At this time of the year, only a tiny 
     
     fraction of our summertime bird pop— 
     
     ulation is with us Still, and the vast 
     
     majority has long since departed for 
     
     sunnier climes. Everyone knows that 
     
     migration, or the regular, annual move— 
     
     ment of an animal population, is very 
     
     important to many species of birds, 
     
    

    
     but few of us appreciate the difficul— 
     
     ties birds must overcome in order to 
     
     reach their destinations. 
     
    

    
     When our favourite songbird leaves his 
     
     post in our backyard in early autumn, 
     
     for example, he faces a' flight of sev— 
     
     eral thousand kilometers, over terrain 
     
     he has probably never seen, and often 
     
     in conditions of high wind and near— 
     
     total darkness. And yet the bird 
     
    
   
   
    
     5L.-._.V:: __ ~v-,._-, 
     
    
   
   
    
     ..-...-.. .-.._: .._-----_-;_:, é-vc.-._.3, 
     
    
   
   
    
     and eventually returns to the same 
     
     backyard for the next breeding season. 
     
    
   
   
    
     The mystery of how birds, with their 
     
     relatively small brains and limited 
     
     analytical abilities, can accomplish 
     
     extraordinary feats of navigation, 
     
    

    
     has engrossed scientists for many years. 
     
     But what has been found is not a simple 
     
     straightforward system, but something 
     
     rather like the Oak Island treasure pit, 
     
     with its multiple traps and pitfalls, 
     
     each designed to reinforce the goal of 
     
     confounding the investigator. 
     
    
   
   
    
     Among the first hypotheses in bird 
     
     navigation research was that birds 
     
     navigate by the sun. And yes, they can 
     
     navigate by the sun: by "knowing" 
     
    

    
     from what point on the horizon the sun 
     
     should rise, and "remembering" how 
     
    

    
     long it has been since sunrise, most bird 
     
     species seem capable of setting their 
     
     bearings with considerable accuracy. 
     
     Similarly, night—flying migrants can 
     
     set an accurate course by the stars. 
     
    
   
   
    
       
     
    
   
   
    
     8 
     
    
   
   
    
     But what if the day (or night) is 
     
    

    
     cloudy? Well, it seems that it has to 
     
    

    
     be very cloudy, at least for some species 
     
     of birds that have been shown to be 
     
    

    
     capable of deducing the position of the 
     
     sun, even on moderately overcast days, by 
     
     detecting plane—polarized light that passes 
     
     through the clouds. 
     
    
   
   
    
     But when the sky is heavily overcast, birds 
     
     continue to migrate, and often seem to 
     
    

    
     find their destinations as quickly as they 
     
    

    
     do in more favourable conditions. And this 5 
     
     is where the picture is muddied. Recent 
     
     research has uncovered a whole arsenal of 
     
     systems and back—up systems, which seem to . 
     
     act as supplements to the primary technique 
     
    
   
   
    
     .- l ---i _...._ l,... ‘_ _._ 
     
    
   
   
    
     -ue :resen: 
     
     time we know little of how the hierarchy 
     
    

    
     of techniques works, and we can only guess 
     
     about many of its components. Some of the 
     
     methods that have been suggested as auxil— 
     
    
   
   
    
     liary navigation aids are as follows: 
     
    
   
   
    
     v- t.--::.-...:.;. --r_:.-5;c..c.-. .1: 
     
    
   
   
    
     Magnetism. There is strong evidence that 
     
     some birds are able to detect the earth's 
     
     magnetic field, and set a compass course 
     
     by it. In one species of gull (the ring— 
     
     billed) magnetic cues are thought to be 
     
     even more important than solar ones. 
     
    
   
   
    
     Odors. Although most birds are believed to 
     
     have no effective sense of Smell, certain 
     
     species of seabirds seem to be’able to 
     
     locate their nest sites, and possibly their 
     
     food supplies, by odor. 
     
    
   
   
    
     Changes in gravity. Variations in the in— 
     
     tensity of gravity, as influenced by the 
     
    
   
   
    
     phase of the moon, seem to affect the or- ‘ 
     
     ientation of pigeons kept under labratory 
     
     conditions. How birds could use cycles of 
     
     gravitational intensity to help in navigation 
     
     is unclear.