Aims: The occurrence patterns of these Siberian passerines in the western Palearctic are examined in order to test the different hypotheses regarding their causes. In particular, it is attempted to check whether the geographical distribution of records may be biased by an uneven distribution of potential observers.
Location: Europe, with particular reference to the Iberian Peninsula and the Atlantic islands of Spain and Portugal.
Methods: The distribution of records by countries is compared with that of the ringing effort currently employed in each. Occurrence patterns are also analysed both by dates and by the locations in which records tend to be concentrated in each country.
Results: Differences between countries in numbers of records are much greater than differences in the distribution of potential observers. On the other hand, autumn arrivals are much earlier on average in Scandinavia and the British Isles than on the Atlantic coasts of central Europe, in the Iberian peninsula or in other Mediterranean areas, where there are also much higher percentages of winter and spring records.
Conclusions: The scarcity of records of these species in southwest Europe can not be accounted for by a low density of potential observers. This, coupled with the differences existing in the mean arrival dates, suggests that just a small fraction of the birds visiting Scandinavia and the British Isles during the autumn move later on through the Iberian Peninsula. On the other hand, the minimal incidence of these species during the spring in the whole of Europe suggests that just a small part of the autumn population spends the winter in Africa. Taking all this into account, a new hypothesis is put forward: that these birds arriving at Europe are mainly juveniles on exploratory migration (Zwischenzug) and that later in the autumn they travel back to Asia directly from northwestern Europe.
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