Analysis of the consequences of climate change and habitat modification on migratory birds.

Habitat alteration and climate change are among the anthropogenic factors that contribute most to the current global decline of biodiversity. Due to their sensitivity to environmental conditions, birds are considered excellent indicators of global change. In particular, long-distance migrants are likely more sensitive to global change than residents or short-distance migrants because they suffer from changes in ecological conditions both at their breeding and wintering quarters The aim of this thesis is to investigate the effects of climate change and habitat modification on population dynamics of migratory birds. The first part of this thesis includes papers investigating the effects of environmental conditions experienced by two long-distance migratory species, the Barn Swallow Hirundo rustica and the Common Swift Apus apus, at their breeding quarters in Northern Italy on their population dynamics. We observed that both the presence of livestock farming and the extent of hayfields close to the breeding site affected colony size of Barn Swallows and that recent variation in these conditions has probably concurred to worsen the demographic decline that this population has suffered. We also investigated the effect of the rearing environment on the survival and growth of Common Swift nestlings in a part of the breeding range of this species where these effects have never been investigated so far. We observed that nestlings’ growth is influenced not only by competition for resources with nest mates, but also by meteorological conditions. However, the effects we documented were different from those observed in more northern parts of the breeding range of the species, thus suggesting geographical variation in the susceptibility of this widespread species to general ecological conditions. In the second part of this thesis we aimed at identifying migration routes and wintering quarters of small-sized migrant birds, and at evaluating the effect of environmental conditions experienced during migration and wintering on population dynamic. We followed two different and complementary approaches. First, we took advantage of the large number of ring recoveries available for the Barn Swallow throughout more than a century to identify changes in the timing of migration and main migration routes of individuals breeding in Europe. Second, we applied miniaturized tracking devices to 94 Barn Swallows to precisely identify, for the first time, timing of migration and position of the wintering areas of each individual. We could also evaluate the impact of the application of these instruments on survival and breeding success. In a further study we combined information on wintering grounds and migration routes with long-term data on population dynamic from Northern Italy and found that environmental conditions encountered during wintering and spring migration are the factors that influence most year-to-year variation in population consistency. Finally, we analyzed a long-term series of ringing data on a short-distance migrant, the European Robin Erithacus rubecula and found that winter temperatures at the breeding grounds influenced spatial and temporal variation in migration propensity and distance among individuals. Overall, our researches confirmed that global change is already affecting bird populations by acting at different stages of their life-cycle. We showed that variations in ecological conditions at breeding quarters seem to influence breeding performances of individuals, while condition experienced during migration and wintering seem to affect their survival. Effective conservation measures for migratory species should therefore aim at protecting both breeding and wintering areas as well as stopover sites along migration routes. The novel analytic frameworks we developed may also be suitable for investigating the effects of climate change on migration across a broad range of species.