Raptor exposure to collision risk in relation to weather and season at a wind farm in tropical Africa

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 .