Spatial segregation in a sexually dimorphic central place forager: Competitive exclusion or niche divergence?

Abstract

1. Sexual competition is increasingly recognized as an important selective pressure driving species distributions. However, few studies have investigated the relative importance of interpopulation versus intrapopulation competition in relation to habitat availability and selection.2. To explain spatial segregation between sexes that often occurs in non-territorial and central place foragers, such as seabirds, two hypotheses are commonly used.The 'competitive exclusion' hypothesis states that dominant individuals should exclude subordinate individuals through direct competition, whereas the 'niche divergence' hypothesis states that segregation occurs due to past competition and habitat specialization.3. We tested these hypotheses in two populations of an extreme wide-ranging and sexually dimorphic seabird, investigating the relative role of intrapopulation and interpopulation competition in influencing sex-specific distribution and habitat preferences.4. Using GPS loggers, we tracked 192 wandering albatrosses Diomedea exulans during four consecutive years (2016-2019), from two neighbouring populations in the Southern Ocean (Prince Edward and Crozet archipelagos). We simulated pseudotracks to create a null spatial distribution and used Kernel Density Estimates (KDE) and Resource Selection Functions (RSF) to distinguish the relative importance of within-versus between-population competition.5. Kernel Density Estimates showed that only intrapopulation sexual segregation was significant for each monitoring year, and that tracks between the two colonies resulted in greater overlap than expected from the null distribution, especially for the females. RSF confirmed these results and highlighted key at-sea foraging areas, even if the estimated of at-sea densities were extremely low. These differences in selected areas between sites and sexes were, however, associated with high interannual variability in habitat preferences, with no clear specific preferences per site and sex. 6. Our results suggest that even with low at-sea population densities, historic intrapopulation competition in wide-ranging seabirds may have led to sexual dimorphism | 2405

Revisiting niche divergence hypothesis in sexually dimorphic birds: Is diet overlap correlated with sexual size dimorphism?

“…Though the functional explanation is open to debate, it can reduce intraspecific resource competition. This explanation is consistent with previous studies of niche divergence that linked it to resource competition in dimorphic species (Orgeret et al., 2021), such as hummingbirds (Maglianesi et al., 2022; Temeles et al., 2010) and woodpeckers (Pasinelli, 2000; Selander, 1966). An alternative interpretation of this observed pattern is that the SSD may have evolved as a consequence of sexual selection.…”

Albatrosses develop attraction to fishing vessels during immaturity but avoid them at old age

“…These results align with the prediction that boldness is part of a suite of behaviours including aggression and dominance in competitive situations [11]. Male wandering albatrosses are probably dominant over females [40], likely owing to their larger size, and bolder birds are often dominant to shyer ones. Our results showing that shy females are less likely to attend boats may be driven by their low competitive ability, resulting in competitive exclusion.…”

Natal legacies cause social and spatial marginalization during dispersal

“…For this, we assessed the spatial overlap between the 50% UD of the different natal groups (supplemented last-hatched, nonsupplemented last-hatched, supplemented early-hatched, non-supplemented early-hatched) with the central foraging area ('CFA'), defined as the 20% UD of all GPS locations between 9 AM and 2 PM, the period of high foraging activity, using the BA index. A 50% UD overlap is a widespread methodology to examine core space use in animals (Orgeret et al, 2021;Rossiter et al, 2002). To conduct a significance test, we simulated 1000 overlaps by randomly exchanging group identity of individuals in each permutation (Farine, 2017) and scored the proportion of the simulated overlaps that were less than or exceeded the observed overlap, where the former indicates aggregation and the latter segregation.…”

“…However, it is important to note that our results do not exclude the existence of a relationship between physical or physiological condition and early dispersal strategies, as demonstrated by previous studies (Kingma et al, 2016;Mares et al, 2014), because red kites can perform long-distance explorations in a matter of days or weeks (Aebischer & Scherler, 2021), and detecting these was not a focus of our approach. Further, natal dispersal in red kites continues for several years after returning from the first migration (Orgeret et al, 2023), so early-life effects on dispersal strategies may become visible only at later phases of natal dispersal. Males spent proportionally more time in the study area than females, consistent with the general dispersal pattern in birds (Greenwood & Harvey, 1982) whereby females tend to disperse further than males.…”