Altimeters on albatrosses: quantifying flight heights for dynamic soaring seabirds

Summary

Offshore wind farm development is expanding into the Southern Hemisphere, the core range of albatross, petrels and shearwaters (Order Procellariiformes). The risk of collision with offshore turbines is poorly understood for Procellariiformes, primarily because of limited data on how high they fly. Timely filling of this data gap supports decision making with better collision estimates and can embed impact mitigation measures into offshore wind farm design (e.g. increasing turbine hub heights) before construction begins in the Southern Hemisphere. Here, we introduce a new method for flight height estimation in Procellariiformes by recording the characteristic patterns that they exhibit during dynamic soaring flight (cyclical ascent and descent) with bird-borne altimeters. During dynamic soaring flight, Procellariiformes regularly skim the sea surface (altitudes <1 m), which is ideal for setting sea level reference pressure in barometric altitude calculation (here termed altimeter ‘zeroing’). We field test our method on breeding Shy Albatross Thalassarche cauta in Tasmania, Australia. As proof of concept, we show that albatross-borne altimeters can record fine-scale vertical movements, such as 10 second dynamic soaring cycles, and wave height and periodicity when albatross float on the sea surface. We found agreement between altimeters zeroed with dynamic soaring and GPS altitudes on mean flight heights of c.4 m for Shy Albatross. Altimeters showed a clearer right skew to the flight height distribution, which is crucial for determining how often albatrosses fly at higher altitudes also may be occupied by turbine blades. As a measure of uncertainty, we estimated that altimeter flight heights had a plausible range of 2.1 m between upper and lower scenarios under our method, while GPS altitudes were characterised by high variance around their mean. Our pilot study was limited to three tracked birds and further study is warranted to validate our methods over a broader sample of individuals and environmental conditions. To account for platform-specific error sources, we recommend complimentary use of altimeters and GPS to estimate seabird flight heights. The tiny power demand of altimeters compared to GPS, makes our method attractive for efficient biologger estimation of flight heights in dynamic soaring seabirds (including >100 Procellariiform species).