The End of the Line for South Africa ’ s National Bird ? Modelling Power Line Collision Risk for the Blue Crane

Abstract

The Overberg wheatbelt population of the Blue Crane (Anthropoides paradiseus) comprises approximately half the global population of this Red-listed species. Blue Cranes are highly susceptible to collisions with overhead power lines, and a spatial risk model was developed to identify high risk lines in the Overberg for proactive mitigation. In this study, I surveyed 199 km of power lines to ground-truth the model before it is widely implemented. Unfortunately, the model was shown to be completely ineffective at predicting collision risk for Blue Cranes. Further GIS modelling was undertaken to test a wider range of landscape and power line variables, but only the presence or absence of cultivated land could usefully identify lines posing a collision risk. Currently, the lack of detailed spatial data and recent Blue Crane demographic information mean that GIS models cannot be used to adequately describe the complex collision problem. On the ground survey, Blue Cranes were the most commonly killed birds found (54% of all carcasses). I used recent carcasses to estimate a Blue Crane collision rate of 0.25 km-1.yr-1 (95% CI 0.10-0.46 km-1.yr-1), corrected for biases, which means that approximately 10% (95% CI 4-18%) of the total Blue Crane population within the Overberg study area is killed annually in power line collisions. While crude, this estimate is extremely high and represents a possibly unsustainable source of unnatural mortality for the Blue Crane. There is urgent need for further research into risk factors and for mitigation measures to be more widely implemented. In addition to Blue Cranes, carcasses of at least 19 other bird species were recovered (including 5 Red-listed or locally endemic taxa), highlighting the wider impacts of power line-induced mortality in the Overberg. After Blue Cranes, Denham’s Bustards (Neotis denhami) were the most numerous species found with a corrected collision rate of 0.06 km-1.yr-1 (95% CI 0.01-0.12 km-1.yr-1), some 30% (95% CI 6-59%) of the total Overberg population. Such a high level of unnatural mortality is of serious concern for this threatened species.