Foraging strategies of an aerial-hawking insectivore, the common noctule bat Nyctalus noctula

Movement is a key signature of life. Yet, the integration of movement ecology and biodiversity concepts was only recently formalize. In this framework, an individual’s movement path and the underlying drivers are used to explain interactions between individuals and eventually species coexistence. Interactions influence the individual’s environment including species assemblage, and thereby feed back on the individual’s movement path. Foraging represents one of the most common movements of many animals, and thus has been of interest for ecologists ever since. Yet, classical foraging ecology predominantly focused on optimality models to explain the behaviour of single foragers, but rarely took into account the interactions between moving individuals. The overarching question of the three studies in this thesis thus was “How can different foraging strategies support coexistence?”. Being highly mobile and showing a large niche overlap with several other species, the insectivorous Common noctule bat Nyctalus noctula (Schreber, 1774) is an ideal model species to study intra- and interspecific interactions during foraging movements. I therefore investigated movement behaviour and space use of N. noctula during aerial foraging, and evaluated the potential role of different foraging strategies for the coexistence of competing bat species in the light of different competitor densities and prey distributions. In chapter one, I asked whether foraging N. noctula adjust their space use to abiotic factors (i.e. moonlight) which might be linked to prey distribution. I used GPS (global positioning system) loggers to investigate the habitat use of nine N. noctula during high and low moonlight intensities. During moonlit nights, N. noctula hunted preferentially over open fields, whereas they avoided open fields in dark nights. I suppose that foraging activity followed changes in insect activity triggered by the lunar cycle. The results suggest that N. noctula might be able to predict cyclic changes in prey distribution. The exploitation of prey aggregations in lit habitats might be an advantage towards competing bat species that are less light tolerant. In chapter two, I asked whether the use of social foraging by N. noctula depends on season, possibly as a response to changes in insect availability. I quantified N. noctula activity at foraging sites in early and late summer during acoustic playbacks of either hunting conspecifics or heterospecifics. N. noctula activity increased during heterospecific playbacks in early summer, but decreased in late summer. There was no clear reaction towards conspecific playbacks, irrespective of the season. The results suggest that external factors determine the strengths of intraspecific and interspecific competition, but that insectivorous bats mitigate different competitive pressures through flexibility in foraging strategy and fine scale space use. I argue that conspecific might impair each other by acoustic interference of echolocation calls and competition for flight space. However, niche segregation might make social foraging with heterospecifics beneficial, given that there is low competition for prey items. In chapter three, I asked whether the foraging strategy of N. noctula depends on the combination of conspecifics density and landscape features that might determine prey distribution. I used combined GPS-ultrasound loggers to record the nightly foraging movements and hunting activity of 27 N. noctula above farmland and forested landscape. Acoustic records also allowed quantification of nearby conspecifics. I deduced two movement states - area restricted movement and directed movement - from the GPS tracks. Above farmland, N. noctula switched to area restricted movements after encounters with conspecifics, and foraging activity was highest during those movements. Above forested landscape, encounters with conspecifics had little influence on the movement behaviour of N. noctula, and foraging activity occurred during directed and area restricted movements alike. N. noctula encountered more conspecifics above the forested landscape than above farmland. I argue that N. noctula was able to integrate prey distribution and competitive pressure when deciding whether or not to pursue a social foraging strategy. The use of a social foraging strategy might be a prerequisite for survival in agricultural landscapes where prey is patchily distributed and ephemeral. In contrast, solitary foraging might be the optimal strategy in forested landscapes that offer evenly distributed prey and support larger populations. In conclusion, the results showed that N. noctula integrated environmental factors that probably influenced prey distribution, adverse effects from intra- and interspecific competition, and public information about prey availability provided by hunting con- and heterospecifics. N. noctula used this compiled information to decide where to forage and whether to forage solitary or socially. The studies highlighted that N. noctula can adjust its foraging strategy context dependently. This flexibility was achieved through dynamic feedbacks between the movement paths and the perceived environment. These dynamic feedbacks may play a pivotal role in promoting the coexistence of competing species. In particular, the similarity of movement behaviours and resulting foraging strategies among conspecifics might stabilize species assemblages through intraspecific competition, while slight differences in the movement behaviour among heterospecifics might allow fine-scale niche segregation and thereby equalize the fitness of coexisting species. I propose that dynamic foraging behaviour might act stabilizing and equalizing not only in insectivorous bats but on assemblages of highly mobile predators in general