Where eagles soar: Fine‐resolution tracking reveals the spatiotemporal use of differential soaring modes in a large raptor

Abstract

Unlike smaller raptors, which can readily use flapping flight, large raptors are mainly restricted to soaring flight due to energetic constraints. Soaring comprises of two main strategies: thermal and orographic soaring. These soaring strategies are driven by discrete uplift sources determined by the underlying topography and meteorological conditions in an area. High‐resolution GPS tracking of raptor flight allows the identification of these flight strategies and interpretation of the spatiotemporal occurrence of thermal and orographic soaring. In this study, we develop methods to identify soaring flight behaviors from high‐resolution GPS tracking data of Verreaux’s eagle Aquila verreauxii and analyze these data to understand the conditions that promote the use of thermal and orographic soaring. We use these findings to predict the use of soaring flight both spatially (across the landscape) and temporally (throughout the year) in two topographically contrasting regions in South Africa. We found that topography is important in determining the occurrence of soaring flight and that thermal soaring occurs in relatively flat areas which are likely to have good thermal uplift availability. The predicted use of orographic soaring was predominately determined by terrain slope. Contrary to our expectations, the topography and meteorology of eagle territories in the Sandveld promoted the use of soaring flight to a greater extent than in territories in the more mountainous Cederberg region. Spatiotemporal mapping of predicted flight behaviors can broaden our understanding of how large raptors like the Verreaux’s eagle use their habitat and how that links to energetics (as the preferential use of areas that maximize net energy gain is expected), reproductive success, and ultimately population dynamics. Understanding the fine‐scale landscape use and environmental drivers of raptor flight can also help to predict and mitigate potential detrimental effects of anthropogenic developments, such as mortality via collision with wind turbines.

Differences in movement and dynamic resource selection between breeders and floaters revealed with an Ornstein-Uhlenbeck space use model

“…Angle of incidence was included for the effect of orographic uplift on eagle space use. It is the deviation of the relative wind from the aspect of a slope and was computed such that aoi ∈ [0, π ] (Murgatroyd et al, 2018); π/ 2 corresponds to a wind orthogonal to a slope, and π a wind perfectly parallel blowing up slope. Wind direction was computed trigonometrically from the meridional and zonal wind components estimated by the NCEP NARR 10 m above the surface.…”

The challenges of estimating the distribution of flight heights from telemetry or altimetry data

“…1). Importantly, depending on the application, researchers might want to study z not h (Pirotta et al 2018, Murgatroyd et al 2018). In the list below, sources of error #3-#5 do not influence z .…”

“…6 in Sanz Subirana et al, 2013), including in wildlife studies (Patterson et al 2008, Johnson et al 2008, Albertsen et al 2015, Brost et al 2015, Buderman et al 2015, Fleming et al 2017). Importantly, these applications are not to be confused with another application of state-space models to movement data, when the focal state variable is a “behavioural state” whose Markovian transitions drive changes in movement rates (Gurarie et al 2016, Pirotta et al 2018, Murgatroyd et al 2018). Instead, when the objective is to correct for positioning errors, the state variable is the position itself.…”

“…For example, flight height data could help documenting the vertical niche and community ecology of aerial foragers (Arlettaz et al 1997, Siemers and Schnitzler 2004). Flight height data quantify the behaviour of flying animals and their flight strategies (Pirotta et al 2018, Murgatroyd et al 2018), and their relationships with environmental factors (e.g., Péron et al 2017). From an applied perspective, we need an accurate, error-free description of the distribution of birds and other animals in the aerosphere to avoid collisions with man-made structures, which is key to aircraft safety and animal conservation, in the current context of increasing human encroachment into the airspace (Lambertucci et al 2015, Davy et al 2017).…”

Dynamic-parameter movement models reveal drivers of migratory pace in a soaring bird

“…Obligate soaring migrants must also consider routes based on their energy landscape (Shamoun-Baranes et al, 2010), the energetic constraints of movement over space (Shepard et al, 2013), which also contributes to a migrant’s location along the behavioral continuum. While soaring migrants can stopover, their energy landscape is more complex; at least as important as foraging resources are meteorological conditions, which can be extremely dynamic and subsidize the energetic cost of flight directly via uplift (Pennycuick, 1971; Alerstam, 1979; Spaar and Bruderer, 1997; Gill, 2007; Duerr et al, 2012; Murgatroyd et al, 2018). Two primary forms of uplift arise by (1) wind interacting with topography to form upslope wind or mountain waves (air currents forming standing waves established on the lee side of mountains; hereafter orographic uplift) and (2) solar heating of the earth’s surface to generate thermal uplift.…”