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Animal populations are often comprised of both foraging specialists and generalists. For instance, some individuals show higher foraging site fidelity (spatial specialization) than others. Such individual differences in degree of specialization can persist over time‐scales of months or even years in long‐lived animals, but the mechanisms leading to these different individual strategies are not fully understood. There is accumulating evidence that individual variation in foraging behaviour is shaped by animal personality traits, such as boldness. Despite this, the potential for boldness to drive differences in the degree of specialization is unknown. In this study, we used novel object tests to measure boldness in black‐legged kittiwakes (Rissa tridactyla) breeding at four colonies in Svalbard and deployed GPS loggers to examine their at‐sea foraging behaviour. We estimated the repeatability of foraging trips and used a hidden Markov model to identify locations of foraging sites in order to quantify individual foraging site fidelity. Across the breeding season, bolder birds were more repeatable than shy individuals in the distance and range of their foraging trips, and during the incubation period (but not chick rearing), bolder individuals were more site‐faithful. Birds exhibited these differences while showing high spatial similarity in foraging areas, indicating that site selection was not driven by personality‐dependent spatial partitioning. We instead suggest that a relationship between boldness and site fidelity may be driven by differences in behavioural flexibility between bold and shy individuals. Together, these results provide a potential mechanism by which widely reported individual differences in foraging specialization may emerge.

1. There is widespread evidence that within populations, specialists and generalists can coexist and this is particularly prevalent in marine ecosystems, where foraging specialisations are evident. 2. While individuals may limit niche overlap by consistently foraging in specific areas, site fidelity may also emerge as an artefact of habitat choice, but both drivers and fitness consequences of site fidelity are poorly understood. 3. Here, we examine an individual metric of site and habitat fidelity, using tracking data collected over 11 years for black-browed albatrosses (Thalassarche melanophris). Fidelity was calculated as the similarity between pairs of foraging zones, quantifying measures for within and between years. Foraging areas were identified using area-restricted search, defined as periods during which birds decrease speed and increase turning. 4. Our results demonstrate that birds were considerably more specialised in the habitat in which they forage than the exact location they use within years, and there was a similar pattern between years. However, despite this, it was site fidelity that explained reproductive success. Within a single year, females which were more faithful to a specific location had higher reproductive success than non-specialists, and between years there was a tendency for both sexes. 5. Our results suggest that black-browed albatrosses are highly faithful in their foraging habitat but it is rather site fidelity that is more clearly associated with reproductive success.

Intra- and inter-specific resource partitioning within predator communities is a fundamental component of trophic ecology, and one proposed mechanism for how populations partition resources is through individual niche variation. The Niche Variation Hypothesis (NVH) predicts that inter-individual trait variation leads to functional trade-offs in foraging efficiency, resulting in populations composed of individual dietary specialists. The degree to which niche specialization persists within a population is plastic and responsive to fluctuating resource availability. We quantified niche overlap and tested the NVH within an Arctic raptor guild, focusing on three species that employ different foraging strategies: golden eagles (generalists); gyrfalcons (facultative specialists); and rough-legged hawks (specialists). Tundra ecosystems exhibit cyclic populations of arvicoline rodents (lemmings and voles), providing a unique system in which to examine predator diet in response to interannual fluctuations in resource availability. Using blood δ13C and δ15N values from 189 raptor nestlings on Alaska’s Seward Peninsula (2014–2019), we calculated isotopic niche width and used Bayesian stable isotope mixing models (BSIMMs) to characterize individual specialization and test the NVH. Nest-level specialization estimated from stable isotopes was strongly correlated with indices of specialization based on camera trap data. We observed a high degree of isotopic niche overlap between the three species and gyrfalcons displayed a positive relationship between individual specialization and population niche width on an interannual basis consistent with the NVH. Our findings suggest plasticity in niche specialization may reduce intra- and inter-specific resource competition under dynamic ecological conditions.

1. The degree of individual specialization in resource use differs widely among wild populationswhere individuals range from fully generalized to highly specialized. This interindividualvariation has profound implications in many ecological and evolutionary processes. A recentreview proposed four main ecological causes of individual specialization: interspecific andintraspecific competition, ecological opportunity and predation.2. Using the isotopic signature of subsampled whiskers, we investigated to what degree threeof these factors (interspecific and intraspecific competition and ecological opportunity) affectthe population niche width and the level of individual foraging specialization in two fur sealspecies, the Antarctic and subantarctic fur seals (Arctocephalus gazella and Arctocephalustropicalis), over several years.3. Population niche width was greater when the two seal species bred in allopatry (low interspecificcompetition) than in sympatry or when seals bred in high-density stabilized colonies (highintraspecific competition). In agreement with the niche variation hypothesis (NVH), higherpopulation niche width was associated with higher interindividual niche variation. However, incontrast to the NVH, all Antarctic females increased their niche width during the interbreedingperiod when they had potential access to a wider diversity of foraging grounds and associatedprey (high ecological opportunities), suggesting they all dispersed to a similar productive area.4. The degree of individual specialization varied among populations and within the annualcycle. Highest levels of interindividual variation were found in a context of lower interspecificor higher intraspecific competition. Contrasted results were found concerning the effect ofecological opportunity. Depending on seal species, females exhibited either a greater or lowerdegree of individual specialization during the interbreeding period, reflecting species-specificbiological constraints during that period.5. These results suggest a significant impact of ecological interactions on the population nichewidth and degree of individual specialization. Such variation at the individual level may bean important factor in the species plasticity with significant consequences on how it mayrespond to environmental variability.

Populations usually considered foraging generalists may include specialized individuals that feed on a restricted subset of the prey spectrum consumed by the population. By analyzing the time series of δ13C and δ15N values in sequential growth layer groups within tooth dentin, we measured population- and individual-level variation in resource use of three populations of Guiana dolphins (Sotalia guianensis)—Caravelas River, Babitonga Bay, and Norte Bay—along a latitudinal gradient in the southwestern Atlantic Ocean. We show that the Guiana dolphin at Caravelas River is a generalist population consisting of individual dietary specialists, likely due to the absence of other resident dolphin populations thus allowing individuals to target prey across a wide range of habitats. The Babitonga Bay population is also composed of individual specialists potentially due to the selective foraging behavior of some individuals on high-quality prey sources within and near the bay. In contrast, the Norte Bay population comprises individual generalists, which likely reflects its distinctive cohesive social organization, coexistence with two other dolphin species, and an opportunistic foraging strategy in response to resource fluctuations inherent to the southern limit of the species distribution. Although the Guiana dolphin is generally considered to be a dietary generalist at the population level, our findings reveal that the total niche width of populations and the degree of individual diet specialization are highly context dependent, suggesting dietary plasticity that may be related to a latitudinal gradient in resource availability and environmental conditions.

Trophic niche width and individual specialization among marine predators are often subjected to seasonal constraints. Differences are expected to arise for sexually dimorphic species exposed to distinct ecological opportunities, as well as intrinsic differences in physiological abilities or energetic requirements. We assess seasonal and sexual differences in isotopic niche overlap and width and analyze inter-individual foraging variation throughout the year of the South American fur seal (Arctocephalus australis). We obtained chronologically ordered δ13C and δ15N data from vibrissae of 25 males and 24 females from southern Brazil and Uruguay. There were significant differences between sexes for both isotopes with males showing higher values (δ13C = − 14.5 ± 0.5‰; δ15N = 18.9 ± 1.2‰) than females (δ13C = − 15.2 ± 0.5‰; δ15N = 17.8 ± 1.2‰), but not among seasons or years. A very small isotopic niche overlap found between sexes (1.1–9.5%), with limited seasonal variation, likely resulted from differences in prey consumption and foraging habitats. Compared to other seasons, females had a wider isotopic niche in spring. While males showed seasonal stability in the foraging niche, females showed some small variation, which is probably influenced by central place foraging. Both sexes had considerable interindividual variation in estimated dietary composition and had different proportions of potential prey contribution at the population level. These findings enlighten the knowledge of South American fur seal trophic ecology, and how they may seasonally and spatially shift their foraging strategies according to their distinct life histories. It ultimately reflects in a small niche partitioning and possibly low intraspecific competition at the species northernmost area of occurrence in the western South Atlantic.

Dietary diversity often varies inversely with prey resource abundance. This pattern, although typically measured at the population level, is usually assumed to also characterize the behavior of individual animals within the population. However, the pattern might also be produced by changes in the degree of variation among individuals. Here we report on dietary and associated behavioral changes that occurred with the experimental translocation of sea otters from a food-poor to a food-rich environment. Although the diets of all individuals were broadly similar in the food-rich environment, a behaviorally based dietary polymorphism existed in the food-poor environment. Higher dietary diversity under low resource abundance was largely driven by greater variation among individuals. We further show that the dietary polymorphism in the food-poor environment included a broad suite of correlated behavioral variables and that the individuals that comprised specific behavioral clusters benefited from improved foraging efficiency on their individually preferred prey. Our findings add to the growing list of examples of extreme individuality in behavior and prey choice within populations and suggest that this phenomenon can emerge as a behavioral manifestation of increased population density. Individuality in foraging behavior adds complexity to both the fitness consequences of prey selection and food web dynamics, and it may figure prominently as a diversifying process over evolutionary timescales.

We evaluated whether existing assumptions regarding the trophic ecology of a poorly‐studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ13C) and nitrogen (δ15N) stable isotope values as a proxy for foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in 13C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e. from smaller to larger prey items) and become enriched in 15N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found some evidence for ontogenetic shifts in habitat‐use (δ13C) for thorny and little skate and diet (δ15N) of thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ15N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked expected spatial variability in the isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ13C and δ15N values, suggesting individuals may use resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g. little and thorny skate), sympatric species (e.g. little and winter skate) exhibited a degree of trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly‐studied, vulnerable group of predators, and highlight a need to re‐examine assumptions pertaining to aspects of their ecology.

The width of a population's resource-use niche is determined by individual diet breadth (“within-individual component”) and the degree of niche partitioning between individuals (“between-individual component”). The balance between these two factors affects ecological stability and evolutionary trajectories, and may shift as ecological opportunity permits broader population niches. Lakes in California’s Sierra Nevada Mountains vary in resource diversity for introduced brook trout (Salvelinus fontinalis) due to elevation, lake morphometry, and watershed features. We compared the relative contributions of within- and between-individual niche components to two measures of the dietary niches of thirteen populations of brook trout: prey taxonomic composition and prey size distribution. For both taxonomic and size diversity of fish diets, population niche width was positively related to both the within- and between-individual components. For taxonomic diversity, the two components increased in parallel, while for size diversity, the between-individual component became more important relative to the within-individual component in populations with the greatest niche widths. Our results support the Niche Variation Hypothesis that populations with broader niches are more heterogeneous among individuals and show that individual niche width and individual specialization can operate in parallel to expand the population niche.

Toothed cetaceans represent a diverse and important component in poorly studied pelagic food webs. We used the variation in δ 13 C and δ 15 N values in tooth dentin and bone collagen sampled from 185 specimens of 23 species (families Delphinidae, Physeteridae, Kogiidae, Phocoenidae, and Ziphiidae) stranded in the western South Atlantic Ocean (WSAO) to estimate the isotopic niche width and assess the interspecific isotopic niche overlap. Two gradients of increasing δ 13 C values among species were evidenced: ocean-coast and high-to-low latitudes. Variation in δ 13 C and δ 15 N values indicated the existence of at least four trophic and habitat-related guilds: neritic high trophic-level predators ( Pseudorca crassidens , Tursiops truncatus gephyreus , T. truncatus truncatus and Phocoena spinnipinis ); oceanic intermediate trophic-level predators ( Orcinus orca , Steno bredanensis , Kogia breviceps , Lagenodelphis hosei , Physeter macrocephalus , Delphinus delphis and Stenella frontalis ), oceanic low trophic-level predators ( Grampus griseus , P. crassidens —mass stranded group, Mesoplodon grayi , M. densirostris , Ziphius cavirostris , Globicephala melas , S. coeruleoalba , and K. sima ), all from the subtropics, and a temperate/polar grouping of neritic/oceanic low trophic-level predators ( Berardius arnuxii , M. layardii , M. hectori and P. dioptrica ). Furthermore, our results also suggest the occurrence of putative ecotypes of O. orca and P. crassidens . The isotopic niche width was highly variable among species, with high niche overlap among several oceanic taxa. The results of the present study suggest a wide variation in trophic position, habitat and, therefore, the ecological role of different odontocete species in the WSAO ecosystems.