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

Abstract

Natal dispersal distance (NDD) is critical in understanding and defining populations and their conservation. It is defined as the linear distance between the natal location and first reproductive (‘effective NDD’) or potential reproductive (‘gross NDD’) location. It is a measure of gene flow and the functional connectivity across generations between individuals breeding in the same or different geographies. NDD is difficult to record in large raptors. GPS‐satellite telemetry has facilitated its recording. Previous Scottish studies showed that gross and effective NDD were apparently equivalent, and an algorithm based on telemetric data could identify the territory settlement timing and location of birds originally tagged as nestlings. We analysed natal dispersal data from 39 Golden Eagles GPS‐tagged in Scotland to estimate NDD. Raw median estimates were 29.8 km for males (n = 22) and 58.6 km for females (n = 17), 38.1 km averaged across sexes. Males had significantly shorter NDD, as theoretically predicted. Our NDD estimates were shorter but not grossly dissimilar to those from the USA, where sex differences in NDD had not been confirmed. Respective sample sizes may underly the latter contrast in confirmation. We also showed that in the absence of data from sexed birds, NDD estimates can be different. Natal dispersal duration was not related to NDD, suggesting that time to prospect a territory opportunity was not associated with the selected territory's distance from the natal site. The previous status of the subsequent settled territory (occupied or vacant) was also not related to NDD. We conclude that sex differences in NDD are important in application to population demography and conservation. Although we found no support for two other potential drivers of NDD (natal dispersal duration and previous territory status), identifying additional influences on NDD is in its infancy in large raptors and deserves more study.

Golden Eagle Populations, Movements, and Landscape Barriers: Insights from Scotland

“…In a sample of thirty-nine birds, the median estimates of the NDD of golden eagles in Scotland were 30 km for males and 59 km for females or 38 km averaged across the sexes, with a significant difference between the sexes. The maximum estimates were 82 km for males and 87 km for females [6]. The records of the JDD were typically much higher [30].…”

“…The NDD is the movement (linear distance) between the fledging (natal) location and the first reproductive or potential reproductive location e.g., the first settled occupied territory [67,68]. In many raptors, especially in genera with larger species such as Aquila and Haliaeetus, most movements affecting gene flow, demography, and population/sub-population limits are most influentially due to the NDD rather than breeding dispersal (movement between successive breeding locations) [6,23,69,70].…”

“…The advent of GPS satellite tagging combined with technological advancements and the increasing deployment of tags has progressed knowledge of many birds' biology [1]. Satellite tracking allows for novel investigations of how golden eagles Aquila chrysaetos use the landscape at a range of scales and during different life history stages (e.g., [2][3][4][5][6][7]). This can include research on geographical barriers that may prevent or limit range expansion or create population/sub-population isolation.…”

“…This can include research on geographical barriers that may prevent or limit range expansion or create population/sub-population isolation. If there are significant barriers to golden eagle movements, there could be demographic and genetic consequences [6,[8][9][10][11] with applied relevance for conservation.…”

“…Beyond recording the natal dispersal distance (NDD) and the distances moved during natal/juvenile dispersal (juvenile dispersal distance, JDD) (e.g., [6,[23][24][25]), this subject has seldom been considered explicitly for golden eagles [17,22]. Despite the species' widespread Holarctic distribution [16], the global opportunities for contemporary research on landscape barriers at finer scales may be limited, or seldom explored, when most focus has been intraor inter-continental [9,10,15,17].…”