Environmental stress may give rise to increased levels of developmental instability measured in terms of fluctuating asymmetry (random deviations from bilateral symmetry in morphological characters), but also deviations from optimal physiological and immunological performance. Linking developmental instability to physiological condition and fitness is an unexplored avenue of research in behavioural ecology which will allow assessment of how environmental conditions mechanistically affect the development of phenotypes and the consequences of such phenotypes for reproductive success. An understanding of the magnitude of the stress factor imposed by migration relative to other stresses in the life history of birds could be used to predict the effects of climatic change on bird populations. Developmental instability in morphological characters is particularly important in migratory birds. Barn swallows are highly sensitive to environmental stress because foraging, migration to Afiica and other activities are highly dependent on weather conditions, and males have an elongated, costly tail due to a directional female mate preference. The main aim of the present project is to determine during which part of the annual cycle of the barn swallow conditions are most stressful as experienced by the swallows themselves, and to make a comparison in this respect between Danish and Spanish populations, which migrate over different distances. Measurements of fluctuating asymmetry will be related to data on physiological condition, immonucompetence and breeding success. Environmental stress will also be simulated by manipulation of tail length. The research training will take place in a stimulating international environment, with Dr A.P. Moeller, Dr N. Saino and Pro? F. de Lope, experts in the fields of environmental stress research, ecophysiology and immunology.