Important experiments

One of the biggest displacement experiments ever undertaken involved Common Starlings Sturnus vulgaris which in autumn migrate west-southwest from northeast Europe through the Netherlands to winter in northern France and southern Britain. Over a period of years, Perdeck (1958, 1967) caught more than 19 000 individuals within the Netherlands, released about 7500 on site to act as controls, and transported 11 500 others by air 500 km south-southeast of the capture site for release in Switzerland. The subsequent ring recoveries from translocated juveniles were on a line west-southwest of the release site and extended for a similar distance as usual (into southern France and northern Iberia). This indicated that the translocated birds had kept their inherent directional preference and normal migration distance, but not corrected for their displacement (Figure 9.2). They therefore migrated parallel to the normal route, and wintered south of the regular wintering area for their population.

The adults, in contrast, which had already experienced the normal wintering area, corrected for the experimental displacement, and headed northwest towards the normal wintering area with which they were familiar. They had evidently 'realised' they were off course at the release site, and took a bearing different from usual in order to correct for this. The age-related (or experience-related) difference in behaviour persisted whether the birds were released in separate juvenile and adult groups or in mixed-age groups. Recoveries from subsequent years showed that, while juveniles tended to return to the new wintering areas reached after their displacement, both age groups continued to return to their original breeding areas (with which they were familiar). The conclusions were that: (1) autumn-migrating adult Common Starlings Sturnus vulgaris used true goal orientation (homing) to reach their wintering areas, whereas the juveniles used one-directional orientation, moving on a fixed bearing for a fixed distance; and (2) juveniles were able to fix both their breeding and their winter quarters in their first year and return there in subsequent years by goal orientation.

In another experiment, Starlings Sturnus vulgaris were transported in the migration season from the Netherlands to Barcelona in eastern Spain (Perdeck 1967), an area much favoured by wintering Starlings. Despite this, the young birds moved on west for several hundred kilometres across the width of Spain. The implications were again that the length of the migratory journey was controlled by internal influence, and that migration continued in the normal direction from the release site while ever the drive to migrate persisted.

Similar displacement experiments were done with Chaffinches Fringilla coelebs, White Storks Ciconia ciconia, Sparrowhawks Accipiter nisus, Hooded Crows Corvus c. cornix, and various ducks. All gave similar results, with a difference in response between juveniles and adults (Drost 1938, Schuz 1938, 1949, 1950, Ruppell 1944, Ruppell & Schuz 1948, Perdeck 1958, Bellrose 1958, Matthews 1968, Wolff 1970, see also Chapter 12). They were consistent with the view that young birds inherited directional preferences which enabled them to head off towards wintering areas appropriate to their population. The fact that adults, with experience of a specific wintering area, could correct for displacement does not detract from this conclusion. It adds to what ring recoveries have often shown: namely that birds with experience of a location can re-find it. They must therefore have

Figure 9.2 Difference in recovery patterns of adult and juvenile Common Starlings Sturnus vulgaris displaced about 500 km south-southeast (Netherlands to Switzerland) off their normal west-southwest migration route through the Netherlands. Birds were caught on migration at site F, transported and released at Basel (R), Z├╝rich (R2) and Genf (R3). Filled circles show recoveries of juveniles and open circles of adults during the ensuing autumn-winter. From Perdeck (1958).

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