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 acc limate 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 Eurosiber ian and Supra-Mediterranean regions in northern Iberia, and the Mediterrane an Lowlands in southern Iberia), which define a gradient of increasing dryn ess southwards. In each region, we selected one representative locality, wh ere we captured individuals during three study years. Methods We reviewed 72 community studies to test whether Iberian robins occ upy fewer forests and decrease in abundance southwards. Because robins are sedentary in the southernmost region, but largely abandon both northern are as in winter, we analysed the variation in migration-related morphology to test for population differentiation in the range boundary. To examine how p opulations cope with environmental variation, we studied four indices of ju venile condition (fluctuating asymmetry, fledgling size, ptilochronology an d size-corrected body mass), each related to nutritional, conditions in a p articular stage of growth, from early development to independence. Results Although robins restricted their range southwards, there was no cha nge in local abundance between regions. Southernmost robins, consistent wit h their sedentary behaviour, had shorter and more rounded wings than northe rn robins, although the populations did not differ in terms of body size. F luctuating asymmetry and fledgling size did not evidence a higher developme ntal stress in peripheral populations, although southernmost robins had a l ower juvenile condition during their independence (as shown by ptilochronol ogy 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 restric tive threshold of quality. In addition, morphological correlates of migrato ry behaviour provided evidence of population differentiation at the range e dge. This introduces the possibility that among-region variations in juveni le condition actually reflect reproductive benefits for migrants, related t o the occupation of seasonally productive habitats, rather than the maladap tation 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 ran ge boundary.