Generalist niche, specialist strategy: the diet of an Australian percichthyid

Dietary analysis revealed that an impoundment population of Australian bass Macquaria novemaculeata holds a generalist niche, but one arising from persistent individual specialization and interindividual variation. This 'individual specialist' strategy appeared adaptive, but the strength of individual specialization was largely independent of variation in diet composition, except during blooms of Daphnia sp. Diet composition and dietary overlap showed only moderate ontogenetic variation, and niche breadth showed no relationship with ontogeny. Macquaria novemaculeata showed an asymmetric predator and prey size distribution, consistent with many aquatic predators, with positive relationships between fish size and average, maximum and minimum prey size. There was no asymmetry in the relative size-based niche breadths of individuals, however, which indicates that the niche is a fixed 'window' of relative prey sizes. The difference in the dietary niche and prey-size relationships of M. novemaculeata at the population and individual levels highlights the necessity of assessing the niche at both these levels.     

Do the evolutionary interactions between moths and bats promote niche partitioning between bats and birds?

Ecological theory suggests that the coexistence of species is promoted by the partitioning of available resources, as in dietary niche partitioning where predators partition prey. Yet, the mechanisms underlying dietary niche partitioning are not always clear. We used fecal DNA metabarcoding to investigate the diets of seven nocturnal insectivorous bird and bat species. Low diet overlap (2%–22%) supported resource partitioning among all species. Differences in diet corresponded with species identity, prey detection method, and foraging behavior of predators. Insects with ultrasonic hearing capabilities were consumed significantly more often by birds than bats, consistent with an evolved avoidance of echolocating strategies. In turn, bats consumed a greater proportion of noneared insects such as spruce budworms. Overall, our results suggest that evolutionary interactions among bats and moths translate to dietary niche partitioning and coexistence among bats and nocturnal birds.     

Seasonal variation in terrestrial resource subsidies influences trophic niche width and overlap in two aquatic snake species: a stable isotope approach

Quantifying diet is essential for understanding the functional role of species with regard to energy processing, transfer, and storage within ecosystems. Recently, variance structure in the stable isotope composition of consumer tissues has been touted as a robust tool for quantifying trophic niche width, a task that has previously proven difficult due to bias in direct dietary analyses and difficulties in integrating diet composition over time. We used carbon and nitrogen stable isotope analyses to examine trophic niche width of two sympatric aquatic snakes, banded watersnakes Nerodia fasciata and black swamp snakes Seminatrix pygaea inhabiting an isolated wetland where seasonal migrations of amphibian prey cause dramatic shifts in resource availability. Specifically, we characterized snake and prey isotope compositions through time, space, and ontogeny and examined isotope values in relation to prey availability and snake diets assessed by gut content analysis. We determined that prey cluster into functional groups based on similarity of isotopic composition and seasonal availability. Isotope variance structure indicated that the trophic niche width of the banded watersnake was broader (more generalist) than that of the black swamp snake. Banded watersnakes also exhibited seasonal variation in isotope composition, suggesting seasonal diet shifts that track amphibian prey availability. Conversely, black swamp snakes exhibited little seasonal variation but displayed strong ontogenetic shifts in carbon and nitrogen isotope composition that closely paralleled ontogenetic shifts in their primary prey, paedomorphic mole salamanders Ambystoma talpoideum. Although niche dimensions are often treated as static, our results demonstrate that seasonal shifts in niche dimensions can lead to changes in niche overlap between sympatric species. Such short‐term fluctuations in niche overlap can influence competitive interactions and consequently the composition and dynamics of communities and ecosystems.     

Feeding ecology of the endangered Valencia toothcarp,Valencia hispanica (Actinopterygii: Valenciidae)

Patterns of food resource utilisation by Valencia toothcarp were studied in a small coastal lagoon in the Iberian Peninsula, its most northern population. Emphasis was placed on feeding activity, diet composition, seasonal and intraspecific variation in diet and feeding strategy. Important features of the overall feeding pattern of the Valencia toothcarp are decreased feeding activity during winter, concentration on benthic invertebrates and narrow dietary breadth, with a focus on gammarid amphipods. The feeding of V. hispanica was independent of seasonal variation in food availability, because its main prey were much the same throughout the year. There were no differences in seasonal niche width and niche overlap between seasons was high. There are also no differences in diets between sexes, although diet composition, niche width and niche overlap differ between juvenile and adult fish. The Valencia toothcarp population from Santes Creus lagoon was formed by specialist individuals, with narrow niche widths, that fed on four preferential prey types, but also consumed some occasional prey.     

Food niche overlap and ecological separation in a multi-species community of shrews in the Siberian taiga

An investigation of the feeding habits and prey availability in a community of seven species of shrew (Insectivora: Soricidae) inhabiting the taiga of Central Siberia was carried out with the aim of quantifying levels of niche overlap and elucidating modes of ecological separation amongst these coexisting species. All species took a wide range of invertebrate prey, and overlap in the numbers of shared prey taxa was high, but differences in dietary composition of certain taxa reduced overlap between most species. Small species fed almost exclusively on small arthropods, mostly Araneae, Chilopoda and Coleoptera, while medium and large-sized species took high proportions of oligochaetes. Prey were mostly taken in proportions approximately equal to their availability, although certain prey appeared to be selected. All shrews took prey in a range of sizes, and the high dietary occurrence of small invertebrates reflected their availability and high encounter rate in field samples. Dietary occurrence of small prey was negatively correlated, and large prey positively correlated, with body size of shrew. Smaller shrews were predominantly ground-surface foragers while larger species were more subterranean, with body size and dietary occurrence of soil prey being positively correlated. Differences in prey size and foraging mode reduced niche overlap between shrew species of widely differing sizes. Each shrew species did not occupy a separate, well-defined food niche. Instead, the community was sub-divided into three functional groups: large and small species which tended towards specialization with relatively low levels of overlap, and intermediate, generalist species with higher levels of overlap.     

18S rRNA metabarcoding diet analysis of a predatory fish community across seasonal changes in prey availability

Predator–prey relationships are important ecological interactions, affecting biotic community composition and energy flow through a system, and are of interest to ecologists and managers. Morphological diet analysis has been the primary method used to quantify the diets of predators, but emerging molecular techniques using genetic data can provide more accurate estimates of relative diet composition. This study used sequences from the 18S V9 rRNA barcoding region to identify prey items in the gastrointestinal (GI) tracts of predatory fishes. Predator GI samples were taken from the Black River, Cheboygan Co., MI, USA (n = 367 samples, 12 predator species) during periods of high prey availability, including the larval stage of regionally threatened lake sturgeon (Acipenser fulvescens Rafinesque 1817) in late May/early June of 2015 and of relatively lower prey availability in early July of 2015. DNA was extracted and sequenced from 355 samples (96.7%), and prey DNA was identified in 286 of the 355 samples (80.6%). Prey were grouped into 33 ecologically significant taxonomic groups based on the lowest taxonomic level sequences that could be identified using sequences available on GenBank. Changes in the makeup of diet composition, dietary overlap, and predator preference were analyzed comparing the periods of high and low prey abundance. Some predator species exhibited significant seasonal changes in diet composition. Dietary overlap was slightly but significantly higher during the period of high prey abundance; however, there was little change in predator preference. This suggests that change in prey availability was the driving factor in changing predator diet composition and dietary overlap. This study demonstrates the utility of molecular diet analysis and how temporal variability in community composition adds complexity to predator–prey interactions.     

Trophic ecology of Atlantic bluefin tuna Thunnus thynnus larvae

Feeding intensity, diet composition, selectivity, energy ingestion and dietary niche breadth of larval Atlantic bluefin tuna Thunnus thynnus were studied on the eastern (Mediterranean) spawning grounds of the species. Larval T. thynnus were collected in the Balearic Archipelago (north-west Mediterranean Sea) during 2004 and 2005 using surveys specific for larval scombrids. Larvae between 2·6 and 8·7 mm standard length (L(S) ) are diurnal feeders, and 94% of the guts collected during daylight hours were full. The mean ±s.d. number of prey per gut was 7·1 ± 5·7, with mean ±s.d. ranging from 3·0 ± 1·6 in the smallest T. thynnus larvae to 11·1 ± 5·8 in 5·0-6·0 mm L(S) larvae. Up to 21 prey were found in a single larval gut (5·0-6·0 mm L(S) ) at the end of the day. Larvae progressively selected larger prey and exhibited increased carbon content concurrent with preflexion development of feeding and locomotory structures. Larvae of 5·0-6·0 mm L(S) exhibited positive selection of cladocerans over other prey (Chesson's index), whereas copepod nauplii dominated the diets of earlier stages. The dietary niche breadth measured increased initially but decreased at c. 5·5 mm L(S) . Appendicularians were found in the diet of larger larval sizes, but no piscivory was observed. Results are discussed in light of the sparse existing data for larval T. thynnus and other larval tuna species.     

Dietary overlap and seasonality in three species of mormoopid bats from a tropical dry forest

Competing hypotheses explaining species’ use of resources have been advanced. Resource limitations in habitat and/or food are factors that affect assemblages of species. These limitations could drive the evolution of morphological and/or behavioural specialization, permitting the coexistence of closely related species through resource partitioning and niche differentiation. Alternatively, when resources are unlimited, fluctuations in resources availability will cause concomitant shifts in resource use regardless of species identity. Here, we used next‐generation sequencing to test these hypotheses and characterize the diversity, overlap and seasonal variation in the diet of three species of insectivorous bats of the genus Pteronotus. We identified 465 prey (MOTUs) in the guano of 192 individuals. Lepidoptera and Diptera represented the most consumed insect orders. Diet of bats exhibited a moderate level of overlap, with the highest value between Pteronotus parnellii and Pteronotus personatus in the wet season. We found higher dietary overlap between species during the same seasons than within any single species across seasons. This suggests that diets of the three species are driven more by prey availability than by any particular predator‐specific characteristic. P. davyi and P. personatus increased their dietary breadth during the dry season, whereas P. parnellii diet was broader and had the highest effective number of prey species in all seasons. This supports the existence of dietary flexibility in generalist bats and dietary niche overlapping among groups of closely related species in highly seasonal ecosystems. Moreover, the abundance and availability of insect prey may drive the diet of insectivores.     

Trophic niche segregation allows range‐extending coral reef fishes to co‐exist with temperate species under climate change

Changing climate is forcing many terrestrial and marine species to extend their ranges poleward to stay within the bounds of their thermal tolerances. However, when such species enter higher latitude ecosystems, they engage in novel interactions with local species, such as altered predator–prey dynamics and competition for food. Here, we evaluate the trophic overlap between range‐extending and local fish species along the east coast of temperate Australia, a hotspot for ocean warming and species range extensions. Stable isotope ratios (δ¹⁵N and δ¹³C) of muscle tissue and stomach content analysis were used to quantify overlap of trophic niche space between vagrant tropical and local temperate fish communities along a 730 km (6°) latitudinal gradient. Our study shows that in recipient temperate ecosystems, sympatric tropical and temperate species do not overlap significantly in their diet—even though they forage on broadly similar prey groups—and are therefore unlikely to compete for trophic niche space. The tropical and temperate species we studied, which are commonly found in shallow‐water coastal environments, exhibited moderately broad niche breadths and local‐scale dietary plasticity, indicating trophic generalism. We posit that because these species are generalists, they can co‐exist under current climate change, facilitating the existence of novel community structures.     

Prey use of wetland benthivorous sunfishes: ontogenetic, interspecific and seasonal variation

The intensity of competitive interactions between fishes is partly determined by prey use and ontogenetic niche shifts. In a wetland where distinct habitat shifts are missing we compared prey use of three generalist benthivorous sunfishes to look for evidence of ontogenetic, interspecific, and “seasonal” variation in prey composition. Diet analysis revealed evidence of diet ontogeny in warmouth (Lepomis gulosus, 30–152 mm standard length, SL), but not in bluespotted sunfish (Enneacanthus gloriosus, 30–47 mm SL) or dollar sunfish (Lepomis marginatus, 30–60 mm SL). Bluespotted and dollar sunfishes consumed small dipteran and amphipod prey and had similar diets in both seasons suggesting a potential for strong interspecific competition. In the dry season, warmouth shifted from using smaller insect prey to larger decapod and fish prey with increasing size. This shift to prey types that were little used by the other species reduced dietary niche overlap with the other sunfishes. After drought and re-flooding (in the wet season), decapods and small fish were less abundant in the wetland and the warmouth ontogenetic shift was less distinct. When matched for gape width, prey composition differed between warmouth and both dollar and bluespotted sunfishes in the wet season, suggesting differences in sunfish foraging modes, but prey use differences were less clear in the dry season when prey were abundant. Both warmouth ontogenetic diet shifts and seasonal variation in prey use (probably mediated by prey abundance) had strong influences on diet overlap and therefore the potential for intra- and interspecific competition between sunfishes in this wetland ecosystem.     

Human activity reduces niche partitioning among three widespread mesocarnivores

Anthropogenic disturbances can constrain the realized niche space of wildlife by inducing avoidance behaviors and altering community dynamics. Human activity might contribute to reduced partitioning of niche space by carnivores that consume similar resources, both by promoting tolerant species while also altering behavior of species (e.g. activity patterns). We investigated the influence of anthropogenic disturbance on habitat and dietary niche breadth and overlap among competing carnivores, and explored if altered resource partitioning could be explained by human‐induced activity shifts. To describe the diets of coyotes, bobcat, and gray foxes, we designed a citizen science program to collect carnivore scat samples in low‐ (‘wildland’) and high‐ (‘interface’) human‐use open space preserves, and obtained diet estimates using a DNA metabarcoding approach. Habitat use was determined at scat locations. We found that coyotes expanded habitat and dietary niche breadth in interface preserves, whereas bobcats and foxes narrowed both niche breadth measures. High human use was related to increased dietary niche overlap among all mesocarnivore pairs, increased coyote habitat overlap with bobcats and foxes, and a small reduction in habitat overlap between bobcats and foxes. The strongest increase in diet overlap was among coyotes and foxes, which was smaller in magnitude than their habitat overlap increase. Finally, coyote scats were more likely to contain nocturnal prey in interface preserves, whereas foxes appeared to reduce consumption of nocturnal prey. Our results suggest that dominant and generalist mesocarnivores may encroach on the niche space of subordinate mesocarnivores in areas with high human activity, and that patterns in resource use may be related to human‐induced activity shifts.     

High-throughput sequencing offers insight into mechanisms of resource partitioning in cryptic bat species

Sympatric cryptic species, characterized by low morphological differentiation, pose a challenge to understanding the role of interspecific competition in structuring ecological communities. We used traditional (morphological) and novel molecular methods of diet analysis to study the diet of two cryptic bat species that are sympatric in southern England (Plecotus austriacus and P. auritus) (Fig. 1). Using Roche FLX 454 (Roche, Basel, CH) high-throughput sequencing (HTS) and uniquely tagged generic arthropod primers, we identified 142 prey Molecular Operational Taxonomic Units (MOTUs) in the diet of the cryptic bats, 60% of which were assigned to a likely species or genus. The findings from the molecular study supported the results of microscopic analyses in showing that the diets of both species were dominated by lepidopterans. However, HTS provided a sufficiently high resolution of prey identification to determine fine-scale differences in resource use. Although both bat species appeared to have a generalist diet, eared-moths from the family Noctuidae were the main prey consumed. Interspecific niche overlap was greater than expected by chance (O(jk) = 0.72, P < 0.001) due to overlap in the consumption of the more common prey species. Yet, habitat associations of nongeneralist prey species found in the diets corresponded to those of their respective bat predator (grasslands for P. austriacus, and woodland for P. auritus). Overlap in common dietary resource use combined with differential specialist prey habitat associations suggests that habitat partitioning is the primary mechanism of coexistence. The performance of HTS is discussed in relation to previous methods of molecular and morphological diet analysis. By enabling species-level identification of dietary components, the application of DNA sequencing to diet analysis allows a more comprehensive comparison of the diet of sympatric cryptic species, and therefore can be an important tool for determining fine-scale mechanisms of coexistence.     

The diet of the Sea Eagle Haliaeetus albicilla and Golden Eagle Aquila chrysaetos in western Scotland

The diets of Sea Eagles Haliaeetus albicilla and Golden Eagles Aquila chrysaetos living sympatrically in western Scotland were studied using pellets and prey remains collected at roosts and nest sites throughout the year. Both species showed changes in diet between winter and summer, and there were differences between species in both seasons. As a rule, Sea Eagles took proportionately more seabirds, waterfowl (ducks and waders) and fish, whilst Golden Eagles took proportionately more rabbits and hares. Sea Eagles had a broader diet than Golden Eagles but a measure of dietary overlap based on taxonomic composition indicated an overlap of more than 90% between the two species.     

Seasonal variation in prey preference,diet partitioning and niche breadth in a rich large carnivore guild

Large carnivore community structure is affected by direct and indirect interactions between intra-guild members. Co-existence between different species within a carnivore guild may occur through diet, habitat or temporal partitioning. Since carnivore species are highly dependent on availability and accessibility of prey, diet partitioning is potentially one of the most important mechanisms in allowing carnivores to co-exist. Intra-guild interactions may vary over time as carnivore prey preference and diet overlap can change due to seasonal changes in resource availability. We conducted scat analysis to compare the seasonal changes in prey preference, diet partitioning and niche breadth of four large carnivore species, namely leopard Panthera pardus, spotted hyena Crocuta crocuta, brown hyena Parahyaena brunnea and wild dog Lycaon pictus in central Tuli, Botswana. Large carnivores in central Tuli display a high dietary overlap, with spotted hyena and brown hyena displaying almost complete dietary overlap and the other carnivore species displaying slightly lower but still significant dietary overlap. Dietary niche breadth for both hyena species was high possibly due to their flexible foraging strategies, including scavenging, while leopard and wild dog showed a relatively low niche breadth, suggesting a more specialised diet. High dietary overlap in central Tuli is possibly explained by the high abundance of prey species in the area thereby reducing competition pressure between carnivore species. Our research highlights the need to assess the influence of diet partitioning in structuring large carnivore communities across multiple study sites, by demonstrating that in prey rich environments, the need for diet partitioning by carnivores to avoid competition may be limited.     

Feeding habits and niche partitioning in a predator guild composed of tigers, leopards and dholes in a temperate ecosystem in central Bhutan

We describe the food habits, niche overlap and prey preferences in a predator guild comprised of tigers Panthera tigris , leopards Panthera pardus and dholes Cuon alpinus in a mountainous region of central Bhutan. Scat analyses revealed that these predators consumed 11 different prey species including livestock and rodents, of which leopards consumed 11. The combined relative occurrence of the three species, sambar Cervus unicolor , muntjac Munticus muntjac and wild pig Sus scrofa , constituted 42.7, 33.7 and 71.1% of the tiger, leopard and dhole diets, respectively, while livestock comprised 44.5, 73.4 and 15.9% of the prey consumed, respectively. Regression equations from earlier feeding trials were used to estimate the relative biomass and the numbers of prey consumed. Results showed that sambar featured more frequently than did muntjac and wild pig in the diets of tiger, leopard and dhole and contributed more relative biomass than did muntjac and wild pig. Sambar, muntjac and wild pig together provided 36.9, 28 and 63.1% of the biomass consumed by tigers, leopards and dholes respectively. All else being equal, there was evidence that all three predators ate livestock less than might have been expected on the basis of the abundance and high biomass of this prey category in the area. There was a high dietary niche overlap between the predators (Pianka's overlap index of 0.58–0.92), with a greater overlap between the two felid species than between the felids and the canid. This study provides evidence of a substantial diet overlap among the three sympatric carnivores, and thus highlights the potential for high intra-guild competition among them, especially given the relatively low density of prey.     

Dietary specialization and fruit availability among frugivorous birds on Sulawesi

Over 20 months I collected data on the diet composition and fruit availability for 23 frugivorous bird species (pigeons, parrots, hornbills and passerines) in two lowland rain forests on Sulawesi, Indonesia. Using these data I determined species dependency on fruit, diet composition, diet breadth and the availability of their fruit resources. Fruit dependency ranged from 44 to 100% of diets across the assemblage, with significant inter-order differences reflecting shared evolutionary history within orders. By contrast, diet breadths did not differ between bird orders owing to high within-order variability, possibly as a result of species-specific foraging strategies. Overall, no significant relationship existed between fruit dependency and diet breadth, which indicated that they are not interchangeable measures of dietary specialization, but that each provided important information on dietary specialization, and enabled assessment of resource requirements and availability. The assemblage consumed the fruits of 120 species representing 40 plant families. Figs ( Ficus spp.) were of great importance across the assemblage and comprised 57% of all fruit-eating records. For hornbills, passerines and pigeons, figs accounted for substantial proportions of monthly feeding records (= 58%) and were consumed whatever the availability of other fruits. Figs were therefore a fundamental part of species' diets, possibly every month, and not merely important during times of relative scarcity of other fruit. Conservation of fig trees both inside and outside protected areas is likely to be important in maintaining healthy populations of figs and the frugivorous birds that depend on them.     

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O_n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ¹⁵N and δ¹³C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, ?17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, ?17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.     

Sexual Dimorphism of Head Size in Phrynocephalus przewalskii: Testing the Food Niche Divergence Hypothesis

Sexual size dimorphism （SSD） is a general phenomenon in lizards, and can evolve through sexual selection or natural selection. But natural selection, which was thought to operate mainly through reducing the competition be- tween the two sexes （niche divergence hypothesis）, gave rise to a lot of controversy. We tested the niche divergence hypothesis in the toad-headed lizard Phrynocephalus przewalskii by comparing diet composition and prey sizes between males and females. The species was found to be sexual dimorphic, with males having relatively larger snout-vent length, head width, head length, and tail length, while females have relatively larger abdomen length. Based on analysis of 93 studied stomachs, a total of 1359 prey items were identified. The most common prey items were formicid, lygaeid and tenebrionid. The two sexes did not differ in the relative proportions of prey size categories they consumed and the dietary overlap based on prey species was high （O = 0.989）. In addition, the meal size, the volume or any maximal dimension of the largest prey item in the stomach was not explained by the sexes. According to our results, food niche divergence might not play an important role in the SSD evolution ofP. przewalskii.     

Differential diet breadths and species coexistence in leafroller-hunting eumenid wasps

There are, at least, three possible ways in which similar species coexist; resource partitioning, interference competition, and exploitation competition. Here, I investigated which way contributed to the coexistence of leafroller-hunting eumenid wasp species. Resource partitioning and, in addition, differential diet breadths proved to promote species coexistence in this case. First, I analyze the prey records and diet overlap of four eumenid species in a local area. The larger two eumenids hunted similar-sized prey items and had similar potential taxonomic prey uses. But the diet breadth of the subsocial eumenid was much wider than that of the solitary one. As a result, the diet overlap between the two large eumenids decreased. This was because the solitary eumenid attend repeatedly to the same hunting site inhabited by one abundant prey species, while the subsocial one made random hunting. On the other hand, the two medium-sized eumenids partitioned resources according to prey size. Secondly, I related these results to prey choice by several other species of eumenid obtained from literature sources. Ten Japanese common eumenids were divided into four groups according to their prey size. In each of the four groups, 2 to 3 wasp species differentiated the habitat (1 group) or coexisted by means of differential diet breadths (parallel with differential sociality, 2 groups).     

Trophic interactions between brown and south polar skuas at Deception Island,Antarctica

It is broadly accepted that the brown skua (Stercorarius antarcticus lonnbergi) competitively excludes the south polar skua (S. maccormicki) from penguin colonies when breeding sympatrically, forcing the latter to feed on marine resources. The purpose of this work was to examine the diets and trophic niche breadths of each species where they co-occur and to determine the degree of overlap. To this end, we analyzed 169 pellets of brown skuas, collected in two different areas (20 individuals), and 152 of south polar skuas, collected in three different areas (18 individuals), on Deception Island, South Shetland Islands, Antarctica, during the austral summer 2000. Pellet analysis often underestimates the amount of easily digestible prey, but allows for comparisons of the relative contributions of different items in the diet. South polar skuas at our study locations consumed seven different food items and had a trophic niche breadth of 0.133 compared to brown skuas that fed on 10 different items and had a trophic niche breadth of 0.078. The niche overlap between the species was 82.1%. Penguins were the principal food source of both species, however, brown skuas fed mostly on chicks, while south polar skuas fed on adults (carcasses). The use of different age classes of penguins as a food source offers an alternative to competitive exclusion, allowing the coexistence of these species on Deception Island.