Highland and lowland forest birds differ in their feather growth rates: a multispecies test in the southwestern Palaearctic

Mountain birds face numerous challenges caused by altitude-dependent environmental seasonality. Although elevation gradients may affect bird morphology, migration strategy and/or phenology of seasonal events (breeding and moulting), the life histories of highland compared with lowland birds have been little explored. In this study we compared the growth rate and mass of the tail feathers of six forest passerine species sampled at different elevations in the Iberian Peninsula (north) and the Maghreb (south) to assess potential differences in moult duration and feather quality between highland and lowland bird populations. As timing is crucial in seasonal environments, we also explored seasonal changes in temperature in the different study areas as a proxy for the duration of the optimal temporal windows available in summer for breeding and moulting. In addition, we compared wing configuration between highland and lowland populations, which could reflect differences in their migration strategy (migratory versus sedentary). The results showed that highland birds had less time available for breeding and moulting, longer and more concave wings, and faster feather growth rates than lowland populations, but we found no consistent patterns of variation in feather mass. These results suggest that selection might have favoured an accelerated moult and a more migratory behaviour in highland populations to cope with the reduction in the optimal temporal window as elevation increases. These patterns are similar to those observed in migratory populations moving along latitudinal gradients to breed in highly seasonal northern environments. We therefore hypothesize that elevational gradients at the southern limit of the Palaearctic could be an important driving force promoting variation in feather growth rate and wing configuration, and probably also other avian life-history characteristics.