Demographic partitioning of dynamic energy subsidies revealed with anOrnstein–Uhlenbeckspace use model

Abstract

In populations across many taxa, a large fraction of sexually mature individuals do not breed but are attempting to enter the breeding population. Such individuals, often referred to as “floaters,” can play critical roles in the dynamics and stability of these populations and buffer them through periods of high adult mortality. Floaters are difficult to study, however, so we lack data needed to understand their roles in the population ecology and conservation status of many species. Here, we analyzed satellite telemetry data with a newly developed mechanistic space use model based on an Ornstein–Uhlenbeck process to help overcome the paucity of data in studying the differential habitat selection and space use of floater and territorial golden eagles Aquila chrysaetos. Our sample consisted of 49 individuals tracked over complete breeding seasons across 4 years, totaling 104 eagle breeding seasons. Modeling these data mechanistically was required to disentangle key differences in movement and particularly to separate aspects of movement driven by resource selection from those driven by use of a central place. We found that floaters generally had more expansive space use patterns and larger home ranges, as well as evidence that they partition space with territorial individuals seemingly on fine scales through differential habitat and resource selection. Floater and territorial eagle home ranges overlapped markedly, suggesting that floaters use the interstices between territories. Furthermore, floater and territorial eagles differed in how they selected for uplift variables, key components of soaring birds’ energy landscape, with territorial eagles apparently better able to find and use thermal uplift. We also found relatively low individual heterogeneity in resource selection, especially among territorial individuals, suggesting a narrow realized niche for breeding individuals, which varied from the level of among‐individual variation present during migration. This work furthers our understanding of floaters’ potential roles in the population ecology of territorial species and suggests that conserving landscapes occupied by territorial eagles also protects floaters.

A hierarchical modelling framework for estimating individual‐ and population‐level reproductive success from movement data

“…The individual level (first stage) of our model is computationally intensive on its own, so the ability to implement it separately (and in parallel) for each individual is crucial to being able to scale the model to estimate population‐level reproductive success. Although only recently introduced to applied ecological statistics (Hooten et al, 2016; McCaslin et al, 2021), recursive Bayes is starting to gain traction in applied studies (Cameron et al, 2021; Eisaguirre et al, 2022; Feuka et al, 2022). Our approach provides another example of its utility in estimating complex, multi‐level ecological models for making both individual‐ and population‐level inference.…”

“…That is, based on previous work (e.g. Eisaguirre et al, 2022), we anticipated likely effect sizes for the movement parameters, in terms of both spread and correlation of movements, and tested the ability of our approach to recover those. Furthermore, the simulated survey data were formulated similar to the real aerial survey data collected for our application.…”

“…For notational simplicity, we let correspond to midnight on 15 April, which implies . We also excluded ‘floaters’ from our sample, which are sexually mature individuals that lack territories and thus do not initiate breeding attempts (Eisaguirre et al, 2022; Penteriani et al, 2011). Consequently, even for individuals in our sample that were not observed engaged in a viable nesting attempt later in the season, because any individuals holding and defending a territory at the beginning of the breeding season are likely attempting to nest (Watson, 2010).…”

“…The telemetry data we use are described in detail by Eisaguirre et al (2022). Briefly, golden eagles were captured and tagged in southcentral Alaska during spring migration 2014-2016, and the solar-powered GPS tags were programmed to take fixes 8-14 times per day, with some consecutive fixes being recorded as frequently as hourly and others as long as 16 h apart.…”

“…However, changes in behaviour can arise as animals perform key life‐history activities. For example, territorial species exhibit marked changes in their movement when they obtain a territory (Eisaguirre et al, 2022), and some taxa, such as ungulates, have been shown to change their movement with the timing of reproductive events (e.g. parturition and offspring mortality; Cameron et al, 2018; DeMars et al, 2013).…”