Facing the wind: The aeroecology of vertebrate migrants

Abstract

The aerosphere is an essential part of the habitat of flying vertebrates. Birds and bats make use of the airspace for daily activities like foraging, commuting, mating, and seasonal movements including migration. In this chapter, we focus on how the properties of the aerosphere affect migration and a few other regular large-scale movements. For animals moving between seasonally favourable habitats across hundreds or thousands of kilometres, the conditions of the aerosphere have a substantial impact on energy and time demands, on orientation and navigation, and finally on survival. Although bats and birds often make similar use of the aerosphere, there is a huge difference in our actual knowledge of these interactions. There are about 4000 migratory bird species comprising 100-150 billion individuals undertaking regular seasonal movements within and between continents and across the oceans. Our understanding of bat migration is much more limited and such estimates are not available, but many bat species and many millions of individuals show similar kinds of migratory behaviour. Atmospheric conditions vary across time and space, including variation in air flow and air temperature, humidity, and density. In this chapter, we emphasize the importance of wind and precipitation as the main factors driving behaviour and evolutionary adaptations. Flying within a moving air space, bats and birds can make use of regular seasonal wind fields, like the trade and anti-trade winds, but they also must deal with irregular events, like heavy storms. In combination with the distribution of their preferred habitats, large-scale atmospheric conditions guide their flight routes and shape their migratory strategies. The timing of individual flight stages is directed by weather conditions, mainly wind and precipitation. Once aloft, individuals may select among varying wind conditions at different flight altitudes to achieve beneficial wind conditions en route. These behavioural patterns have a strong effect on the time needed to move between suitable habitats, but probably more importantly on overall energy demand and thus foraging time/cost and survival. While birds are known to explore heights up to 8000 m asl during migration, bats are generally restricted to heights below 3000 m asl, possibly due to differences in lung morphology. On the other hand, many bats can withstand harsh weather conditions by using torpor, while birds may have to leave or starve. Birds and bats are confronted with the regular occurrence of both predictable (e.g. wind support deviations from target) and unpredictable (e.g. storms) displacements and are therefore equipped with excellent orientation capabilities (Chap. 6). This chapter provides the background of what we really know about the role of the aerosphere for the migration of birds and bats and where we still marvel.