The Ornithodolite as a tool to quantify animal space use and habitat selection; a case study with birds diving in tidal waters

Animal-attached technologies can be powerful means to quantify space-use and behaviour, however, there are also ethical implications associated with capturing and instrumenting animals. Furthermore, tagging approaches are not necessarily well-suited for examining the movements of multiple individuals within specific, local areas of interest. Here, we assess a method of quantifying animal space use based on a modified theodolite with an inbuilt laser rangefinder. Using a database of > 4,200 tracks of migrating birds, we show that detection distance increases with bird body mass (range 5 g - >10 kg). The maximum distance recorded to a bird was 5500 m and measurement error was ≤ 5 m for targets within this distance range; a level comparable to methods such as GPS tagging. We go on to present a case study where this method was used to assess habitat selection in seabirds operating in dynamic coastal waters close to a tidal turbine. Combining positional data with outputs from a hydrographic model revealed that great cormorants (Phalacrocorax carbo) appeared to be highly selective of current characteristics in space and time; exploiting areas where mean current speeds were < 0.8 m s-1, and diving at times when turbulent energy levels were low. These birds also orientated into tidal currents during dives. Taken together, this suggests that collision risks are low for cormorants at this site, as the two conditions avoided by cormorants (high mean current speeds and turbulence levels), are associated with operational tidal turbines. Overall, we suggest that this modified theodolite system is well-suited to the quantification of movement in small areas associated with particular development strategies, including sustainable energy devices.