Age of First Territory Settlement of Golden Eagles Aquila chrysaetos in a Variable Competitive Landscape

Abstract

With life-history traits involving high survival, low reproductive output, years of natal dispersal and deferred maturity, the population ecology and behaviour of large raptors which occur at low densities can be difficult to study. The age at which large raptors first settle on a prospective breeding territory receives relatively little attention, but is a key metric in population modelling including, for example, projections of reintroduction projects. It can also be a barometer of the “health” of populations and the availability of breeding opportunities. The advancement of GPS-telemetry has proved invaluable in gaining insights into several aspects of large raptor ecology and behaviour. Age of first territory settlement (AFTS) is one such aspect. AFTS is important in modelling population trajectories and considering individuals’ lifetime reproductive success. We used an algorithm based on GPS-records from dispersing Golden Eagles tagged as nestlings in Scotland to estimate AFTS. While the lifespan of GPS-tags can bias against settlement dates of older birds, they can also potentially reveal settlement ages difficult or impossible to discern from other methods. We found a range of ages for AFTS, including those in their second calendar year; much younger than previously documented by other methods. Ground-truthing – when possible and if inevitably slightly delayed – confirmed territory occupation on field-based survey criteria. We found that eagles settled younger in vacant territories and when older in occupied existing territories. Birds’ sex had no effect on AFTS. Birds which dispersed earlier from their natal territory (indicative of “quality” from some previous research) had no association with AFTS. Our results indicate that within technological temporal limits GPS-data can provide for accurate and precise estimations of AFTS including early settlement not consistently or precisely recorded by other methods. Within our study’s variable competitive landscape we found that AFTS was associated with the availability of territorial opportunities but not with the timing of dispersal. These findings have consequences for studying and understanding large raptor population dynamics.

Sex difference in natal dispersal distances of Golden Eagles Aquila chrysaetos in Scotland

“…2020, 2021, 2022, Whitfield et al . 2022a). Nestlings were tagged when 50–70 days old (Hoechlin 1976, Peterson 1997) with transmitter weights and harnesses less than the 3% lower recommended maximum of body weight (Phillips et al .…”

“…2019, Whitfield et al . 2022a). Gross dispersal refers to the permanent movement of individuals to a new location irrespective of whether they reproduce after dispersing.…”

“…In many raptors, especially genera with larger species such as Aquila and Haliaeetus , the majority of movements affecting gene flow and demography are most influentially due to NDD (Newton 1979, Struwe‐Juhl & Grünkorn 2007, Whitfield et al . 2009a, 2022a, Millsap et al . 2014).…”

“…In our study population there appears to be equivalency in gross and effective dispersal (Whitfield et al . 2022a; notably table 1 and supplementary appendix A).…”

“…This variable was highly influential for the time taken by dispersing young birds to settle, with younger birds settling sooner in vacant territories (Whitfield et al . 2022a). Hypothetically, a lower NDD may also be associated with a vacant prospective territory.…”