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.
Highlights
Dispersal is a critical but poorly understood behaviour in animal population dynamics (Clobert et al, 2009)
In preliminary analyses there was no correlation between post-fledging dependence period (PFDP) and post-dispersal days to the estimated Age of first territory settlement (AFTS)
Young Golden Eagles in occupied territories are less likely to reproduce or attempt to reproduce beyond basic occupancy (Steenhof et al, 1983; Sánchez-Zapata et al, 2000; Pedrini and Sergio, 2001; Whitfield et al, 2004a), and presumably thereby spend further years building to reproduction, like the White-tailed Eagle Haliaeetus albicilla (Murgatroyd et al, 2018), which rarely switches territories once settled (Whitfield et al, 2009a)
Summary
Dispersal is a critical but poorly understood behaviour in animal population dynamics (Clobert et al, 2009). Age of first territory settlement (AFTS) is an important variable in understanding several features of large raptor populations, not just NDD estimates. Our study used a novel algorithm, based on GPS satellite telemetry, accounting for these known features of movement behaviour, to determine when a dispersing Golden Eagle Aquila chrysaetos had settled on (occupied) a prospective breeding territory, to estimate AFTS. AFTS is indicative of a territorial landscape free of occupants due to other causes These causes typically involve persistent deaths of subadults and notably, adults, from persecution or other adverse anthropogenic influences (Balbontín et al, 2003; Whitfield et al, 2004a,b; Penteriani et al, 2005a,b). As an influential measure in lifetime reproductive success (Newton, 1989) AFTS should be earlier in birds which began dispersal earlier if they were of higher quality
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