Adults 13 5
Juveniles 0 45 Winter (January)
Adults 4 8
Juveniles 3 21
Difference between age groups in autumn: x2 =40.9, P < 0.001; in winter x2 = 2.2, P < 0.13. From Myers et al. (1988).
birds from long distances is more or less restricted to migratory stock, as shown from comparison of results from migratory and sedentary populations of the same species, such as White-crowned Sparrows Zonotrichia leucophrys (Mewaldt 1964) or Herring Gulls Larus argentatus (Matthews 1968). Experiments with completely sedentary species, such as House Sparrows Passer domesticus, gave no returns from further than about 12 km. In general, the proportions of individuals that returned declined with increase in distance of displacement and, allowing for distance, adults returned in greater proportions than juveniles.
In most homing experiments, even after allowing for expected mortality, the proportion of displaced birds that returned successfully to the capture site was often low. Some birds may have had no fixed home at the time, so had no incentive to return to the capture site; or if they returned, they may have stayed too short a time to be recaught and identified. Moreover, the navigational skills of individuals are likely to have varied because of differences in innate ability and experience. Such variation is well established in homing pigeons, only a small proportion of which are capable of achieving really long-distance returns at high speed (Matthews 1968). Nevertheless, the ability of birds to return to a known site, either after migration or artificial displacement, indicates that individuals are able to find their way back to the place where they were trapped, provided that they are motivated to do so and are tested within the limits of their abilities. Whether migratory or non-migratory, many birds evidently have a map sense, which they can use outside the migration seasons as well as within. The same is true for many other kinds of animals subject to displacement experiments.
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