The extraordinary adaptations of birds to contend with atmospheric conditions during their migratory flights have captivated ecologists for decades. During the 21st century technological advances have sparked a revival of research into the influence of weather on migrating birds. Using biologging technology, flight behaviour is measured across entire flyways, weather radar networks quantify large-scale migratory fluxes, citizen scientists gather observations of migrant birds and mechanistic models are used to simulate migration in dynamic aerial environments. In this review, we first introduce the most relevant microscale, mesoscale and synoptic scale atmospheric phenomena from the point of view of a migrating bird. We then provide an overview of the individual responses of migrant birds (when, where and how to fly) in relation to these phenomena. We explore the cumulative impact of individual responses to weather during migration, and the consequences thereof for populations and migratory systems. In general, individual birds seem to have a much more flexible response to weather than previously thought, but we also note similarities in migratory behaviour across taxa. We propose various avenues for future research through which we expect to derive more fundamental insights into the influence of weather on the evolution of migratory behaviour and the life-history, population dynamics and species distributions of migrant birds.

Large ecological barriers such as oceans and deserts have considerably shaped the migratory strategies of birds. The ecological barriers posed by the Alps, the Mediterranean Sea, and the Sahara seem to prevent most long-distance migrants from flying on a direct southward course from Europe to Africa. Migratory routes toward southwest and southeast prevail. These two flyways differ with respect to topography, refueling possibilities, and wind conditions. Aiming at a better understanding of the evolution of both flyways in spite of differing conditions, we studied potential survival of passerine birds on their first autumn migration from northern Europe to tropical Africa by means of a computer simulation. Considering real wind conditions at 850 mb (approximately 1500 m above sea level), the survival rates of birds with southeasterly (SE) migratory directions were much higher than those of birds with southwesterly (SW) directions. With the possibility to choose the altitude (from four levels) with the most favorable wind, both SE and SW migrants had similar high survival, but only with refueling opportunities in northwest (NW) Africa for SW migrants. Our results suggest that the southwestern flyway depends on the selection of days, but especially altitudes, with favorable wind conditions and on refueling opportunities in NW Africa. The SE flyway is privileged by the frequent favorable wind conditions for crossing the eastern Mediterranean Sea and the Egyptian desert, where refueling sites are almost absent. Both autumn migration routes would be unlikely without wind assistance.

Being airborne is considered to be energetically more costly as compared with being on the ground or in water. Birds migrating or foraging while airborne are thought to spend some time resting on the ground or water to recover from these energetically demanding activities. However, for several decades ornithologists have claimed that some swifts may stay airborne for almost their whole lifetime. Here we present the first unequivocal evidence that an individual bird of the Alpine swift (Tachymarptis melba) can stay airborne for migration, foraging and roosting over a period of more than 6 months. To date, such long-lasting locomotive activities had been reported only for animals living in the sea. Even for an aerodynamically optimized bird, like the Alpine swift, flying requires a considerable amount of energy for continuous locomotive control. Our data imply that all vital physiological processes, including sleep, can be perpetuated during flight.

Nocturnal avian migration flyways remain an elusive concept, as we have largely lacked methods to map their full extent. We used the network of European weather radars to investigate nocturnal bird movements at the scale of the European flyway. We mapped the main migration directions and showed the intensity of movement across part of Europe by extracting biological information from 70 weather radar stations from northern Scandinavia to Portugal, during the autumn migration season of 2016. On average, over the 20 nights and all sites, 389 birds passed per 1 km transect per hour. The night with highest migration intensity showed an average of 1621 birds km ^–1 h ^–1 passing the radar stations, but there was considerable geographical and temporal variation in migration intensity. The highest intensity of migration was seen in central France. The overall migration directions showed strong southwest components. Migration dynamics were strongly related to synoptic wind conditions. A wind‐related mass migration event occurred immediately after a change in wind conditions, but quickly diminished even when supporting winds continued to prevail. This first continental‐scale study using the European network of weather radars demonstrates the wealth of information available and its potential for investigating large‐scale bird movements, with consequences for ecosystem function, nutrient transfer, human and livestock health, and civil and military aviation.