Migrating in flocks may improve direction finding, allowing for the emergence of either skilled or experienced leaders or of collective decisions leading to optimal 'mean directions'. This view is supported by field observations showing that the navigational accuracy in migrating flocks of birds often exceeds that expected from the navigational abilities of single individuals (Thorup & Rab0l 2001, Simons 2004), or that the directional scatter decreased with increase in group size (Rab0l & Noer 1973). It is also supported by an experimental comparison of the orientation of single homing pigeons compared with small flocks, the latter showing less scatter in directions and homing times (Tamm 1980). Moreover, in many bird species, young migrate from their natal areas at the same time as experienced adults and, in some species, notably cranes, swans and geese, the young travel in family parties with their parents. This raises the possibility that, in some species, young birds could learn migration routes from experienced individuals, and that such knowledge could be passed down the generations by cultural transmission. Individuals migrating in flocks often call continually during the journey, which may help to maintain flock cohesion, especially at night or in mist.
Research on some species has shown that young birds can sometimes be influenced in migratory behaviour by the example of others. For instance, 754 young White Storks Ciconia ciconia were transported from the Baltic region (where storks normally migrate southeast) and released at migration time in western Germany (from where storks normally migrate southwest) (Schuz 1950). In contrast to the experiment described earlier, in which the release of White Storks was made after the local birds had left, these releases were made while the local birds were still present. The resulting ring recoveries from the released birds showed a strong tendency toward the southwest, approximating the direction taken by the local storks, and contrary to the southeast direction prevalent in the homeland. Similar results were achieved in North America with 131 Canada Geese Branta canadensis and 213 ducks of various species (Williams & Kalmbach 1943). Inherent directional preferences were apparently overridden by social influences. This is in any case evident in the males of some duck species, which pair with females in wintering areas, and then accompany their partner to her natal area to breed (Chapter 17).
In addition, migration can even be induced in non-migratory stock by the example of other migratory individuals of the same species. Eggs obtained from English non-migratory Mallards Anas platyrhynchos were hatched in Finland and the Baltic region (Valikangas 1933, Putzig 1938). The resulting 116 young were allowed freedom with the local migratory Mallard. They gave 19 recoveries up to 2300 km away, all within the normal winter range of the host populations. In contrast, other young released in autumn after the local young had left remained during winter near the release points. Once again, innate behaviour was apparently overruled by social influences.
Different conclusions could be drawn from experiments on gulls. In some cross-fostering experiments, migratory Lesser Black-backed Gulls Larus fuscus were raised by resident Herring Gulls L. argentatus, and vice versa (Harris 1970). The young of both species became imprinted on their foster species. In autumn, however, the young Lesser Black-backed Gulls migrated as normal, even though they had associated with Herring Gulls until then, and later returned to pair with them. In contrast, the cross-fostered Herring Gulls were mostly sedentary as was usual for their species in this region, but some moved further south than expected (Harris 1970). The implication was that, in these species, in contrast to the waterfowl, genetic influences overrode social ones.
The knowledge that young geese, swans and cranes appear to find their specific wintering localities by accompanying their parents has been used to re-establish populations and migration routes in regions from which former populations had been eliminated (Ellis et al. 2003). Captive-bred young were taught to follow ultralight or microlight aircraft (in the same way that they would naturally follow their parents), and then at the appropriate time, they were led on a long 'migration' to an appropriate wintering area. For example, young Whooping Cranes Grus americana were taught to fly behind an ultralight plane, and released on former breeding areas in central Wisconsin. The plane was then used to guide these young from their release area 1800 km south to former wintering grounds in Florida that had not been used for many years. The cranes over-wintered successfully there. In the spring they returned unaided to their release area, and in autumn migrated again to their new wintering site. Apparently one journey was enough to fix a migration route upon these young birds. Further releases were made subsequently and, at the time of writing, the new population consists of 64 individuals, some of which have nested. In another experiment, Sandhill Cranes Grus canadensis did not necessarily fly the exact route in spring that they had used on their first autumn training flight. In general, the birds used the most direct route rather than repeat the circuitous path necessitated by motorised craft needing to refuel and avoid obstacles (Ellis et al. 2003). This implied that particular landscape details were not of primary importance in the directed migration of these birds.
From 15 similar experiments conducted during 1990-2001, which also included Canada Geese Branta canadensis and Trumpeter Swans Cygnus buccinator, most of the birds reached a new wintering area chosen for them and returned on their own to their starting area next spring (Sladen et al. 2002, Ellis et al. 2003). When Canada Geese were trained to follow an ultralight aircraft for southward migrations of 680 km or 1320 km, 81% returned in the next spring to their locality of training. Again, the birds evidently learnt the route from their first guided journey. These birds probably had an innate migratory direction, like other birds that have been studied, but social factors influenced the actual route and stopover sites used on migration and, in some species, even whether migration occurred.
While many bird species migrate in flocks, others migrate singly, so could gain little or nothing from other individuals. This applies to the cuckoos mentioned earlier, in which the young also depart 3-4 weeks later than adults, and find their own way to wintering areas, presumably relying entirely on inherent directional preferences and time programmes.
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