Abundance distribution, morphological variation and juvenile condition of robins, Erithacus rubecula (L.), in their Mediterranean range boundary

Aim It has been suggested that populations nearing the border of a species’ range should be maladapted because they occupy progressively less suitable environments. In some instances, however, peripheral populations might acclimate or even be adapted to local conditions. We studied Iberian robins to evaluate whether southern, peripheral populations become more restrictive in selecting their habitats, are locally differentiated or show evidence of maladaptation. Location We divided the Iberian Peninsula into three regions (the Eurosiberian and Supra‐Mediterranean regions in northern Iberia, and the Mediterranean Lowlands in southern Iberia), which define a gradient of increasing dryness southwards. In each region, we selected one representative locality, where we captured individuals during three study years. Methods We reviewed 72 community studies to test whether Iberian robins occupy fewer forests and decrease in abundance southwards. Because robins are sedentary in the southernmost region, but largely abandon both northern areas in winter, we analysed the variation in migration‐related morphology to test for population differentiation in the range boundary. To examine how populations cope with environmental variation, we studied four indices of juvenile condition (fluctuating asymmetry, fledgling size, ptilochronology and size‐corrected body mass), each related to nutritional conditions in a particular stage of growth, from early development to independence. Results Although robins restricted their range southwards, there was no change in local abundance between regions. Southernmost robins, consistent with their sedentary behaviour, had shorter and more rounded wings than northern robins, although the populations did not differ in terms of body size. Fluctuating asymmetry and fledgling size did not evidence a higher developmental stress in peripheral populations, although southernmost robins had a lower juvenile condition during their independence (as shown by ptilochronology and residual body mass). Main conclusions The distribution of abundance found in this study supports the idea that Iberian robins could select habitats above a somehow restrictive threshold of quality. In addition, morphological correlates of migratory behaviour provided evidence of population differentiation at the range edge. This introduces the possibility that among‐region variations in juvenile condition actually reflect reproductive benefits for migrants, related to the occupation of seasonally productive habitats, rather than the maladaptation of sedentary, peripheral populations. From this perspective, and in view of the patterns of distribution and morphology that we found, southern Iberian robins might be independent populations locally adapted in the range boundary.