The foraging decisions of a central place foraging seabird in response to fluctuations in local prey conditions

During reproduction, seabirds need to balance the demands of self- and offspring-provisioning within the constraints imposed by central place foraging. To assess behavioral adjustments and tolerances to these constraints, we studied the feeding tactics and reproductive success of common murres (also known as common guillemots) Uria aalge , at their largest and most offshore colony (Funk Island) where parents travel long distances to deliver a single capelin Mallotus villosus to their chicks. We assessed changes in the distance murres traveled from the colony, their proximate foraging locations and prey size choice during two successive years in which capelin exhibited an order of magnitude decrease in density and a shift from aggregated (2004) to dispersed (2005) distributions. When capelin availability was low (2005), parental murres increased their maximum foraging distances by 35% (60 to 81 km) and delivered significantly larger capelin to chicks, as predicted by central place foraging theory. Murres preferred large (>140 mm) relative to small capelin (100–140 mm) in both years, but unexpectedly this preference increased as the relative density of large capelin decreased. We conclude that single prey-loading murres target larger capelin during long foraging trips as parents are 'forced' to select the best prey for their offspring. Low fledgling masses suggest also that increased foraging time when capelin is scarce may come at a cost to the chicks (i.e. fewer meals per day). Murres at this colony may be functioning near physiological limits above which further or sustained adjustments in foraging effort could compromise the life-time reproductive success of this long-lived seabird.     

Prey foraging of red titi monkeys, Callicebus cupreus, in comparison to sympatric tamarins, Saguinus mystax and Saguinus fuscicollis

We compared the prey capture strategies of red titi monkeys, Callicebus cupreus, with those of sympatric mustached, Saguinus mystax, and saddleback tamarins, Saguinus fuscicollis, to examine whether animal prey is important in niche differentiation between these Neotropical primates. We collected data on strata and substrate use during foraging, on prey searching and capturing, and on prey type of two C. cupreus groups and one group each of S. mystax and S. fuscicollis during a 5-month field study in northeastern Peruvian Amazonia. Our results showed that C. cupreus differed both from S. mystax and S. fuscicollis in prey capture strategies: (1) C. cupreus used lower forest strata for prey search and capture than S. mystax and higher forest strata than S. fuscicollis. (2) C. cupreus captured prey on a higher variety of substrates than S. mystax and more often on open microhabitats compared to S. fuscicollis. (3) C. cupreus captured prey more often directly than S. mystax and rarely by manual search, in contrast to S. fuscicollis. (4) C. cupreus fed exclusively on arthropods and focused on Hymenoptera, in contrast to both tamarin species that focused on Orthoptera and included vertebrates in their diet. These findings indicate that animal prey plays a role in niche differentiation between C. cupreus and S. fuscicollis/S. mystax and might facilitate the coexistence of these three sympatric species.     

Vision and the foraging technique of Great Cormorants Phalacrocorax carbo: pursuit or close‐quarter foraging?

Predatory diving birds, such as cormorants (Phalacrocoracidae), have been generally regarded as visually guided pursuit foragers. However, due to their poor visual resolution underwater, it has recently been hypothesized that Great Cormorants do not in fact employ a pursuit-dive foraging technique. They appear capable of detecting typical prey only at short distances, and primarily use a foraging technique in which prey may be detected only at close quarters or flushed from a substratum or hiding place. In birds, visual field parameters, such as the position and extent of the region of binocular vision, and how these are altered by eye movements, appear to be determined primarily by feeding ecology. Therefore, to understand further the feeding technique of Great Cormorants we have determined retinal visual fields and eye movement amplitudes using an ophthalmoscopic reflex technique. We show that visual fields and eye movements in cormorants exhibit close similarity with those of other birds, such as herons (Ardeidae) and hornbills (Bucerotidae), which forage terrestrially typically using a close-quarter prey detection or flushing technique and/or which need to examine items held in the bill before ingestion. We argue that this visual field topography and associated eye movements is a general characteristic of birds whose foraging requires the detection of nearby mobile prey items from within a wide arc around the head, accurate capture of that prey using the bill, and visual examination of the caught prey held in the bill. This supports the idea that cormorants, although visually guided predators, are not primarily pursuit predators, and that their visual fields exhibit convergence towards a set of characteristics that meet the perceptual challenges of close-quarter prey detection or flush foraging in both aquatic and terrestrial environments.     

Optimal foraging as the criteria of prey selection by two centrarchid fishes

The nature of prey selection by two centrarchids (white crappie and bluegill) is presented as a model incorporating optimal foraging strategies. The visual field of the foraging fish as represented by the reactive distance is analysed in detail to estimate the number of prey encounters per search bout. The predicted reactive distances are compared with experimental data. The energetic cost associated with fish foraging behaviour is calculated based on the sequence of events that takes place for each prey consumed. Comparisons of the relative abundance of prey species and size categories in the stomach to the lake environment indicated that both white crappie and bluegill (length < 100 mm) strongly select prey utilising an energy optimization strategy. In most cases, the fish exclusively selected large Daphnia ignoring evasive prey types (Cyclops, Diaptomids) and small cladocera. This selectivity is the result of fish actively avoiding prey with high evasion capabilities even though they appear to be high in energetic content and having translated this into optimal selectivity through capture success rates. The energy consideration and visual system, apart from the forager's ability to capture prey, are the major determinants of prey selectivity for large-sized bluegill and white crappie still at planktivorous stages.     

Optimal foraging on perilous prey: risk of bill damage reduces optimal prey size in oystercatchers

Intake rate maximization alone is not always sufficient in explainingprey size selection in predators. For example, bivalve-feedingoystercatchers regularly select smaller prey than expected ifthey aimed to maximize their intake rate. It has been proposedthat to these birds large prey are "risky," in the sense thatbirds may damage their bills when feeding on large bivalves.Large bivalves yield more energy, but according to this hypothesisthis is achieved at the expense of energy yield in the longterm when (1) the risk of bill damage increases with prey sizeand (2) foraging with a damaged bill is less effective. In accordancewith this hypothesis, we show that captive oystercatchers feedingon large cockles experienced a high probability of bill tipdamage, while bill damage was absent when cockles were small.Moreover, among free-living oystercatchers the prevalence ofbill damage was correlated with mean cockle size near the capturesite, and the data on captive birds fit in this pattern. Foodintake of captive oystercatchers feeding exclusively on cockleswas reduced by 23% after bill damage, and free-living birdswith damaged bills had 14 g lower mass. Because lower body masswas associated with higher mortality probability, these resultsindicate long-term costs associated with feeding on large cockles.We conclude that the risk of bill damage can potentially explainwhy oystercatchers avoid large bivalves and that oystercatchersmay maximize long-term intake rate by selecting prey sizes thatare "suboptimal" from a short-term rate-maximizing point ofview.     

Size-dependent predation by snakes: selective foraging or differential prey vulnerability?

I staged replicate encounters between unrestrained lizards andsnakes in outdoor enclosures to examine size-dependent predationwithin the common garden skink (Lampropholis guichenoti). Yellow-facedwhip snakes (Demansia psammophis) forage widely for activeprey and most often consumed large skinks, whereas death adders(Acanthophis antarcticus) ambush active prey and most oftenconsumed small skinks. Small-eyed snakes (Rhinoplocephalusnigrescens) forage widely for inactive prey and consumed bothsmall and large skinks equally often. Differential predationmay reflect active choice by the predator, differential preyvulnerability, or both. To test for active choice, I presentedforaging snakes with an inert small lizard versus an inertlarge lizard. They did not actively select lizards of a particularbody size. To test for differential prey vulnerability, I quantifiedvariation between small and large lizards in behavior thatis important for determining the outcome of predator—prey interactions. Snakes did not differentiate between integumentarychemicals from small and large lizards. Large lizards tendto flee from approaching predators, thereby eliciting attackby the visually oriented whip snakes. Small lizards were moremobile than large lizards and therefore more likely to passby sedentary death adders. Additionally, small skinks were more effectively lured by this sit-and-wait species and less likelyto avoid its first capture attempt. In contrast, overnightretreat site selection (not body size) determined a lizard'schances of being detected by small-eyed snakes. Patterns ofsize-dependent predation by elapid snakes may arise not becauseof active choice but as a function of species-specific predatortactics and prey behavior.     

Linking piscivory to spatial–temporal distributions of pelagic prey fishes with a visual foraging model

A visual foraging model (VFM) used light-dependent reaction distance and capture success functions to link observed prey fish abundance and distribution to predation rates and the foraging performance of piscivorous cutthroat trout Oncorhynchus clarki in Lake Washington (WA, U.S.A.). Total prey density did not correlate with predation potential estimated by the foraging model for cutthroat trout because prey were rarely distributed in optically favourable conditions for detection. Predictions of the depth-specific distribution and timing of cutthroat trout foraging were qualitatively similar to diel stomach fullness patterns observed in field samples. Nocturnal foraging accounted for 34–64% of all prey fish consumption in simulations for 2002 and 2003. Urban light contamination increased the access of nocturnally foraging cutthroat trout to vertically migrating prey fishes. These results suggest that VFMs are useful tools for converting observed prey fish density into predictions of predator consumptions and behavioural responses of predators to environmental change.     

Hunting strategies in wild common marmosets are prey and age dependent

We investigated the hunting strategies of wild common marmosets (Callithrix jacchus) to determine whether the strategies differed among animals of different age classes and/or prey type. The study was conducted in a fragment of Atlantic Rain Forest, situated 40 km from Recife (PE/Brazil). Twenty‐seven individuals from four social groups were observed. Captured prey items were divided into three categories. The hunting strategies of the common marmosets were ranked into four categories. The acquisition of larger prey (items more than 2.0 cm) involved the appropriate body movements and postures that concealed the approaching marmosets, whereas the acquisition of smaller prey (items under 2.0 cm) involved less concealing behaviors. Furthermore, adults and juveniles (age ≥5 months) were more capable of capturing larger prey than were younger (1–2 months) or older infants (3–4 months). Although older infants were successful in capturing certain prey, they often failed when they attempted to capture larger prey that jumped and/or used flight to escape. The results suggest that both the experience of the monkeys and escape behavior of the prey affect predation efficiency in wild common marmosets. Am. J. Primatol. 72:1039–1046, 2010. © 2010 Wiley‐Liss, Inc.     

Prey size,prey perishability and group foraging in a social spider

Summary Selection might favor group foraging and social feeding when prey are distributed in patches that do not last long enough for a solitary individual to consume more than a small fraction of them (Pulliam and Millikan 1982; Pulliam and Caraco 1984). Here we considered the foraging behavior of a social spider, Anelosimus eximius, in light of this ephemeral resource hypothesis. This species builds large webs in which members cooperate to capture a wide variety of different sizes and types of prey, many of which are very large. The capture success of this species was very high across all prey sizes, presumably due to the fact that they foraged in groups. Group consumption times in natural colonies for all prey larger than five mm were less than the time that dead insects remained on the plastic sheets that we used as artificial webs. Solitary consumption estimates, calculated from the rate at which laboratory individuals extracted insect biomass while feeding, were the same as the residence times of insects on artificial webs in the field for insects between 6 and 15 mm in length and were significantly longer than the persistence of insects on plastic sheets for all larger insects. Large prey, that contribute substantially to colony energy supplies, appeared to be ephemeral resources for these spiders that could not be consumed by a single spider in the time they were available. These factors made the food intake of one spider in a group less sensitive to scavenging by others and could act to reinforce the social system of this species.     

Light intensity, prey detection and foraging mechanisms of age 0 year yellow perch

The ability of age‐0 year yellow perch Perca flavescans to detect prey using visual and mechano‐sensory input was examined during laboratory feeding trials at varying light intensities. Perch were highly effective predators and captured Daphnia pulicaria with 94% overall foraging success at light levels ranging from 0 to 3400 lx. Maximum average reaction distances (5·0 ± 0·8 cm, mean ±  s . e .) occurred in front of the fish at 3000 lx and significantly decreased as light intensities fell to <2 lx, with minimum reaction distances (2·8 ± 0·1 cm) observed in the dark. Following chemical ablation of the lateral line, yellow perch showed a significant reduction in reaction distance when compared to the untreated fish at 3000 lx, suggesting that the lateral line may augment visual prey detection at high light levels. A model was created to predict reaction distances for fish feeding with multiple sensory systems that can be applied to a variety of photic environments. This study provides a better understanding of the contribution of vision and the lateral line to prey detection, and relates the reaction distance of age‐0 year yellow perch to light intensities similar to those experienced in nature.     

Foraging behaviour and prey discrimination in the bluespotted maskray Dasyatis kuhlii

A study observing the foraging behaviours and prey discrimination of a common demersal stingray, the bluespotted maskray Dasyatis kuhlii was performed under controlled laboratory conditions. A selection of prey species and masses were offered at depths of 10 and 50 mm in sand. Foraging efficiency and prey selection at both burial depths were compared. Dasyatis kuhlii selected the ghost shrimps, Trypaea australiensis and T. australiensis >2·5 g, range ± 0·2 g though foraging errors represented by prey being excavated and not consumed suggested a limited discriminatory ability at the point of detection. Burial depth did not influence prey species, mass selection or discriminatory ability.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

Seasonal versatility in the feeding ecology of a group of titis (Callicebus coimbrai) in the northern Brazilian Atlantic Forest

The feeding behavior of a group of titis (Callicebus coimbrai) was monitored over an annual cycle at a site in northeastern Brazil. Behavioral data were collected in scan samples (1-min scan at 5-min intervals), and complementary data on fruit availability and new leaf cover were collected. Feeding time accounted for 28.9% of daily activity. Fruit was the principal item of the diet (61.2% of records) and the primary category in all months except September, when it was surpassed by leaves. Young leaves were the second most important category (20.0%). The consumption of seeds and insects was prominent in November and December. Fifty-two plant species were exploited, and the Elaeocarpaceae, Myrtaceae, Sapotaceae, and Passifloraceae provided the vast majority (86.0%) of plant feeding records. The phenological record did not provide a good measure of fruit availability, but a strong correlation (r(s) =0.902, P<0.0001, n=12) was found between the consumption of leaves and the exploitation of lianas each month. Lianas accounted for 28.2% of plant feeding records, and predominated between August and December. This suggests that lianas may represent a key factor in the ability of the species to tolerate the intense habitat fragmentation found throughout its geographic range.     

Long-term consequences of changes in territory quality on feeding and reproductive strategies of vervet monkeys

1. Food availability and quality are important determinants of mammalian reproductive success, and long-term changes in food availability were assessed for their impact on diets and reproduction of three adjacent groups of vervet monkeys in Amboseli, Kenya in two periods spanning an interval of 9 years.
2. Diets were largely restricted to the products of two species of acacia trees ( Acacia xanthophloea and Acacia tortilis ), with food selection primarily determined by availability (tree density, size and seasonal production of foods).
3. Over this period the overall abundance of major foods, measured through absolute species density, declined while territory size increased.
4. Despite significant changes in food plant densities, diets remained relatively stable, suggesting a component of consistency in diet choice. Limited options or high costs for incorporation of novel foods are suggested as factors maintaining this stability, with deleterious consequences in the face of very long-term habitat changes.
5. This study suggests that the habitat deterioration, assessed by reduction in food densities, initiated local group extinction. An increased energy expenditure in foraging, high mortality and low reproductive rates ultimately led to a population crash under conditions of reduced food availability.     

Taking movement data to new depths: Inferring prey availability and patch profitability from seabird foraging behavior

Detailed information acquired using tracking technology has the potential to provide accurate pictures of the types of movements and behaviors performed by animals. To date, such data have not been widely exploited to provide inferred information about the foraging habitat. We collected data using multiple sensors (GPS, time depth recorders, and accelerometers) from two species of diving seabirds, razorbills (Alca torda, N = 5, from Fair Isle, UK) and common guillemots (Uria aalge, N = 2 from Fair Isle and N = 2 from Colonsay, UK). We used a clustering algorithm to identify pursuit and catching events and the time spent pursuing and catching underwater, which we then used as indicators for inferring prey encounters throughout the water column and responses to changes in prey availability of the areas visited at two levels: individual dives and groups of dives. For each individual dive (N = 661 for guillemots, 6214 for razorbills), we modeled the number of pursuit and catching events, in relation to dive depth, duration, and type of dive performed (benthic vs. pelagic). For groups of dives (N = 58 for guillemots, 156 for razorbills), we modeled the total time spent pursuing and catching in relation to time spent underwater. Razorbills performed only pelagic dives, most likely exploiting prey available at shallow depths as indicated by the vertical distribution of pursuit and catching events. In contrast, guillemots were more flexible in their behavior, switching between benthic and pelagic dives. Capture attempt rates indicated that they were exploiting deep prey aggregations. The study highlights how novel analysis of movement data can give new insights into how animals exploit food patches, offering a unique opportunity to comprehend the behavioral ecology behind different movement patterns and understand how animals might respond to changes in prey distributions.     

Environmental enrichment and prior experience of live prey improve foraging behaviour in hatchery‐reared Atlantic salmon

Atlantic salmon salmo salar L. parr were reared for 3 months under standard hatchery conditions or in a structurally enriched tank (containing plants, rocks and novel objects). Half of each of these fish had prior exposure to live prey in the form of live bloodworm while the other half were fed hatchery‐pellets. After 12 days all fish were tested on a novel live prey item (brine shrimp). A significant interaction between the two factors (prior exposure to live prey and rearing condition) revealed that foraging performance was only enhanced in fish that had been reared in a complex environment and exposed to live prey. It appears that the ability to generalize from one live prey type to another is only enhanced in fish that had been reared in an enriched environment. The findings support the assertion that the provision of enriched environments in combination with exposure to live prey prior to release may significantly improve the post‐release survival rates of hatchery‐reared fishes. As both the environmental enrichment and the prior foraging experience procedures were comparatively simple, the provision of such pre‐release experiences are likely to prove cost effective to hatcheries.     

Visual configuration of two species of Falconidae with different foraging ecologies

Significant interspecific differences in avian vision occur, even in congeneric species, and these have been correlated with differences in the perceptual challenges associated with foraging. Although diurnal raptors are assumed to be mainly visually guided in their foraging, they differ markedly in their foraging tactics and this may result in different visual demands. Among the Falconidae (Falconiformes), most falcons forage mainly on the wing for highly mobile prey, whereas caracaras forage on the ground for carrion and insects. We assessed whether Saker Falcon Falco cherrug and Southern Caracara Caracara plancus differ in their visual abilities by determining the visual fields and foveal characteristics of both species. Using an ophthalmoscopic reflex technique, we found a higher degree of binocular overlap in the caracaras than in the falcons. The high binocular overlap (47°) of the Southern Caracara may facilitate object manipulation (e.g. moving rocks) when foraging. We used an ultra‐high resolution spectral‐domain optical coherence tomography to determine foveal characteristics. We found two foveas (depressions in the retina where high visual resolution is expected) in the falcons (one central and one temporal) but only a central fovea in the caracaras. The presence of a shallower temporal fovea in Saker Falcons may help to fixate visually upon a highly mobile prey item during pursuit. We conclude that these differences in visual field configurations and foveal characteristics reflect different foraging demands, suggesting that the extraction of visual information is finely tuned to the demands of their foraging tactics.     

Estimating piscine prey size from partial remains: testing for shifts in foraging mode by juvenile bluefish

Knowledge of prey sizes consumed by a predator aids in the estimation of predation impact. Young-of-the-year bluefish, Pomatomus saltatrix, attack their prey tail-first and often bite their prey in half; this poses a unique problem in determining prey sizes from stomach content analysis. We developed a series of linear regressions to estimate original prey lengths from measurements of eye diameter and caudal peduncle depth for striped bass, Morone saxatilis, bay anchovy, Anchoa mitchilli, American shad, Alosa sapidissima, blueback herring, Alosa aestivalis, Atlantic silverside, Menidia menidia, and white perch, Morone americana. We then used these regressions to estimate original prey sizes from pieces of prey found in stomachs of bluefish collected in the Hudson River estuary from 1990–1993. Lengths of prey that were swallowed whole were compared to estimated lengths of prey that were consumed in pieces. Lengths of prey that were consumed in pieces were larger than prey that were consumed whole. We determined the prey length/predator length ratio at which bluefish began shifting from swallowing their prey whole to partial consumption. Shifting occurred at a ratio of approximately 0.35 irrespective of prey species, suggesting that prey length plays an important role in predator foraging decisions and may contribute to gape limitations. Shifts in foraging mode effectively reduce gape limitation and allow bluefish to consume larger prey sizes which may increase their effect on prey populations.