Return of displaced birds to wintering sites

Turning to winter, some of the largest displacement experiments involved ducks (McIlhenny 1934, 1940). In the United States, Mallard Anas platyrhynchos, Northern Pintail A. acuta and Green-winged Teal A. carolinensis use as main fly-ways the Atlantic and Pacific coasts and the Mississippi valley. In one experiment, 440 ducks were taken from their winter quarters in the Mississippi valley, ringed and released on the coasts. Of 90 subsequently shot, 79 had returned to the Mississippi flyway. No distinction was made in this experiment between adults and juveniles or between recoveries in the same or subsequent winters. However, in another experiment in which 895 drake Mallard were transported from the Mississippi flyway about 1800 km to the Pacific flyway, recoveries in the same winter of both juveniles and adults were mostly in the area of release. Recoveries in subsequent winters showed that birds transferred as juveniles mostly maintained their westward displacement, whereas birds transferred as adults mostly returned to the original site (Bellrose 1958). Evidently, both age groups could return to areas previously visited, but the adults had greater attachment to their original area, having spent more time there than juveniles.

Displacement of wintering White-crowned Sparrows Zonotrichia leuco-phrys from San José in California gave some remarkable returns in later years (R. Mewaldt 1964; Figure 9.4). Of 411 birds displaced by aircraft 2900 km east-southeast to Baton Rouge in Louisiana, 26 were recaught the following winter in the same small garden at San José where they were originally captured. They formed 21% of the 123 which would normally have been expected to survive and return to the traps if no displacement had been made. In the next winter, of 660 birds transported 3860 km eastward to Laurel, Maryland, 15 were recaught at the capture site in the following season. Of special interest were six of 22 birds displaced to Laurel after they had already returned from Baton Rouge. All birds are presumed to have returned to their breeding areas in northwestern North America in the interim, and one was found during the spring migration period in an intermediate locality; again adults returned in greater proportion than juveniles. The numbers were small, but the annual mortality in this species was around 50%, and low site fidelity as well as poor navigation may have depressed the recorded return rates.

The rate of return next year by young sparrows to the capture site varied according to date of displacement. Those displaced before mid-January were less likely to return, possibly because site attachment had not occurred before then. In addition, however, birds displaced early in winter must live for a longer period than those displaced in late winter before their return can be observed next year (Ralph & Mewaldt 1975, 1976).

In another study, 30 adult and 69 juvenile Sanderlings Calidris alba were transplanted from Bodega Bay in California 200 km south (Myers et al. 1988). Overall, 60% of adults but only 4% of juveniles later returned and settled in Bodega Bay,

Figure 9.4 Long-distance displacements of Golden-crowned Sparrows Zonotrichia atricapilla and White-crowned Sparrows Z. leucophrys gambelii from a wintering site at San Jose in California to Baton Rouge (Louisiana) and Laurel (Maryland). Shaded area depicts the breeding range of the populations under study. The birds were presumed to have returned to their breeding areas after displacement before they returned and were recaptured in the original wintering area. A recovery of a bird displaced to Laurel was reported from the spring migration period to the northwest. After R. Mewaldt (1964).

Figure 9.4 Long-distance displacements of Golden-crowned Sparrows Zonotrichia atricapilla and White-crowned Sparrows Z. leucophrys gambelii from a wintering site at San Jose in California to Baton Rouge (Louisiana) and Laurel (Maryland). Shaded area depicts the breeding range of the populations under study. The birds were presumed to have returned to their breeding areas after displacement before they returned and were recaptured in the original wintering area. A recovery of a bird displaced to Laurel was reported from the spring migration period to the northwest. After R. Mewaldt (1964).

either in the same or a subsequent winter (Table 9.1). The age difference was most apparent among birds moved in autumn (from which no juveniles returned) (X2 = 40.9, P < 0.001), but was non-existent among birds moved in winter. By that time the juveniles had been in Bodega Bay long enough to have become attached to the site, and returned in the same proportion as adults. The adults were already familiar with the site in autumn, from having spent their previous winters there. Clearly, the time spent at a site influenced the tendency to return there after displacement.

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