Assessing temporal and spatial variation in the diet of the Common Kestrel (Falco tinnunculus) using web-sourced photography

Sympatric populations of the western fence lizard, Sceloporus occidentalis and the sagebrush lizard, Sceloporus graciosus were compared with respect to morphological measurements, and habitat and resource utilization. An allopatric population of S. occidentalis was studied to determine whether any differences among sympatric populations result from species—specific differences or from a shift in characteristics of S. occidentalis because of the presence of a congener. The presence of sympatric S. graciosus does not alter the size of prey selected by S. occidentalis. Sceloporus graciosus selects a significantly smaller mean length of prey (4.6 mm) than S. occidentalis (5.4 mm). Two factors theoretically contribute to the variation in size of prey taken by a population: each individual shows variation in length of prey, and morphological variation in size of adult lizards results in differences among individuals in average length of prey. The first factor explains the variation in size of prey taken by these populations, because there is no correlation between mean prey length and adult body length for either species. Although there is a significant difference between the two species in mean size of prey taken, extensive overlap in prey utilization is revealed directly by the high overlap values between species (66%) and indirectly from the low value of niche separation to niche width ratio. The significantly larger mean prey size of adults to immatures reduces the intraspecific competition between age classes. Adult S. graciosus, with a smaller mean prey length than S. occidentalis, overlaps the most with immatures, particularly S. occidentalis immatures. The significantly larger head size of sympatric compared with allopatric S. occidentalis is not explained by character displacement in trophic apparatus to decrease overlap between species in prey utilization because the same size prey is selected in both areas and by both sexes. The larger head size of S. occidentalis males in sympatry may increase species identification between species in push—up displays. Sympatric S. occidentalis and S. graciosus segregate themselves significantly between habitats, and within a habitat between structural microhabitats. This minimal spatial overlap between sympatric species allows their coexistence despite extensive overlap in prey utilization.

C. analis had the most diverse diet. 0. snyderi and A. lateralis had roughly similar levels of diversity in their diets, although they concentrated on different food items (0. snyderi on amphipods and polychaetes, and A. lateralis on shrimp and fishes). There were no statistical differences in diet composition between large and small fish within species. Size classes of C. analis had the greatest average diversity of food while size classes ofA. lateralis had the lowest average diversity. 0. snyderi, the smallest species, generally took the smallest prey while A. lateralis, the species with the largest gape, took the largest prey. Variation in prey size was smallest for 0. snyderi and largest for A. lateralis. Much of the variation in prey sizes eaten by each species was due to the large average variation in prey sizes eaten by individual size classes. Information on food habits is combined with earlier data on intertidal distributions of the fishes. Differences among species in the use of food and space resources are viewed as a way for species to minimize, and as a possible result of, competition. Some degree of complementarity in use of resource dimensions (tidepool category, food category, prey size) also indicates that competition may have influenced patterns of resource use shown by these fishes.

Gape-limited predators, such as snakes, might rely on phenotypic plasticity to cope with variation in prey size, but experimental studies have found mixed evidence for plasticity in snake head morphology. Our study sought to determine whether variation in prey size and feeding rate induce head size plasticity in Northern Water Snakes (Nerodia sipedon). We avoided limitations of previous studies by growing all neonates to a consistent final size (400 mm snout–vent length [SVL]), and uncoupling feeding rate and prey size as potential factors influencing head size through four feeding treatments: big–fast (one large prey item every week), big–slow (one large prey item every other week), small–fast (two small prey items every week), and small–slow (one small prey item every week). Snakes in fast treatments grew faster than snakes in slow treatments, but there were no strong effects of feeding rate or prey size treatments on multiple head size metrics. Females grew faster than males, however, and had longer jaw lengths at 400 mm SVL than did males. Unlike previous studies, our results do not support the presence of phenotypic plasticity in head size in N. sipedon, indicating that head size plasticity in snakes can be species and context specific.

The four different assemblages of marsupial dasyuroid carnivores in Australia, comprising four species on the island of Tasmania and three different two-species assemblages on the mainland of Australia, present an ideal opportunity to test the theories of competitively based morphological character displacement. With males and females treated as separate morphospecies, four trophic characters that relate to killing and feeding behavior and skull length, as a measure of body size, were examined for evidence of morphological patterning, latitudinal size variation, and sexual dimorphism. The competitive basis for character displacement in intra- and interspecific variation in prey size was sought from dietary analysis at a field site. Even spacing in canine strength, but not in other characters, was found among species and sexes of quolls (Dasyurus viverrinus and D. maculatus), but not including the devil (Sarcophilus laniarius) or the thylacine (Thylacinus cynocephalus), in Tasmania. Even spacing was found in temporalis muscle size for all combinations of the four Tasmanian species. Canine breaking strength and muscle strength driving the canines can be construed as the most important trophic characters involved in prey capture, and therefore in determining the upper size of prey that can be handled. Even spacing was found in Tasmania, where there are two larger species in the guild, but not in any of the three mainland assemblages where the larger species of carnivores have been absent for many generations. The basis for the observed morphological patterning in killing behavior, evidence for even spacing in prey size, and partial character release on the mainland are consistent with the hypothesis of competitive character displacement. These size relationships result from a complex interplay of factors. Latitudinally correlated size variation has influenced an increase in body size and canine strength in D. viverrinus and an increase in body size in D. maculatus in Tasmania. This is countered by decreased canine strength in D. maculatus in Tasmania resulting from competitive character displacement. The result is character convergence. Sexual selection also influences size relationships.

The four different assemblages of marsupial dasyuroid carnivores in Australia, comprising four species on the island of Tasmania and three different two-species assemblages on the mainland of Australia, present an ideal opportunity to test the theories of competitively based morphological character displacement. With males and females treated as separate morphospecies, four trophic characters that relate to killing and feeding behavior and skull length, as a measure of body size, were examined for evidence of morphological patterning, latitudinal size variation, and sexual dimorphism. The competitive basis for character displacement in intra- and interspecific variation in prey size was sought from dietary analysis at a field site. Even spacing in canine strength, but not in other characters, was found among species and sexes of quolls (Dasyurus viverrinus and D. maculatus), but not including the devil (Sarcophilus laniarius) or the thylacine (Thylacinus cynocephalus), in Tasmania. Even spacing was found in temporalis muscle size for all combinations of the four Tasmanian species. Canine breaking strength and muscle strength driving the canines can be construed as the most important trophic characters involved in prey capture, and therefore in determining the upper size of prey that can be handled. Even spacing was found in Tasmania, where there are two larger species in the guild, but not in any of the three mainland assemblages where the larger species of carnivores have been absent for many generations. The basis for the observed morphological patterning in killing behavior, evidence for even spacing in prey size, and partial character release on the mainland are consistent with the hypothesis of competitive character displacement. These size relationships result from a complex interplay of factors. Latitudinally correlated size variation has influenced an increase in body size and canine strength in D. viverrinus and an increase in body size in D. maculatus in Tasmania. This is countered by decreased canine strength in D. maculatus in Tasmania resulting from competitive character displacement. The result is character convergence. Sexual selection also influences size relationships.

Diets of stream‐dwelling salmonids vary at multiple spatial and temporal scales, but the relative importance of different sources of variation has rarely been evaluated for a population. In a small coastal basin in central California, we sampled diets of steelhead/Rainbow Trout Oncorhynchus mykiss monthly for a year from two reaches that differed in the presence of streambed travertine (calcium carbonate) to determine the relative magnitude of spatial, seasonal, and ontogenetic variation in consumption and diet composition. Seasonal parameters (cosine function terms for months in periodic regression and multivariate models) and fish size (FL) were greater sources of variation than differences between the two study reaches, and seasonal and ontogenetic patterns were evident despite high interindividual variability. Seasonal parameters explained the most variation in diet composition, which varied in an orderly pattern over the year. Consumption was highest in spring and summer for both aquatic and terrestrial prey, and seasonal parameters accounted for more variation than fish size for consumption of aquatic prey but this was reversed for terrestrial prey and total consumption. The main ontogenetic trend was increased consumption of terrestrial invertebrates: from 15‐20% of the energy consumed by 60–100‐mm fish to 60% of the energy consumed by fish larger than 160 mm. In addition, diet breadth (taxonomic richness and prey size) and the degree of variation among individuals also increased with fish size. Diet differences between sites were relatively small and included higher consumption of aquatic invertebrates at the site without travertine in 4 months and modest differences in the relative abundances of common prey taxa. Although seasonal and ontogenetic patterns were evident, interindividual variation was the dominant scale of diet variation, and diets of individual fish ranged from specialized to generalized relative to the population. Based on these findings, O. mykiss appeared to be opportunistic feeders that took advantage of diverse aquatic and terrestrial prey available in these study reaches.

The squid Loligo forbesii is the only cephalopod species currently targeted by fisheries in the northern NE Atlantic. An active predator, it feeds primarily on fish, crustaceans and cephalopods. During 15 years since the only previous large-scale study of the diet of this species in Scottish waters, there have been substantial changes in marine fish abundances. The present study evaluates sources of variation (temporal, ontogenetic) in diet composition and prey size preferences of L. forbesii, including a comparison of contemporary (July 2006–June 2007) and historical (1990–1992) dietary datasets. Results revealed significant size-related and seasonal variation in diet composition and prey size. Teleost fish of the families Ammodytidae and Gobiidae were eaten by squid of all sampled sizes, although occurrence of gobies was generally more frequent in smaller squids, while Gadidae were eaten more frequently by larger squids. Cannibalism was also more frequent in larger squids. Compared to the 1990–1992 dataset, clupeid fish were less important in the diet of squid in 2006–2007, while the importance of gobies increased, and the size of gobies eaten also increased. The trend in gadoids differed according to the index used: their frequency of occurrence was considerably higher in 2006–2007 than in 1990–1992, but their numerical importance was slightly lower. In general, results provided little evidence that changes in the diet of L. forbesii correspond with changes in fish abundance, at least at the scales at which these are measured.

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...

Time constraint on food choice in provisioning blue tits, Parus caeruleus: the relationship between feeding rate and prey size

Functional responses are central to predator–prey dynamics and describe how predation varies with prey abundance. Functional responses often are measured without regard to prey size (i.e., body mass) or the temperature dependence of feeding rates. However, variation in prey size within populations is ubiquitous, and predation rates are often both size and temperature-dependent. Here, we assessed functional responses of larvae and adult Harmonia axyridis on the 1st, 2nd, and 3rd instars of the prey Spodoptera litura across a range of temperatures (i.e., 15, 20, 25, 30, and 35°C). The type and parameters of the functional responses were determined using logistic regression and fitted to the Roger's random predator equation. The magnitude of predation varied with the predator and prey stage, but prey predation increased with warming and predator age. Predation by the female and 4th instar of H. axyridis on the 1st instar of prey was greater, followed by the 2nd and 3rd instar of prey S. litura. No predation occurred on the larger prey for the 1st, 2nd, and 3rd instars of H. axyridis. The larvae and adult H. axyridis produced a type II (hyperbolic) functional response curve across all temperatures and the three prey types they consumed. Space clearance rates, handling time, and maximum predation rates of H. axyridis changed with temperature and prey size, increasing with temperature and decreasing with prey size, suggesting more predation will occur on younger prey. This study indicates an interactive role of temperature and prey/predator size in shaping functional responses, which might complicate the planning of effective biocontrol strategies against this serious pest.

The functional response is a critical link between consumer and resource dynamics, describing how a consumer's feeding rate varies with prey density. Functional response models often assume homogenous prey size and size-independent feeding rates. However, variation in prey size due to ontogeny and competition is ubiquitous, and predation rates are often size dependent. Thus, functional responses that ignore prey size may not effectively predict predation rates through ontogeny or in heterogeneous populations. Here, we use short-term response-surface experiments and statistical modeling to develop and test prey size-dependent functional responses for water bugs and dragonfly larvae feeding on red-eyed treefrog tadpoles. We then extend these models through simulations to predict mortality through time for growing prey. Both conventional and size-dependent functional response models predicted average overall mortality in short-term mixed-cohort experiments, but only the size-dependent models accurately captured how mortality was spread across sizes. As a result, simulations that extrapolated these results through prey ontogeny showed that differences in size-specific mortality are compounded as prey grow, causing predictions from conventional and size-dependent functional response models to diverge dramatically through time. Our results highlight the importance of incorporating prey size when modeling consumer-prey dynamics in size-structured, growing prey populations.

Temporal variation in prey selection by adult European sardine (Sardina pilchardus) in the NW Mediterranean Sea

Resource partitioning due to interspecific differences in phenotype is a key component of ecological and evolutionary theory, but the relationship between morphology and resource use is poorly understood for most species. In addition, ecologists often assume that morphological differences cause distinct resource preferences between species. Using mechanistic models that combine bill morphology and kinetics, I show that filter-feeding dabbling ducks face a morphology-mediated trade-off between particle size selection and water filtration rate. When detritus is absent, mallards (Anas platyrhynchos) and northern shovelers (Anas clypeata) should maximize their intake rates and exhibit high overlap in prey size. When prey and detritus co-occur, species should separate prey from detritus by size, leading to reduced intake rates and size-based prey partitioning. Models for both species correctly predicted variation in water filtration rates, particle retention probabilities, and prey ingestion rates due to variation in prey size, the presence of detritus, and experimental modification of bill morphology. Because species have both shared and distinct resource preferences, duck communities should exhibit strong density-dependent niche shifts (i.e., centrifugal dynamics), a finding that contradicts previous studies that assumed that ducks have distinct resource preferences only. Centrifugal dynamics may be widespread among filter feeders because of the common cost of separating prey from detritus.

Resource partitioning due to interspecific differences in phenotype is a key component of ecological and evolutionary theory, but the relationship between morphology and resource use is poorly understood for most species. In addition, ecologists often assume that morphological differences cause distinct resource preferences between species. Using mechanistic models that combine bill morphology and kinetics, I show that filter‐feeding dabbling ducks face a morphology‐mediated trade‐off between particle size selection and water filtration rate. When detritus is absent, mallards (Anas platyrhynchos) and northern shovelers (Anas clypeata) should maximize their intake rates and exhibit high overlap in prey size. When prey and detritus co‐occur, species should separate prey from detritus by size, leading to reduced intake rates and size‐based prey partitioning. Models for both species correctly predicted variation in water filtration rates, particle retention probabilities, and prey ingestion rates due to variation in prey size, the presence of detritus, and experimental modification of bill morphology. Because species have both shared and distinct resource preferences, duck communities should exhibit strong density‐dependent niche shifts (i.e., centrifugal dynamics), a finding that contradicts previous studies that assumed that ducks have distinct resource preferences only. Centrifugal dynamics may be widespread among filter feeders because of the common cost of separating prey from detritus.

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 9:13-22 (2010) - DOI: https://doi.org/10.3354/ab00225 Seasonal shift in the foraging niche of Atlantic puffins Fratercula arctica revealed by stable isotope (δ15N and δ13C) analyses April Hedd1,*, David A. Fifield1,2, Chantelle M. Burke1, William A. Montevecchi1, Laura McFarlane Tranquilla1, Paul M. Regular1, Alejandro D. Buren1, Gregory J. Robertson3 1Cognitive and Behavioural Ecology, Psychology Department, Memorial University, St. John’s, Newfoundland A1C 3X9, Canada 2Canadian Wildlife Service and 3Science and Technology Section, Environment Canada, 6 Bruce St., Mount Pearl, Newfoundland A1N 4T3, Canada *Email: ahedd@mun.ca ABSTRACT: We measured tissue stable nitrogen (δ15N) and carbon (δ13C) values to investigate the extent of seasonal and age-related variation in the foraging ecology of Atlantic puffins. For adults, there was considerable seasonal variation in the foraging niche. A generalized, lower trophic level (TL) diet during the winter moult was replaced by a highly specialized, higher TL diet during summer when birds were rearing chicks. The seasonal δ15N enrichment of 2.36‰ is consistent with an increase of 0.65 TL. Output from a 2-source single isotope (δ15N) mixing model suggested that adult diets were composed largely of zooplankton during winter with a switch to capelin (or other higher TL prey) in summer. A concurrent decrease in δ15N variability suggests that adult diets are more varied in winter. Trophic position of first-year birds was similar across summer and fall, as they went from being provisioned by parents at the colony to feeding independently. Both isotope mixing model and stomach contents analysis suggested that first-year birds relied largely (>85%) on capelin (or other higher TL fish) in fall. In summer, the trophic position of adults was significantly higher than that of nestlings; however, greater δ15N variability suggested that nestling diet was mixed overall. Shipboard surveys indicated an offshore movement of puffins post-breeding; however, this distributional shift was not reflected in δ13C signatures of adults. The dietary shift and increased TL of adult Atlantic puffins from winter to summer is consistent with reports for puffin species in the Pacific, suggesting that puffins may undergo the most extensive seasonal trophic shift among the auk species. KEY WORDS: Seasonal diet shift · Winter diet · Stable isotope analysis · δ15N · δ13C · Trophic level · Fratercula arctica Full text in pdf format PreviousNextCite this article as: Hedd A, Fifield DA, Burke CM, Montevecchi WA and others (2010) Seasonal shift in the foraging niche of Atlantic puffins Fratercula arctica revealed by stable isotope (δ15N and δ13C) analyses. Aquat Biol 9:13-22. https://doi.org/10.3354/ab00225 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 9, No. 1. Online publication date: March 25, 2010 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2010 Inter-Research.