MIGRATIONS OF WATERFOWL 



ried past this critical distance in homing experiments, they must search 

 randomly* for familiar cues, consuming their time in wandering rather 

 than in loitering. 



When they have passed this critical point, birds may perceive the 

 changes in the sun's position, then becoming aware of the direction home, 

 which they adopt at once. Using two teams of experienced pigeons, Mat- 

 thews (1955b: 77) found that: (1) In birds released 10 miles from home, 

 "a coarse orientation resulted which could be clearly attributed to memory 

 of landmarks since it became generally better at closer distances." (2) In 

 birds released 35 miles from home, there was a "near random scatter, as- 

 sociated as in the other nonorientated releases with slow returns." (3) When 

 released at 50 miles from home, "good orientation has been achieved by 

 these and many other pigeons." Perhaps 50 miles is near the critical point 

 in this "distance effect" for pigeons. In the Swallow, Wodzicki and Wojtu- 

 siak (Wojtusiak, 1949:101) determined that 120 kilometers (74.5 miles) 

 was the critical point. Griffin's homing experiments with Leach's Petrel 

 (1940:72) showed that birds released at 65 miles and 85 miles from their 

 nesting places came home much less efficiently than those set free 135 

 miles away. No doubt the critical point may vary with the species, with 

 seasons, and with latitude, but the evidence suggests that it may rest 

 roughly between 50 and 100 miles from home. 



One wonders why migrants, such as petrels and swallows, if they find 

 their way home to a familiar range in spring, become lost only 30 or 50 

 miles away when artificially displaced. We must remember that the dis- 

 placement in the homing experiment breaks the thread of continuity link- 

 ing past with present. For birds released at a great distance, the direction 

 home in terms of the sun is perceived, and they approach home in the 

 same manner as migrants. Those set free at closer range may be cued only 

 to compass direction and, without awareness of the way home, must wan- 

 der. In our own human experience, the whole of a range is often more 

 familiar than its parts. Regardless of how I approach the vast Delta marsh 

 from a distance, I can always perceive the way home. But more than once 

 I have become lost within the marsh, not far from familiar landmarks. 



Hitchcock (1952:285) points out that this distance effect may support 

 the Coriolis theory of navigation. "If one makes the assumption that a 

 homing bird wants to return at once," he says, "poor homing performance 

 at short distances is difficult to explain on the basis of visual orientation 



• See Griffin ( 1952a) and Wilkinson ( 1952) for a discussion of random movements. 

 200 



