Movement ecology of aerial insectivorous birds : Individual-based studies on swifts and nightjars

Sammanfattning: Many birds are predisposed to annually undertake movements over large temporal and spatial scales. They have an efficient flight machinery, demonstrate impressive capacities in storing and burning fuel, and they exhibit necessary orientation and navigational skills. Avian migrants are still limited in the amount of time and energy they can allocate towards their movements and need to balance migration-related costs against expenditures associated with reproduction and maintenance. Different specialisations towards a migratory life will likely evolve depending on the ecology of the bird. My work aimed to investigate the ecology of large-scale movements of three species of aerial insectivorous birds by using miniaturised technology to record individual birds´ position and activity during the non-breeding season. The common swift Apus apus and the pallid swift A. pallidus, are two highly aerial species that through their morphological and behavioural modifications are predisposed to a life in the air. Both species were documented to be air-borne throughout the non-breeding season which in the common swift results in a continuous flight for about ten months. Spending this time ranging over the African continent where large-scale weather systems generate an asynchronous seasonal landscape the swifts could potentially continuously track spatiotemporal variations in resources. However, the birds prioritised longer periods of residency briefly interrupted by shorter movement phases indicating that resource patchiness and local knowledge may be important determinants of the swifts´ movement patterns.The crepuscular and nocturnal European nightjar Caprimulgus europaeus showed in contrast to the swifts a very limited activity pattern. It is a visual hunter that mainly forages during dusk and dawn, although on moonlit nights this temporal constraint is reduced. Throughout the non-breeding season I found a clear periodic pattern in both migratory flights and foraging activities suggesting that the European nightjar may strive to increase the migration speed by allocating the fuelling towards moonlit nights. Influenced by wind conditions en route, the tracked birds undertook loop-migrations where the spring routes were consistently west of autumn routes. The detoured routes were at least in spring beneficial for a migrant striving to reduce the amount of energy and time spent on migration due to an associated reduction in barrier crossing. This optimal detour varied with breeding longitude and resulted in a spatial linkage between the breeding site and the stopover locations associated with the barrier crossing. This indicates that any spatial variations in this important part of the annual cycle may influence population trends differently across the breeding population. Technical advances and miniaturisations of data-recording devices have enabled individual-based studies on more and more species and will likely reveal predicted as well as completely surprising examples of bird migrations in many years to come. In this thesis I have explored some of the large-scale movement patterns observed in a group of aerial insectivorous birds. Some of the results and conclusions reveal peculiarities of this specific group of birds while other findings, such as the selection of migratory routes, should be of a more general interest in the field of bird migration.