Average Prey Size Research Articles

Diet and prey size preference in Razorbills Alca torda breeding at Stora Karlsö, Sweden

The Razorbill Alca torda is one of three auk species breeding in Sweden, often occurring in colonies with Common Guillemot Uria aalge. The largest colony in the Baltic Sea is situated on the island Stora Karlsö west of Gotland. During the chick-rearing phase, Razorbills forage at sea and return to feed the young with single or multiple prey carried sideways in the bill, unlike Common Guillemots that carry a single fish in the middle of the bill. Clupeids, especially sprat Sprattus sprattus, are the main prey of Baltic Sea Razorbills, but studies are scarce. Here, we investigate the diet preference of Razorbills at Stora Karlsö with regard to prey taxonomy and size, analysing photographs and applying morphometric equations. We show that most feeding attempts involved a single clupeid (88% of known fish taxonomy), in most cases likely sprat. The average prey size (115 mm) and prey species was similar to that of Common Guillemots in the same colony. The variation in size was much smaller in the Razorbill diet than the size distribution of clupeids at sea, indicating a strong prey size selectivity. The Razorbills’ high specialisation underlines the importance of maintaining strong populations of clupeids in their foraging area, as these fish are also targeted by industrial fisheries.

Diet and trophic interactions of Mediterranean planktivorous fishes

Diet and trophic interactions of seven species of planktivorous fishes: European anchovy Engraulis encrasicolus, European sardine Sardina pilchardus, round sardinella Sardinella aurita, European sprat Sprattus sprattus, red bandfish Cepola macrophthalma, damselfish Chromis chromis and bogue Boops boops were studied in the Bay of Marseille (North-Western Mediterranean Sea) from March to September 2017. Taxonomic composition and size distribution of prey were studied using stomach content analysis, and compared to prey availability determined by continuous zooplankton sampling at a fixed point. Frequently consumed items included copepods, decapod larvae and fish eggs. Comparatively, E. encrasicolus consumed more calanoid copepods (i.e. Centropages spp.), S. pilchardus, S. aurita and S. sprattus consumed more harpacticoid copepods (i.e. Microsetella/Macrosetella spp.), C. macrophthalma consumed more decapod larvae, C. chromis consumed more pteropods and B. boops consumed more benthic polychaetes. Sardina pilchardus consumed the widest diversity of prey. Prey size distribution and average prey size significantly differed among species. The prey–predator size ratio (PPSR) was highest for B. boops and lowest for S. aurita. Carbon and nitrogen stable isotope composition of species overlapped, probably due to the ingestion of particulate organic matter from the same sources at the base of the planktonic food web in the Bay of Marseille. Furthermore, trophic niche overlap supported the hypothesis of potential trophic competition between Engraulidae and Clupeidae.

Clovis through Big Sandy Technological Response to the Younger Dryas in Northern Alabama

ABSTRACT Kuhn and Miller (2015) suggest that changes in projectile point life histories could result from (a) increased replacement costs due to limited raw material access, or (b) a decline in average hunting returns. In northern Alabama, where raw material access can be held constant, we use variation in biotic structures to make predictions for life histories of Clovis to Big Sandy projectile points over the course of the Younger Dryas (12,900–11,700 calendar years ago). Then, using data from the Alabama Paleo Point Survey and private collections, we find a decrease in projectile point size through time, and fluctuations in resharpening. These trends follow changes in forest structure, average prey size, and hunting returns. We conclude that there are no abrupt technological changes coeval with the Younger Dryas onset. However, with the Younger Dryas terminus and subsequent Holocene warming, foragers in northern Alabama made significant changes in projectile point technology.

Mouth gape determines the response of marine top predators to long-term fishery-induced changes in food web structure

Here, we analyse changes throughout time in the isotopic niche of the Franciscana dolphin (Pontoporia blainvillei), the South American fur seal (Arctocephalus australis) and the South American sea lion (Otaria flavescens) from the Río de la Plata estuary and adjacent Atlantic Ocean to test the hypothesis that fishing may modify the diet of small-gape predators by reducing the average size of prey. The overall evidence, from stable isotope and stomach contents analyses, reveals major changes in resource partitioning between the three predators considered, mainly because of an increased access of Franciscana dolphins to juvenile demersal fishes. These results are consistent with the changes in the length distribution of demersal fish species resulting from fishing and suggest that Franciscana dolphin has been the most benefited species of the three marine mammal species considered because of its intermediate mouth gape. In conclusion, the impact of fishing on marine mammals goes beyond the simple reduction in prey biomass and is highly dependent on the mouth gape of the species involved.

Size‐based interactions and trophic transfer efficiency are modified by fish predation and cyanobacteria blooms in Lake Mývatn, Iceland

Abstract Trophic cascade studies have so far mostly focused on changes in the abundance, biomass, or average size of prey and predators. In contrast, individual size‐based interactions, playing a key role in the trophic structure and functioning of aquatic ecosystems, have been less explored. We conducted a 3‐month in situ experiment in Lake Mývatn, Iceland, with two fish treatments (with and without fish, Gasterosteus aculeatus). After the first month of the experiment, Anabaena blooms appeared in the lake. We studied the effects of fish predation and occurrence of cyanobacteria blooms on the individual size structure (i.e. the distribution of the number of organisms over a size range) of zooplankton and phytoplankton. We also assessed the potential consequences for trophic transfer efficiency (TTE) (measured as the predator to prey biomass ratio) in the planktonic food web. Our results showed that fish predation and cyanobacteria bloom had a negative relationship with size diversity of zooplankton, which became dominated by small‐sized individuals in both cases. The phytoplankton size diversity changed over time particularly due to the blooming of large‐sized Anabaena, and its increase was apparently mainly driven by changes in resources. Low zooplankton size diversity related to fish predation reduced TTE, particularly in the enclosures with fish. This may be because low zooplankton size diversity represents a lower partition of resources among consumers, thereby decreasing the trophic energy transfer. With the occurrence of Anabaena bloom, high phytoplankton size diversity coincided with a lower energy transfer in all enclosures likely due to reduced zooplankton grazing when large‐sized colony‐forming Anabaena dominated. In conclusion, our results indicate that both top‐down and bottom‐up forces significantly influence the size structure of planktonic communities. The changes in size structure were related to shifts in the energy transfer efficiency of the Lake Mývatn food web. Thus, our study underpins the importance of taking into account size‐based interactions in the study of trophic cascades, particularly in a warming climate where strong planktivorous fish predation and frequent cyanobacteria blooms may occur.

Adaptive flexibility in the feeding behaviour of brown trout: optimal prey size.

Browntrout, Salmotrutta Linnaeus,1758, is a species of significant conservation and socio-economic importance. A consequence of this importance is the enormous amount of literature that has been published on the species in the last few decades. In general terms, brown trout has been considered as a size-selective predator, even though it is able to feed on a wide range of prey sizes. Nevertheless, there are still some gaps in our knowledge, for example the theoretical relationship between prey numbers and prey sizes eaten by the fish need to be addressed. This research aimed to study optimal prey size in the environment (benthos and drift) as well as the potential relationship between prey size and two other feeding variables (prey numbers and stomach fullness). Additionally, ontogenetic shifts in these variables were addressed. Brown trout showed a clear preference for 4- to 6-mm-length prey, although the use of prey larger than 10-mm length may be feasible. The similarity of the prey size frequency distribution between the environment (benthos and drift) and the diet in some cases was considerable (from 57.7% to 95.9%). Moreover, the results revealed that the feeding strategy can be related to prey size and the numbers of prey eaten by the brown trout; as food size decreased, prey numbers increased. On the contrary, the correlation between the average prey size and fish length was positive but statistically nonsignificant. A significant ontogenetic shift, in terms of prey size sorted by age classes, was found in only two of eight studied populations. No clear relationship between prey size and stomach fullness was found. The feeding strategy of this species is flexible and clearly influenced by the size frequency distribution of potential prey: trout fed on either small numbers of large prey or large numbers of small, and theoretically low energy, prey. Our approach covers a general subject in trophic ecology and animal behaviour that may be applicable to other fish species to improve our understanding of predator feeding behaviour.

Microstructure and cross-sectional shape of limb bones in Great Horned Owls and Red-tailed Hawks: how do these features relate to differences in flight and hunting behavior?

The Red-tailed Hawk and Great Horned Owl are two species of raptor that are similar in body size, diet, and habitat. Both species use their hindlimbs during hunting, but differ in foot morphology, how they approach and immobilize prey, and the average size of prey captured. They also differ in primary flight style: the Red-tailed Hawk uses static soaring and the Great Horned Owl uses flap-gliding. The objectives of this study were to characterize the microstructure and cross-sectional shape of limb bones of these species and examine the relationship with flight and hunting behaviors. The mid-shaft of six limb bones from six individuals of each species was sampled. The degree of bone laminarity (proportion of circular primary vascular canals) and cross-sectional geometric parameters were calculated. In both species, the humerus and femur exhibited features that suggest high resistance to torsional loading, whereas the tibiotarsus and phalanges had a shape more likely to resist compression and bending in a specific plane. The femur of the Red-tailed Hawk exhibited higher laminarity and larger polar moment of area than that of the Great Horned Owl. The tibiotarsus was more elliptical than that of the Great Horned Owl. The hawk approaches prey from a more horizontal axis, takes prey of greater mass, and is more likely to pursue prey on the ground, which could potentially be causing more torsional loads on the femur and bending loads on the tibiotarsus. In addition, differences in polar moment of area of the phalanges between the species could relate to differences in foot morphology or digit length. The humerus and ulna of the flap-gliding Great Horned Owl are more elliptical than the static soaring Red-tailed Hawk, a shape that may better resist the bending loads associated with a larger amount of flapping.

Diet of the Exotic American Bullfrog,Lithobates catesbeianus, in a Stream of Northwestern Baja California, Mexico

We studied the diet of the exotic American bullfrog (Lithobates catesbeianus) in Arroyo San Carlos, located in northwestern Baja California, Mexico, during spring and summer 2009. Analysis of 64 stomach contents revealed 15 prey types, of which the exotic red crayfish (Procambarus clarkii) and terrestrial isopods (Armadillidiidae) were the most significant items (according to an index of their relative importance), making up 64% and 31% of the bullfrog's diet, respectively. Diet differed between the sexes, with red crayfish eaten more frequently by males (84%) than females (69%). Bullfrog diet changed with body size, with smaller individuals (≤86 mm) containing woodlice (61%) as the dominant prey type and larger individuals (≥129 mm) containing red crayfish (97%) as the dominant prey type. The average prey size consumed was significantly correlated with body length.

Trophic links and the relationship between predator and prey body sizes in food webs

The relationship between predator and prey body sizes is an important property of food webs with potential implications for community dynamics and ecosystem functioning. To shed more light on this issue I here analyze the relationships between prey size, predator size and trophic position of consumers, using body size estimates of 697 species in 52 food webs. First I show that the relationship between predator and prey body sizes across many systems can be different from, and potentially obscure the true relationship within systems. More specifically, when data from all webs are aggregated average prey size is positively correlated to predator size with a regression slope less than unity, suggesting that predators become less similar in size to their average prey the larger the predator is, and consequently that the relative size difference between a predator and its prey should increase with the trophic position of the consumer. However, despite this I find the predator-prey body mass ratio to be negatively correlated to the trophic position of the consumer within many webs. The reason for this is that the across-webs pattern is not representative for the within-web relationship. Second, I show that the pattern observed is not compatible with a simple null-model for the distribution of trophic links between predators and prey. The observed relationship between predator size and mean prey size is for most webs significantly steeper than that predicted by the cascade model. Furthermore, the observed relationship also deviates significantly (but less so) from an ecologically more realistic model for the distribution of trophic links (the niche model). The results contradict the traditional Eltonian paradigm that predator-prey body mass ratios do not vary consistently across trophic levels. It is concluded that more studies are needed to establish the generality of the results and explore its dynamical implications.

Intersexual Trophic Niche Partitioning in an Ant-Eating Spider (Araneae: Zodariidae)

BackgroundDivergence in trophic niche between the sexes may function to reduce competition between the sexes (“intersexual niche partitioning hypothesis”), or may be result from differential selection among the sexes on maximizing reproductive output (“sexual selection hypothesis”). The latter may lead to higher energy demands in females driven by fecundity selection, while males invest in mate searching. We tested predictions of the two hypotheses underlying intersexual trophic niche partitioning in a natural population of spiders. Zodarion jozefienae spiders specialize on Messor barbarus ants that are polymorphic in body size and hence comprise potential trophic niches for the spider, making this system well-suited to study intersexual trophic niche partitioning.Methodology/Principal FindingsComparative analysis of trophic morphology (the chelicerae) and body size of males, females and juveniles demonstrated highly female biased SSD (Sexual Size Dimorphism) in body size, body weight, and in the size of chelicerae, the latter arising from sex-specific growth patterns in trophic morphology. In the field, female spiders actively selected ant sub-castes that were larger than the average prey size, and larger than ants captured by juveniles and males. Female fecundity was highly positively correlated with female body mass, which reflects foraging success during the adult stage. Females in laboratory experiments preferred the large ant sub-castes and displayed higher capture efficiency. In contrast, males occupied a different trophic niche and showed reduced foraging effort and reduced prey capture and feeding efficiency compared with females and juveniles.Conclusions/SignificanceOur data indicate that female-biased dimorphism in trophic morphology and body size correlate with sex-specific reproductive strategies. We propose that intersexual trophic niche partitioning is shaped primarily by fecundity selection in females, and results from sex-differences in the route to successful reproduction where females are selected to maximize energy intake and fecundity, while males switch from foraging to invest in mating effort.

Feeding ecology ofSymbolophorus californiensislarvae (Teleostei: Myctophidae) in the southern transition region of the western North Pacific

The feeding habits of myctophid larvae ofSymbolophorus californiensiswere examined in the southern transition region of the western North Pacific where the main spawning and nursery grounds ofS. californiensisare formed. This species is a key component of the pelagic ecosystems of this region, and their larvae attain one of the largest sizes among myctophids. To analyse gut contents larvae, including most life history stages after yolk-sac absorption (3.7 to 22.2 mm body length (BL)), were collected in the upper 100 m layer in 1997 and 1998. Feeding incidence was higher during the day than at night (53.1–92.3% versus 0–5.6%), and daytime feeding incidence increased gradually with larval growth. Larvae fed mainly on copepods of various developmental stages. Larvae ofS. californiensisshowed an ontogenetic change in their diet: larvae ≤7.9 mm BL (i.e. preflexion stage) fed mainly on copepod eggs and nauplii, while the larvae ≥8 mm BL consumed mainly calanoid copepodites such asPseudocalanusandParacalanusspp. In the largest size-class (16–22.2 mm BL), the furcilia stage of euphausiids was also an important prey item. There was an increase in the average prey size with growth in larvae ≤11.9 mm BL, while the number of prey eaten positively correlated with growth in larvae ≥12 mm BL. The trophic niche breadth also increased with larval growth, which would ensure a wide range of available food resources for the larger size-class larvae.

Stepwise Increases in Maximum Prey Size of Larval Creek Chubs, Semotilus atromaculatus, in an Urbanized Ohio Stream

Abstract The diet of larval Semotilus atromaculatus (Creek Chubs) was examined in fish collected from an urbanized stream with a limited food base. Chironomids comprised nearly 90% of food items. They appeared in the gut of early larvae and continued to be the main food source as size increased; cladocerans were the second most abundant food item. Both average and maximum prey size were examined. Overall, average prey size increases significantly with standard length (SL). Maximum prey size is gape-limited twice, from the early to mid-mesolarval stage and again in the late mesolarval stage. Significant differences were observed in the maximum size of chironomids ingested among fish of four size ranges, 14.1 mm SL, indicating maximum prey size increases rapidly at 8.0 mm SL, 9.6 mm SL, and again at 14.1 mm SL, yet maximum prey size within each group remains constant. The degree of cranial ossification and fin development at these break points was examined wit...

Predation by brown trout (Salmo trutta) along a diversifying prey community gradient

Predation has a fundamental role in aquatic ecosystems, but the relative importance of factors governing prey selection by predators remains controversial. In this study, we contrast five lakes of a subarctic watershed to explore how prey community characteristics affect prey selection and growth rate of the common top predator, brown trout ( Salmo trutta ). The lakes constitute a distinct gradient of different coregonid prey fish, ranging from monomorphic common whitefish ( Coregonus lavaretus ) to polymorphic whitefish co-occurring with vendace ( Coregonus albula ). The brown trout was a morph–species- and size-specific pelagic predator, selecting the small-sized, pelagic whitefish morph or vendace over the benthic whitefish morphs. In all lakes, the average prey size increased with predator size, but small-sized prey were also included in the diet of large predators. The selection of small-sized, pelagic prey fish appeared to be a favourable foraging strategy for the brown trout, yielding higher growth rates and an earlier ontogenetic shift to piscivory. The findings emphasize that piscivory appear to be shaped by the diversity, size-structure, and abundance of available prey in a given community.

Invariant size selection of blue mussels by roach despite variable prey size distributions

Predation is a key trophic component with a potentially large influence on rocky shore community organization. We studied prey size selection by roach Rutilus rutilus, feeding on blue mussels Mytilus edulis in the northeastern Baltic Sea. In this region, roach feed extensively on abun- dant populations of blue mussels living at the very edge of their range primarily set by low salinity conditions. The study area is characterised by a marked decline in mean mussel size from the saltier west to the less salty east. We predicted that average prey size of roach would decrease with decreas- ing availability of larger prey but increase with increasing size of the predator. The size of mussels ingested by roach ranged from 2 to 28 mm, largely covering the size distribution of blue mussels in the area. In accordance with our prediction, body size of the predator was the foremost factor deter- mining patterns of prey selection and mean prey size increased steadily with increasing size of the predator. Roach were size selective, preferring median and large mussels in proportions different from their accessibility in the habitat. Contrary to our expectations, no differences between the areas were seen regarding prey size selection. These results contrast with optimal foraging theory predict- ing that predators under higher prey densities decrease the proportion of less profitable prey. Our results suggest that roach have the potential to significantly affect the blue mussel dynamics in areas with poor prey availability and we predict that ongoing environmental change in the Baltic will likely increase its impact.

Royal Tern (Sterna maxima) Chick Diet at Fisherman Island National Wildlife Refuge, Virginia

Abstract Royal Tern (Sterna maxima) chick diet at Fisherman Island National Wildlife Refuge, Virginia, was investigated in 2003 and 2004. Primary objectives were to document common prey species, annual and seasonal variation in prey selection, and seasonal variation in prey size based on average bill length in this species. Over 2200 Royal Terns bred on this island each year of the study. Single items of prey species carried in the bill by adult terns feeding their chicks were identified using 8 × 42 binoculars. A total of 11,566 and 33,646 prey items were identified during 44 and 87.5 h of systematic observations in 2003 and 2004, respectively. Terns foraged largely on anchovy (Anchoa spp.) early in the season, then switched to herrings (Clupeidae); average prey size also increased seasonally each year. Prey switching was similar to that of Royal Terns in North Carolina (but with drum as well as herring), indicating regional adaptations to forage fish availability by this species along the mid-Atlantic s...

Seasonal and sex-specific prey composition of black-footed catsFelis nigripes

During a 6-year field study on the game farm ‘Benfontein’ in the central Republic of South Africa 1725 prey items were observed consumed by 17 free-ranging habituated black-footed catsFelis nigripes Burchell, 1824. Average prey size was 24.1 g. Eight males fed on significantly larger prey (27.9 g) than 9 females (20.8 g). Fifty-four prey species were classified by their average mass into 8 different size classes, 3 for mammals, 3 for birds, 1 for amphibians/reptiles, and 1 for invertebrates. Small mammals (5–40 g) constituted the most important prey class (39%) of total prey biomass followed by larger mammals (>100 g; 17%) and small birds (<40 g; 16%). Mammals and birds pooled comprised 72% and 26% of total prey biomass, respectively, whereas invertebrates and amphibians/reptiles combined constituted just 2% of total prey mass consumed. Three seasons of 4-months duration were recognized. Heterotherm prey items were unavailable during winter, when larger birds and mammals (> 100 g) were mainly consumed. Small rodents like the large-eared mouseMalacothrix typica, captured 595 times by both sexes, were particularly important during the reproductive season for females with kittens. Male black-footed cats showed less variation between prey size classes consumed among climatic seasons. This sex-specific difference in prey size consumption may help to reduce intra-specific competition.

Feeding ecology and habitat of the threespine stickleback, Gasterosteus aculeatus microcephalus, in a remnant population of northwestern Baja California, México

Abstract – The feeding ecology and habitat of the threespine stickleback (Gasterosteus aculeatus microcephalus Girard, 1854) was studied from November 1996 to May 1997 in a remnant population of northwestern Baja California, México. The analysis of the stomach content of 179 individuals (25 to 56 mm standard length [SL]) showed a diet dominated by cyclopoid copepods (43.8%) and chironomid larvae (39.1%). Diet in relation to size and sex of the fish was dominated by copepods in autumn and winter and by chironomid larvae during spring. Diet overlap (Schoener’s index) was significant (≥60%) between fish size‐classes in January, March and April and between sexes for most sampling months. The average size of prey consumed was independent of fish mouth size. The feeding strategy of the threespine stickleback shifted from opportunist in winter to specialist in spring.

Effects of Web Design on the Prey Capture Efficiency of the Uloborid Spider Octonoba sybotides under Abundant and Limited Prey Conditions

The influence of web design on prey capture efficiency was investigated in the orb-webs of Octonoba sybotides under different prey conditions (abundant and limited). The insects captured by artificial traps under limited prey conditions were smaller than those captured under abundant prey conditions. O. sybotides webs with spiral stabilimenta (SP webs) have a narrower mesh and a larger catching area than webs with linear stabilimenta (LN webs). Previous studies have shown that the form of the stabilimentum is changed in response to the internal energetic state of individual spiders. Food-deprived spiders tend to construct SP webs, and food-satiated spiders tend to construct LN webs. The total thread length of SP webs is significantly longer than that of LN webs; so food-deprived spiders seem to invest more energy in foraging than do satiated spiders. When prey was abundant, prey sizes and capture rates for SP webs and LN webs were similar. LN webs may have an advantage over SP webs in prey capture when prey is abundant, because SP webs appear to be more costly than LN webs. On the other hand, SP webs caught more prey than LN webs under limited-prey conditions, when the prey was smaller than under abundant-prey conditions. The average size of prey captured on SP webs was smaller than that on LN webs; SP webs seem to catch more prey than LN webs by catching smaller prey efficiently in prey-limited conditions. Since the food-deprived and food-satiated conditions of spiders in the field may correspond to the potential prey abundance in their habitat, O. sybotides seems to change its web dimensions in order to capture prey efficiently under different prey conditions.

What did Virgil's swallows eat?

Juturna drives Turnus’ chariot now here now there, hoping to throw off Aeneas’ pursuit, but he follows the twisted circles (tortos orbes, 12. 481) of her course. Virgil compares her to a black hirundo flying through a rich man's house out into the colonnades and then round the pools or fishtanks. Hirundo can mean swallow, martin, or even swift. All these birds eat insects and air-borne spiders; they do not eat human food. The common swallow chiefly eats flies, and feeds the nestlings on flies; it also eats wasps and bees. Its average prey size is much greater than the house martin's. Virgil's hirundo gathers pabula parua for the nestlings. W F. J. Knight in the Penguin translation writes ‘tiny scraps of food’; C. Day Lewis translates ‘crumbs of food’. If Virgil meant scraps of meat or crumbs of bread, stolen from the rich man's dinner table, then Virgil did not know what these birds eat.

The diets of Malaysian swiftlets

Four species of swiftlet (Aves: Apodidae), two of which produce ‘edible nests’, occur sympatrically in Malaysia. Food boluses were analysed to investigate (1) diet differences and resource partitioning among the four species and (2) the influence of habitat on the diet of the Glossy Swiftlet Collocalia esculenta. Hymenoptera and diptera were the most abundant prey in all diets. Black‐nest Swiftlets Aerodramus maximus were extremely selective; 89% of the items taken were hymenoptera, of which 97% were flying ants. White‐nest Swiftlets A. fuciphagus and Mossy‐nest Swiftlets A. salanganus took more than twice as many prey items as either of the other species. Their diets were taxonomically diverse and differed only in the average prey size taken, Mossy‐nest Swiftlets taking the larger prey. Glossy Swiftlets took a higher proportion of coleoptera than did the other species. For the habitat comparisons, diptera were the main constituents of the Glossy Swiftlet diet in an urban habitat (71% of total prey items), whereas hymenoptera predominated in forest habitat (42% of total prey items). The results suggest that diet separation occurs among the four species of Malaysian swiftlet, in relation to both species and habitat. The implications of potentially habitat‐driven dietary changes are discussed for the edible‐nest swiftlets.