Four years research of Black Harriers breeding within the Eastern Cape wind farm (RE Simmons and M Martins unpubl. data) has also revealed that harriers are more susceptible to mortality during breeding because the proportion of time they fly within the blade (rotor) swept area (BSA) rose from 0% to 35% as provisioning birds, particularly males, commute to and from foraging areas.
There may be several reasons for this: unlike European harriers, breeding Black Harriers do not appear to move away from operational farms, and second Black Harriers exhibit very wide-scale movements with breeding birds moving on average 16.4 km from their nests (range 7.1 to 33.4 km) with 90% kernel estimates of territory size (92.7 ± 66.6 km
2 and 147.8 ± 205.4 km
2 for breeders and non-breeders respectively: Garcia-Heras et al. 2019). As such, these are territory sizes are more akin to eagle-sized territories than a medium sized raptor like a harrier and may expose them to more wind farm environments than a more sedentary species. This is supported by our remote sensing study that indicated that birds on migration, particularly in the Eastern Cape and Lesotho were more likely to die than those on their breeding grounds. At least three birds succumbed to over head transmission lines, as judged by their proximity to the lines and in one case an x-ray indicating trauma to the spinal vertebrae
http://blackharrierspace.blogspot.com/2016/01/x-raying-black-harriers.html