1008 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-16, NO. 5, SEPTEMBER 1980 z MAGNETIC ORIENTATION IN HOMING PIGEONS (invited) zyx Charles Walcott Abstfact -Homing pigeons appear to use both a "map" and "compass" s y s t e m to find their way home. Under sunny skies, the sun is used as a compass. But when the sun is obscured, pigeons appear to use the earth's magnetic field. The sensory basis of the pigeon's map is unknown but the disorientation ,of pigeons at magnetic anomalies under sunny conditions suggests that the map may be partlybased on magnetic cues. Recently small deposits of the mineral magnetite have been found in two regions of the pigeon's head. It is conceivable that this material may somehow be involved in the pigeon's sensitivity to magnetic fields. I. INTRODUCTION Homing pigeons have been bred for years for their ability to return quickly to their home l o f t s . Pigeon racers win or lose hundreds or even thousands of dollars on the speed with which their birds can return. As zyxwvutsrqponmlkj a result they practice a rigorous selection process-- only birds that return home breed and those that return slowly are candidates for pigeon pot pie. zyxwvutsr The result of this process is a bird that onecantakehundreds of miles away from its loft in any direction and which, on being released, will promptly f l y home. The question is of course, how does it know which direction to fly? And despite years of investigation we still don't know the answer to this question. Recently, however, an in- creasing amount of evidence has been found that the earth's magnetic field plays some role in the pigeons' ability to find its way home. Inthispaper I will attempt to summarize that evidence. (Emlen [l] Keeton [Z] and Schmidt-Koenig [3] all provide an excellent review of the literature). 11. COMPASS SYSTEMS Pigeonsreleasedundersunnyconditionsappear to use the sun as a compass; shifting a birds internal clock by 6 hours leads to a 90" e r r o r i n the homeward direction [4]--just what we would predict for such an error in the pigeon's estimation of the sun's position. This experiment also suggests that the pigeon's orien- tation is a two step process: a pigeon released at an unfamiliar location must first determine the direction i n which home lies. W e c a l l t h i s f i r s t s t e p the "Map" or "navigation," although we don't mean to imply that a pigeon has a map in the sense of a road map or chart--merely that the bird somehow finds the direction to home from the release site. Once a bird makes the determination of which direction to fly, it uses its sun compass to find that direction.[5] The results of the clock shifting experiment support this idea of a two step process because the shift that one would pre- dict from the error inthe bird's internal clock is always with respect to the direction of the control birds which for most release sites is generally toward home. Repeatingthisclockshiftexperimentunderover- cast skies when the sun is not visible results in pigeons that head directly for home, indicating that Manuscript received March 8, 1980. The author is with the Department of NeurobiologyandBehavior, The State University of New York at Stony Brook, Long Island, New York 11794. the birds cannotsee the sun through the overcast and that no other components of the birds' orientation system are upset by the 6 hour clock shift. [61 Glueing small magnets on the pigeon's back revealed that under overcast skies, pigeons were often disoriented by the magnets but not by brass bars of equal size and weight. [7] If a pair of small coils were placed around the pigeon's head and enough current from a small mercury cell flowed through the coils to induce an earth strength magnetic field around the pigeon's head, the pigeon's behavior de- pended on the polarity of the induced field. If a magnetic compass placed between the two coils i n the same position as the pigeon's head, had its south seeking end pointing up, the coil was termed a "sup" and pigeons released withsuch a coil under overcast homed normally. In contrast pigeons with thecurrentflowreversed ("Nup'' c o i l s ) frequently flew 180" away from the correct home direction. [8] A pigeonwearing a pair of coils is subject to two p= 0000 / - Fig. 1. A pair of coils which induced an earth- strength field of about 0.6 Gauss around the pigeon's head had a substantial effect on the pigeon's orienta- tion under overcast skies. Each dot around the circle represents the direction in which a pigeon vanished from the release point. The arrow in the center of the circle is the mean vector of the distribution, its length is proportional to the degree of clumping of the release bearings. The "p" value represents the probability that the observed distribution of vanishing bearings is no different from a random dis- tribution. The direction to the home l o f t is at the top of the diagram. The top set of circles repre- sents the behavior of birds on their first exposure to coils. Pigeons that returned from thereleases were then equipped with coils of the opposite polari- ty and released for a second time; their vanishing bearings are shown in the lower circles. 0018-9464/80/0900-1008$00.75 zyxwvu 0 1980 IEEE