Surviving to winter: strategies of wintering birds in the Iberian Peninsula.
Published: Volume 51(1), June 2004. Pages 133-168.
Original Title: Sobrevivir al invierno: estrategias de las aves invernantes en la península Ibérica
- Aims: Climatic conditions during the winter are typically harsher for birds, which have to counteract deteriorating weather by different physiological or behavioural adaptations, or simply by moving to other areas of milder weather (e.g. migrating). The aim of this paper is to review the different adaptations and strategies of birds for surviving the winter, with a special emphasis on the peninsular winter quarters, one of the most suitable geographic areas for the reception of a very important contingent of birds from the western Palaearctic. This suitability is the result of its southern geographic situation which provides a milder climate, of the canalising of the main presaharian migratory routes, and of its topographic variety, diverse landscape and heterogeneity of environments.
Results and Conclusions: Key variables explaining the winter distribution of birds in Iberian Peninsula are altitude, vegetation structure and geographic location. Parameters describing land bird communities (density, richness and diversity) can equally be predicted by the same variables. Aquatic environments are affected by the basin in which they are located and by rainfall patterns, which in turn determine the levels of grouping of the different contingents of birds. In order to offset the higher energetic requirements of winter, birds have developed different adaptations: increase the size of fat reserves, store food, increase metabolic rate, and develop different specialized behaviours. The Iberian Peninsula offers a wide range of trophic possibilities: herbs, seeds, bulbs, tubers and rhizomes; fruits, berries and acorns; pine seeds and other arboreal food resources, and invertebrates. In aquatic environments, there are important vegetable resources, aquatic invertebrates and fishes. Finally, the same wintering birds may appear as an important food resource for raptors. Human life and its consequences (garbage and rubbish dumps, fish farms, etc.) have favoured changes in the feeding habits of several species. The exploitation of all these natural resources may give rise to different patterns of spatial use. One of the behavioural strategies most common during winter is the aggregation of individuals in flocks, albeit that some species may hold territorial patterns. Winter flocking entails different costs and benefits, which may be compensated by different strategies such as the formation of heterospecific groups, communal roosting, the establishment of social hierarchies, or the share-out of space by the different classes. Wintering contingents may fluctuate greatly from year to year because of phenomena such as thermal movements, irruptions and altitudinal movements. Winter may have important consequences on the evolutionary patterns of different species, either because some species may pair in this period, or because of the strong selection pressures exerted on them because of the harder weather conditions. Therefore, the consequences of what may happen in winter do not only affect this season, and its study may be of great relevance to the understanding of the whole natural and life history of the different species.