The finding that captive migrants, during periods of migratory restlessness, flutter in their cages with an appropriate heading, suggested that migratory directions were also under genetic control. This could be easily tested on Blackcaps because in Europe this species shows a migratory divide, with those from the western part of the continent migrating southwest in autumn and those from the eastern part southeast. Moreover, the eastern populations migrate around the eastern edge of the Mediterranean to East Africa, so they must change their course (by about 60° clockwise) from southeast to south-southwest about halfway through their autumn journey. Western populations migrate southwest and winter mainly in the western Mediterranean region; they therefore have a shorter journey and do not obviously change direction en route.
With birds from two localities lying west and east of the migratory divide, hand-raised and tested under identical conditions, this behavioural difference did indeed seem to have a genetic basis (Figure 20.3). In orientation cages, Blackcaps from western Germany kept a constant southwest course throughout the season, as expected from their relatively direct route to the Iberian Peninsula and northwest Africa, whereas those from eastern Austria started with a southeast course in September-October, and changed within a 10-day period to a south-southwest course in November (Helbig et al. 1989). It seemed that young Blackcaps not only inherited from their parents a general starting direction, but a fairly detailed time-direction programme appropriate to their dog-leg migration route. Moreover, when Blackcaps from both sides of this divide were cross-bred, the firstgeneration hybrids moved directly southward, behaving in a phenotypically intermediate fashion between both parent groups, with no difference in the amount of directional spread (Helbig 1991b). These findings thus confirmed the genetic basis of migratory directions and, through cross-breeding, revealed a phe-notypically intermediate mode of inheritance. The implication is that, if birds from southwest and southeast migrating populations interbred where they meet, their offspring would inherit intermediate, and probably inappropriate, directional preferences. Any selection against the hybrids could help to maintain the differences between populations, perhaps leading eventually to taxonomically distinguishable subspecies (Chapter 22).
Evidence for an intra-seasonal change in direction, genetically determined and endogenously programmed, was also found in Garden Warblers Sylvia borin (Gwinner & Wiltschko 1978) and Pied Flycatchers Ficedula hypoleuca (Beck & Wiltschko 1988) in Europe, and in Yellow-faced Honeyeaters Lichenostomus chrys-ops in Australia (Munro & Wiltschko 1993).
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