Here we develop a method for differentiating between local and migratory Robins during winter in southern Spain, which we applied to estimate their relative proportion in the population wintering in the Campo de Gibraltar area (southern Spain) in 1998-99. To do so, we considered the variation in wing length and wing formula between two partially migratory populations (Álava area, northern Spain, and Guadarrama area, central Spain) and one sedentary population (Campo de Gibraltar). From these, we obtained a discriminant function (Tables 1 and 4) which allowed us to classify correctly almost 80% of the Robins measured (n = 135; Tables 2 and 3). Wing length (third primary) showed the highest contribution to between-group discrimination (Fig. 1). By using this method, we estimated that around 58% (n = 134) of the Robins wintering in the Campo de Gibraltar area in 1998-99 belonged to the sedentary population. This proportion was much larger in the breeding habitats of these local populations (70% in forests) than in the habitats that are occupied during the winter only (40% in scrublands), suggesting a differential occupation of both environments by foreign Robins and the displacement of part of the residents away from their breeding areas during the winter.
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