Flight heights in ibis and spoonbills: implications for collision risk

Context Millions of birds worldwide have become victims of airspace collisions with aircraft, wind turbines, power lines and other infrastructure. Mobile bird species using grassland, agricultural and urban habitats are at higher risk, including large wading waterbird species such as ibis, spoonbills, egrets and herons that are priorities for conservation. Aim This work aimed to improve understanding of ibis and spoonbill flight characteristics as a first step in assessing species vulnerability to collision, and developing risk mitigation. Methods We used high-accuracy GPS telemetry data to quantify (a) flight heights of three aggregate-nesting waterbird species, i.e. straw-necked ibis (Threskiornis spinicollis), Australian white ibis (T. molucca) and royal spoonbill (Platalea regia), and (b) variations in flight heights and modes in relation to atmospheric conditions for straw-necked ibis as a focal species. Key results Across all species and movements, flights mostly occurred at heights of between 150 and 550 m above ground level (AGL). Long-distance movements by straw-necked ibis reached a maximum height of 2800 m AGL; however, most flights (75%) occurred below 1000 m. Soaring and gliding were driven by the intensity of thermal uplifts and associated with longer-distance flight legs. Where thermal uplift was absent, birds flapped at relatively low and constant heights compared to when uplift was present. For straw-necked ibis, 29% of all flight fixes were in the rotor swift zone of wind turbines (20–250 m), but this figure increased to 53% if only flapping flights were considered. Flight heights broadly overlapped with general aviation zones, notably during aircraft take-off and landing phases. Conclusions There are clearly collision risks associated with wind turbines and aircraft flight zones when considering the flight characteristics and ecology of large aggregate-nesting waterbirds such as ibis and spoonbills. Implications When assessing spatially and temporally explicit scenarios of risk for such species, we suggest that several factors should be considered, including (a) atmospheric, weather and seasonal conditions, (b) common routes or flyways used during long-distance movements, (c) the locations of important nesting sites and associated foraging sites, (d) the locations of important stopover and overwintering sites, and (e) the timing of flights.