer to this question is not clear but one experiment provides a clue. Pigeons transported to their release site in sealed containers supplied only with filtered air from tanks are unable to return home whereas their cohorts, riding in the same truck in cages open to the outside, return home without difficulty. Perhaps birds need to "smell" their environment on the outbound leg of their journey in order to make it safely back to their starting point. This result implies that birds learn an odor "map" of their home environment by associating specific odors with wind directions. If this is the case, then manipulating the odor "map" should affect homing behavior, and a group of researchers in Italy did just that. They raised birds in lofts whose natural air and its odors were reversed 180E from their normal directions using giant fans and air diverters. Birds raised in these "odor-reversed" conditions went in the opposite direction from home when released from an unfamiliar site.

Where does all this leave us? In science, our judgement is at any time based on the weight of evidence. In the case of homing behavior by pigeons, the accumulated evidence points to not one but several cues--visual, olfactory, time, and magnetic--used by these animals to guide them during their incredibly accurate homeward journeys. But the story is not likely complete; recent data from several labs suggest that homing pigeons may under certain circumstances use very low frequency sound information-- sounds coming from ocean waves and weather fronts--to get home. The jury is still out on this but if true, it is amazing even to this professional animal watcher.

Nathan Tublitz

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