Mirrorimage Migration

At a broad geographical scale, juveniles of many species appear annually in localities that indicate they have followed a direction that is a mirror-image of the correct direction. Mirror-image migration supposedly occurs when a departing bird takes a correct bearing with respect to the north-south axis, but chooses the wrong east-west (right-left) side of that axis. Instead of migrating southwest, say, the bird migrates southeast, with distance, timing and other aspects of the migration remaining unchanged. This apparent situation was studied in some northern North American warblers, about 30 species of which normally migrate far to the east of California, but have been recorded there in autumn, with Blackpoll Warblers Dendroica striata and Palm Warblers D. palmarum together forming more than half the records. Simple drift on westerly winds could not account for this pattern, for species would then be expected to occur in numbers proportional to their overall abundance, and over a much wider span of latitude than that over which they occur. Instead, such records are more confined geographically, and more consistent with the hypothesis of mirror-image navigation (De Sante 1973, 1983a, 1983b, Patten & Marantz 1996). Moreover, vagrant Blackpoll Warblers that were tested in orientation cages showed both the correct migratory direction and the mirror image of that direction. Other apparent mirror-image migrants to California from eastern North America include the Brown Thrasher Toxostoma rufum and Dickcissel Spiza americana (Table 10.4).

The same hypothesis has been proposed to explain the occurrence of various western North American species, including Dendroica warblers, in eastern North America in spring, which would imply a mirror-image error in the direction taken from wintering areas (McLaren 1981). Some 38 records of western Dendroica warblers lay on a direct mirror-image route at around 45° from wintering areas in western Mexico northwest towards Nova Scotia (McLaren 1981). The same mechanism could explain the spring occurrence of southwestern species, such as Green-tailed Towhee Pipilo chlorurus, Rock Wren Salpinctes obsoletus and Band-tailed Pigeon Columba fasciata on Nova Scotian Islands.

Other possible examples of mirror-image misorientation include: (1) the occurrence in western Europe in autumn of Eurasian species that normally migrate broadly southeast or east then south from their breeding areas (such as Arctic

Warbler Phylloscopus borealis, Greenish Warbler P. trochiloides, Richard's Pipit Anthus novaeseelandiae and Pechora Pipit A. gustavi) (Table 10.2); (2) the occurrence of Citrine Wagtail Motacilla citreola that normally migrates from eastern Siberia south-southwest in autumn but occasionally reaches eastern Australia, suggesting mirror-image flights to the south-southeast; (3) vagrants of numerous species that breed in far eastern Siberia and normally migrate south-southwest to Southeast Asia, but occasionally follow a mirror-image route to western Alaska and down the North American coastline. On this last route, Eurasian Wigeon Anas penelope and Tufted Duck Aythya fuligula are regular, while many others are occasional (Table 10.4). Re-examination of off-route migration records elsewhere may yield further examples of possible mirror-image misorientation.

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