Abstract
Wind energy is vital for climate change mitigation, but poses risks to some bird species, particularly raptors. This study investigated raptor flight behaviour at the Kipeto Wind Farm in southern Kenya and exposure to potential collision risk related to time of day, season and weather. Observers monitored raptor flights from 09:00–18:00 daily for five years, beginning January 2020, and assigned flight time to height classes above, below or at collision risk height (CRH, 30–180 m). Raptors were categorised as falcons/kestrels, medium raptors, large raptors, vultures or Palaearctic migrants. Vultures accounted for 44% of all raptor sightings, with passage rates of 0.3–0.6 birds per hour, 86% of flights entirely above CRH and only 3% of flight time at CRH overall. Migrants also mainly flew above turbines (5.1% of time at CRH), with resident species spending more flight time at CRH (11.0%, 18.6% and 22.8% for large raptors, medium raptors, and falcons/kestrels respectively). Vulture time at CRH was least during the cool, dry mid-year months and peaked around the short rains in November–December. Other than migrants, other raptor categories did not show strong seasonal patterns in activity. Except for falcons/kestrels, time at CRH was highest before 11:00 for all categories. Controlling for confounding factors, vulture and migrant passage rates at CRH were elevated at lower barometric pressures, owing to greater overall activity. Resident raptors (falcons, medium and large raptors) made more flights at CRH in dry and sunny weather, or (for large raptors) when wind speed was low. For vultures, large raptors and migrants, activity patterns appear related to physical requirements for soaring flight. Species' actual collision risk at a site will depend on many factors, not only on time spent at collision risk height. A better understanding of raptor flight activity in relation to environmental factors nevertheless helps inform risk assessment and mitigation for wind energy developments.
Wind energy is essential for mitigating climate change, but it poses risks to some bird species, particularly raptors. This study examined the flight behaviour of raptors at the Kipeto wind farm in southern Kenya and their exposure to a potential risk of collision depending on the time of day, season, and weather conditions. Observers monitored raptor flights from 9 a.m. to 6 p.m. daily for five years, starting in January 2020, and classified flight times based on altitude above, below, or at the risk of collision (HRC, 30–180 m). Raptors have been classified as falcons, medium-sized raptors, large raptors, vultures or Palearctic migrants. Vultures accounted for 44% of all raptor sightings, with passage rates of 0.3 to 0.6 birds per hour, with 86% of flights taking place entirely over the HRC, and only 3% of the flight time at the HRC. Migrants also flew primarily over wind turbines (5.1% of the time at the HRC), with resident species spending more time flying at the HRC (11.0%, 18.6%, and 22.8% for large raptors, medium-sized raptors, and falcons, respectively). The time spent by vultures at HRC was lowest during the cool, dry mid-year months and peaked during the short rainy season in November-December. With the exception of migrants, the other categories of raptors did not show strong seasonal variations in their activity. With the exception of falcons, the time spent at the HRC was higher before 11 a.m. for all categories. Taking into account confounding factors, vulture and migratory passage rates at the HRC were higher when barometric pressure was lower, due to greater overall activity. Resident raptors (hawks, medium and large raptors) made more flights in the HRC during dry, sunny weather, or (for large raptors) when wind speeds were low. For vultures, large raptors and migrants, the activity seems to be linked to the physical demands of gliding. The actual risk of a species colliding at a site will depend on many factors, not just the time spent at a height at risk of collision. A better understanding of raptor flight activity in relation to environmental factors nevertheless helps to inform risk assessment and mitigation for wind projects.