A simple topographical model to predict Golden Eagle Aquila chrysaetos space use during dispersal

Abstract

Many large raptors exploit or rely on anabatic and orographic winds which provide vertical lift, to supplement or provide the energy fuelling flight. Airspace is therefore a critical habitat for such large raptors and its use is subject to the underlying terrestrial topography, because particular topographical features are more likely to provide wind‐energetic lift. Accordingly, ridges and/or ‘rugged topography’ are common preferred features in habitat use by large raptors. Our study aimed to provide a simple model of space use for a large raptor, the Golden Eagle Aquila chrysaetos, based on thousands of GPS telemetry records during juvenile dispersal of 92 birds tagged as nestlings between 2007 and 2016 across upland Scotland. Model development was based on the hypothesis that four topographical variables would be influential: slope, aspect, altitude and distance from ridge. The telemetry dataset was divided into training and two testing components. The first testing set was derived by a temporal split resulting in approximately equal sample size on records and some temporal overlap in individuals’ records with training data. The second testing set involved no individuals from the training set. Aspect was removed early in training model development because it was not influential. The model found that young Golden Eagles preferred, or used according to availability, space above slopes greater than 10°, at an altitude of ≥ 300 m, and within 300 m of a ridge. The test data were highly correlated with those from the training data in the model variables, and performance as regard to expected preferences from the model was improved in both test datasets, indicating the model was robust. Given the apparent universal nature of large raptor dependence on topography, that topography is relatively immutable according to time and use, and that topographical data are readily available, we commend our approach to other habitat preference studies of Golden Eagles and other large raptors elsewhere.

Non-territorial GPS-tagged golden eagles Aquila chrysaetos at two Scottish wind farms: Avoidance influenced by preferred habitat distribution, wind speed and blade motion status

“…Use of lower values enhances data precision and accuracy. Conservatively we used only fixes < 3.5 HDOP ( cf < 10 HDOP [ 106 ]) in keeping with the 50-m pixel resolution of our habitat preference predictions [ 46 ] and see later. We also excluded records if altitude data were unavailable or potentially inaccurate (> 6 km).…”

“…Telemetry records included only those during natal dispersal (or juvenile dispersal: [ 107 – 109 ]: hence, after dispersal from the natal territory [ 109 , 110 ] and before breeding territory settlement [ 46 ] (see also Fig 1 ). Taking the centre of Stronelairg as the ‘destination’ location, the 23 tagged birds originated from nests 5–216 km (mean 67 km) distant; to Dunmaglass centre, natal nests were 15–233 km (mean 71 km) away.…”

“…Underlying topography can affect the way in which anabatic and/or orographic processes provide external wind energy sources as uplift [ 43 – 46 ]. Soaring birds on migration may exploit different uplift sources to when settled on wintering and breeding grounds, or to resident birds [ 37 , 44 , 47 , 48 ].…”

“…As facultative or obligate dependents on external wind energy to fuel flight, soaring birds’ flight behaviour is also often conditional on weather [ 12 , 19 , 58 ], especially wind speed and direction for those exploiting orographic uplifts from topographic features [ 12 , 44 – 46 , 48 ]. Focus via specific turbines on relationships between wind, topography, birds’ flight behaviour and collision risk may be highly informative [ 8 – 10 , 37 , 48 , 54 , 55 ].…”

An evidence‐based assessment of the past distribution of Golden and White‐tailed Eagles across Wales

“…A post-hoc assessment of the species identities assigned to place-name records by the DFA showed an interesting congruence between the species identity and elements of the place-name relating to geographical features known to be associated with that species. For example, place-names in upland Wales incorporated components such as "cefn" (Welsh for ridge), "crug" (hill-rock), "bryn" (hill), and "gwaun" (moorland); all elements that can be associated with Golden Eagles (Fielding et al, 2020). Place-names in lowland areas included "allt" (Welsh for a wooded hillside), "coed" (forest or wood), "nant" (stream), and "llyn" (lake); habitat features associated with White-tailed Eagles (Sansom et al, 2016).…”