Some bird species, leaving a particular point on the migration route, show much greater spread in their migration directions than others, depending partly on the locations and distributions of their respective wintering areas. This is obvious both from observations of birds on migration, and from subsequent ring recoveries (Busse 2001). However, in some species, it is also apparent that juveniles on their first autumn journey show much greater spread in departure directions than adults. Such an age difference has been reported from Swedish or Finnish ringing recoveries for Spotted Flycatchers Muscicapa striata (angular spread 8-18° for adults, 13-23° for juveniles), Willow Warblers Phylloscopus trochilus (9° for adults, 21° for juveniles), Honey Buzzards Pernis apivorus (8° for adults, 12° for juveniles) and others (Alerstam 1990b, Fransson & Stolt 2005). This difference in autumn directional spread between adults and juveniles is also obvious among birds tested in orientation cages to find their directional preferences (De Sante 1973, Able 1977, Moore 1994). If juveniles were to continue migrating on their more extreme directions, many would end up well outside the usual migration range, as is again evident from observations and ring recoveries. However, it is not clear whether the juveniles that take the wrong direction suffer greater mortality, and so are less represented in the adult samples, or whether they learn from their mistakes, and perform better in later life. In any case, one plausible hypothesis is that some autumn vagrancy is simply a consequence of the wide range of directional preferences normally existing within some bird populations and evident chiefly in juveniles. Directional errors have been classed as 'disorientation' (inability to follow any consistent direction) or 'misorientation' (ability to follow a consistent direction, but not the correct one) (De Sante 1983a).
Because the directional tendencies of individuals are known to be partly under genetic control (Chapters 12 and 20), extreme individuals could be classed as mutants. We can expect some directional mutants to arise in every generation, but in general mutation rates are too low to account for the frequency with which some long-distance vagrants occur at particular sites. Thus, annual mutation alone within each year's crop of young could not be expected to account for the relatively large numbers of Richard's Pipits Anthus novaeseelandiae, Yellow-browed Warblers Phylloscopus inornatus and Pallas's Warblers P. proregulus which occur each autumn in western Europe, or of Blackpoll Warblers Dendroica striata and other species in California. Some other explanation is therefore required, either a cumulative effect of favourable mutations, increasing through the generations, or frequent non-genetically controlled mistakes in directional preferences. While such 'misoriented' individuals could in theory turn up in any direction from the usual range, in practice they tend to concentrate in specific directions, showing at least two distinct types of pattern, as described below.
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