Prey Patches Research Articles

Oviposition Behavior of the Predatory Midge Aphidoletes aphidimyza in Response to Aphid Patch Quality

Female Aphidoletes aphidimyza confronts serious challenges from both aphid prey and conspecifics. These challenges constitute strong selective pressures on the predatory midge to have an adaptive oviposition strategy. We did laboratory experiments to investigate clutch size of A. aphidimyza in response to the aphid Rhopalosiphum padi (density and age) and the presence of conspecific eggs (density) and larvae (age) in aphid colonies. The results of our study show that A. aphidimyza female adjusts its foraging effort by assessing the quality of prey patches. The number of eggs laid increased in response to aphid density. However, patches consisting of older aphids received fewer eggs. The number of eggs laid decreased in response to the presence of conspecific eggs, and in response to the presence of 2-day old conspecific larvae. Our study reveals deterrent effects on A. aphidimyza oviposition decisions on clutch size in response to older aphids and the presence of more conspecific eggs and older larvae.

Northern flicker mates foraging on renewing patches within home ranges avoid competition not by separate niches but by segregation

Foraging with others can reduce predation risk but may increase competition for resources. Species, sex, or age classes may use different niches to reduce competition for food, which may be enforced by social dominance or may lead to ecological specialization. An alternate hypothesis is that individuals using resource patches that renew over time intentionally avoid recently depleted patches by avoiding each other. We tested these hypotheses by simultaneously radio-tracking northern flicker (Colaptes auratus) mates. Members of a pair rarely foraged in the same patch, spending over 90 % of the time >50 m apart. Counter to the ecological specialization hypothesis, the types of ground cover used for foraging and the distance of foraging patches from cover and edge did not differ between males and females. Mates were more spatially segregated on the home range than random and males foraged closer to the nest than females, but we never saw agonistic behaviors counter to the dominance hypothesis. Both sexes avoided returning to the same foraging location repeatedly, perhaps because of prey depletion. We suggest that members of a pair avoided foraging close to each other to reduce encounters with depleted prey patches. We emphasize the importance of simultaneous tracking of mates to identify foraging strategies and potential niche segregation at the home range scale.

Spatial foraging segregation by close neighbours in a wide-ranging seabird.

Breeding seabirds are central-place foragers and therefore exploit food resources most intensively nearer their colonies. When nesting aggregations are close to one another density-dependent competition is likely to be high, potentially promoting foraging segregation (i.e. neighbouring colonies may segregate to search for food in different areas). However, little is known about spatial segregation in foraging behaviour between closely adjacent colonies, particularly in species that are wide-ranging foragers. Here, we tested for foraging segregation between two sub-colonies of a wide-ranging seabird, Cory's shearwater Calonectris borealis, separated by only 2 km, on a small Island in the North Atlantic. During the 2010 chick-rearing period, 43 breeding adults of both sexes were simultaneously sampled at both sub-colonies. A GPS logger was deployed on each individual and removed after several foraging trips at sea. Blood samples (plasma and red blood cells) were collected from each tracked individual for stable isotope analysis. Results indicated partial spatial segregation between the two sub-colonies during local foraging trips (i.e. those of ≤1 day duration and 216 km from the colony) accounting for 84.2% of all trips recorded. The location of the breeding sub-colony influenced the direction of travel of birds during local trips resulting in sub-colony-specific foraging areas. Although the oceanographic conditions associated with the foraging range of the two sub-colonies differed, no differences were found in the habitat exploited and in their estimated diets. This suggests that birds concentrated their feeding activity in patches of similar habitat and prey during the chick-rearing period.

Familiarity affects social network structure and discovery of prey patch locations in foraging stickleback shoals.

Numerous factors affect the fine-scale social structure of animal groups, but it is unclear how important such factors are in determining how individuals encounter resources. Familiarity affects shoal choice and structure in many social fishes. Here, we show that familiarity between shoal members of sticklebacks (Gasterosteus aculeatus) affects both fine-scale social organization and the discovery of resources. Social network analysis revealed that sticklebacks remained closer to familiar than to unfamiliar individuals within the same shoal. Network-based diffusion analysis revealed that there was a strong untransmitted social effect on patch discovery, with individuals tending to discover a task sooner if a familiar individual from their group had previously done so than if an unfamiliar fish had done so. However, in contrast to the effect of familiarity, the frequency with which individuals had previously associated with one another had no effect upon the likelihood of prey patch discovery. This may have been due to the influence of fish on one another's movements; the effect of familiarity on discovery of an empty ‘control’ patch was as strong as for discovery of an actual prey patch. Our results demonstrate that factors affecting fine-scale social interactions can also influence how individuals encounter and exploit resources.

Selectivity by planktivorous fish at different prey densities, heterogeneities, and spatial scales

Studies in artificially confined, small‐scale experimental systems with a homogeneous prey distribution suggest that selectivity in planktivorous fish for more conspicuous and rewarding prey (such as adult Daphnia with eggs in the brood cavities) is stronger at high than at low prey population density. We examined whether prey selectivity is altered when fish are allowed to feed in larger scale systems and on more heterogeneously distributed prey. The experimental system was comprised of 1000 liter tanks (one, four, or eight), interconnected to allow free movement of roach (Rutilus rutilus, eight per tank), with different densities of Daphnia prey, distributed with varying levels of homogeneity. As reported previously, the selectivity of roach increased at higher Daphnia densities, but only in small‐scale systems where prey distribution was homogeneous. However, the opposite was found in large‐scale systems with heterogeneous prey distribution, i.e., greater selectivity at lower prey densities. The reversed slope of the selectivity‐on‐prey‐density regression was linked to increased swimming speed at low prey density, where the fish, less constrained by the limited size of the experimental system, accelerate, possibly in an effort to compensate for low encounter rate and to more rapidly locate patches of prey. More selective feeding on scarce and patchy prey may result from the decreased relative visibility of smaller prey and from the increased capture costs due to the need to expend more energy for postcapture accelerations to regain a higher speed.

Experimental determination of the spatial scale of a prey patch from the predator’s perspective

Foraging theory predicts that predators should prefer foraging in habitat patches with higher prey densities. However, density depends on the spatial scale at which a "patch" is defined by an observer. Ecologists strive to measure prey densities at the same scale that predators do, but many natural landscapes lack obvious, well-defined prey patches. Thus one must determine the scale at which predators define patches of prey. We estimated the scale at which guppies, Poecilia reticulata, selected patches of zooplankton prey using a behavioral assay. Guppies could choose between two prey arrays, each manipulated to have a density that depended on the spatial scale at which density was calculated. We estimated the scale of guppy foraging by comparing guppy preferences across a series of trials in which we systematically varied the scale associated with "high" prey density. This approach enables the application of foraging theory to non-discrete habitats and prey landscapes.

Modelling the Effects of Prey Size and Distribution on Prey Capture Rates of Two Sympatric Marine Predators

Understanding how prey capture rates are influenced by feeding ecology and environmental conditions is fundamental to assessing anthropogenic impacts on marine higher predators. We compared how prey capture rates varied in relation to prey size, prey patch distribution and prey density for two species of alcid, common guillemot (Uria aalge) and razorbill (Alca torda) during the chick-rearing period. We developed a Monte Carlo approach parameterised with foraging behaviour from bird-borne data loggers, observations of prey fed to chicks, and adult diet from water-offloading, to construct a bio-energetics model. Our primary goal was to estimate prey capture rates, and a secondary aim was to test responses to a set of biologically plausible environmental scenarios. Estimated prey capture rates were 1.5±0.8 items per dive (0.8±0.4 and 1.1±0.6 items per minute foraging and underwater, respectively) for guillemots and 3.7±2.4 items per dive (4.9±3.1 and 7.3±4.0 items per minute foraging and underwater, respectively) for razorbills. Based on species' ecology, diet and flight costs, we predicted that razorbills would be more sensitive to decreases in 0-group sandeel (Ammodytes marinus) length (prediction 1), but guillemots would be more sensitive to prey patches that were more widely spaced (prediction 2), and lower in prey density (prediction 3). Estimated prey capture rates increased non-linearly as 0-group sandeel length declined, with the slope being steeper in razorbills, supporting prediction 1. When prey patches were more dispersed, estimated daily energy expenditure increased by a factor of 3.0 for guillemots and 2.3 for razorbills, suggesting guillemots were more sensitive to patchier prey, supporting prediction 2. However, both species responded similarly to reduced prey density (guillemot expenditure increased by 1.7; razorbill by 1.6), thus not supporting prediction 3. This bio-energetics approach complements other foraging models in predicting likely impacts of environmental change on marine higher predators dependent on species-specific foraging ecologies.

Intra- and Trans-Generational Costs of Reduced Female Body Size Caused by Food Limitation Early in Life in Mites

BackgroundFood limitation early in life may be compensated for by developmental plasticity resulting in accelerated development enhancing survival at the expense of small adult body size. However and especially for females in non-matching maternal and offspring environments, being smaller than the standard may incur considerable intra- and trans-generational costs.Methodology/Principal FindingsHere, we evaluated the costs of small female body size induced by food limitation early in life in the sexually size-dimorphic predatory mite Phytoseiulus persimilis. Females are larger than males. These predators are adapted to exploit ephemeral spider mite prey patches. The intra- and trans-generational effects of small maternal body size manifested in lower maternal survival probabilities, decreased attractiveness for males, and a reduced number and size of eggs compared to standard-sized females. The trans-generational effects of small maternal body size were sex-specific with small mothers producing small daughters but standard-sized sons.Conclusions/SignificanceSmall female body size apparently intensified the well-known costs of sexual activity because mortality of small but not standard-sized females mainly occurred shortly after mating. The disadvantages of small females in mating and egg production may be generally explained by size-associated morphological and physiological constraints. Additionally, size-assortative mate preferences of standard-sized mates may have rendered small females disproportionally unattractive mating partners. We argue that the sex-specific trans-generational effects were due to sexual size dimorphism – females are the larger sex and thus more strongly affected by maternal stress than the smaller males – and to sexually selected lower plasticity of male body size.

Blue whales respond to simulated mid-frequency military sonar

Mid-frequency military (1–10 kHz) sonars have been associated with lethal mass strandings of deep-diving toothed whales, but the effects on endangered baleen whale species are virtually unknown. Here, we used controlled exposure experiments with simulated military sonar and other mid-frequency sounds to measure behavioural responses of tagged blue whales (Balaenoptera musculus) in feeding areas within the Southern California Bight. Despite using source levels orders of magnitude below some operational military systems, our results demonstrate that mid-frequency sound can significantly affect blue whale behaviour, especially during deep feeding modes. When a response occurred, behavioural changes varied widely from cessation of deep feeding to increased swimming speed and directed travel away from the sound source. The variability of these behavioural responses was largely influenced by a complex interaction of behavioural state, the type of mid-frequency sound and received sound level. Sonar-induced disruption of feeding and displacement from high-quality prey patches could have significant and previously undocumented impacts on baleen whale foraging ecology, individual fitness and population health.

Turbulence disturbs vertical refuge use byChaoborus flavicanslarvae and increases their horizontal dispersion

Summary Small-scale turbulence is a key factor in increasing encounter rates but also dispersing prey patches of planktonic organisms and can thus contribute to their distribution. Our hypotheses were that turbulence can (i) disturb the vertical refuge use of fourth instar Chaoborus flavicans larvae and (ii) also disperse them horizontally. Both were tested experimentally, across a gradient of turbulence. The vertical experiments were conducted in 46-L cylinders, providing the larvae a vertical refuge by darkening the bottom layer. The horizontal experiments were conducted in a 200-L aquarium, providing them a horizontal refuge by darkening one end of the aquarium. With both set-ups, two light treatments were tested: uniform darkness as a control and the bottom layer of the cylinder or either end of the aquarium darkened to provide a refuge. In addition to non-turbulent treatment, five different root-mean-square (RMS) velocities were generated, ranging from 0.6 to 10.2 cm s−1 in the vertical and to 10.9 cm s−1 in the horizontal experiment. Increasing turbulence had a significant effect on the distribution of C. flavicans larvae in both vertical and horizontal directions and increased the proportion of larvae in the non-preferred illuminated environment. In the vertical direction, the proportion of larvae in the refuge decreased linearly with increasing turbulence, showing a decline from 84% under calm conditions to 61% at the highest RMS velocity (10.2 cm s−1). The effect of turbulence was more prominent in the horizontal direction; without turbulence, 86% of the larvae occupied the refuge, but the proportion declined to 32% at 5.4 cm s−1 RMS velocity. In experiments with uniform darkness, the distribution of chaoborids was not affected by turbulence either in the vertical or in the horizontal direction. Our results indicate that turbulence has substantial effects on refuge use and distribution of C. flavicans larvae. Increasing turbulence can greatly diminish the ability of prey species to use their vertical refuges. Once away from their vertical refuge, chaoborids are highly vulnerable to horizontal dispersion, thus upsetting their horizontal escape behaviour. Small-scale turbulence is therefore likely to have considerable effects on food-web dynamics in lakes.

The learning mechanism underlying public information use in ninespine sticklebacks (Pungitius pungitius).

Elucidating the mechanisms by which animals learn from others is central to understanding the evolution of behavioral adaptations and the constraints that limit options when gathering information about the environment. Here we present findings from three experiments that investigated the psychological mechanisms underlying public-information use in ninespine sticklebacks (Pungitius pungitius). Using a prey patch choice assay we compared two candidate processes, local enhancement and stimulus enhancement. These experiments revealed (a) that fish only selected socially demonstrated prey patches via local enhancement, (b) that even in the absence of any confounding influence of local enhancement there was no evidence for stimulus enhancement in patch choice, and (c) sensitization rather than associative learning underlies the observed public information use. Our findings suggest that local and stimulus enhancement are distinct processes, and that local enhancement is not merely a subcategory of stimulus enhancement, as has previously been argued in the literature. We offer several reasons why learning via stimulus enhancement may not have arisen in ninespine sticklebacks, and speculate that the evolution of this learning mechanism may be favored by specific properties of social structure.

Echolocation in Blainville’s beaked whales (Mesoplodon densirostris)

Here we use sound and movement recording tags to study how deep-diving Blainville's beaked whales (Mesoplodon densirostris) use echolocation to forage in their natural mesopelagic habitat. These whales ensonify thousands of organisms per dive but select only about 25 prey for capture. They negotiate their cluttered environment by radiating sound in a narrow 20° field of view which they sample with 1.5-3 clicks per metre travelled requiring only some 60 clicks to locate, select and approach each prey. Sampling rates do not appear to be defined by the range to individual targets, but rather by the movement of the predator. Whales sample faster when they encounter patches of prey allowing them to search new water volumes while turning rapidly to stay within a patch. This implies that the Griffin search-approach-capture model of biosonar foraging must be expanded to account for sampling behaviours adapted to the overall prey distribution. Beaked whales can classify prey at more than 15m range adopting stereotyped motor patterns when approaching some prey. This long detection range relative to swimming speed facilitates a deliberate mode of sensory-motor operation in which prey and capture tactics can be selected to optimize energy returns during long breath-hold dives.

Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 479:283-302 (2013) - DOI: https://doi.org/10.3354/meps10209 Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey Kelly J. Benoit-Bird1,*, Brian C. Battaile2, Chad A. Nordstrom2, Andrew W. Trites2 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon 97331, USA 2Marine Mammal Research Unit, University of British Columbia, Room 247, AERL, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada *Email: kbenoit@coas.oregonstate.edu ABSTRACT: Marine prey often occur in hierarchical mosaics whereby small, high-density patches are nested inside of larger, lower density aggregations. We tested the extent to which the foraging behavior of a marine predator (northern fur seal Callorhinus ursinus) could be explained by the hierarchical patch structure of a dominant prey species (juvenile walleye pollock Theragra chalcogramma) in the eastern Bering Sea. Comparing the movements of satellite-tracked fur seals with ship-based acoustic surveys of prey revealed that fur seals did not randomly search for prey, but instead showed deviations in the distribution of step-lengths (distances between their foraging patches) corresponding to the distances between aggregations of prey. Scales of prey distribution varied between Bering Sea shelf and deep-water slope habitats, while spatial scale distributions of fur seals showed corresponding changes, indicating that their search strategies were not innate patterns decoupled from the environment. Fur seals tended to avoid the smallest prey patches in both shelf and slope habitats. They also avoided prey patches that were separated by large distances. Fur seals responded to several levels of prey patchiness simultaneously, resulting in strong correlations between predator and prey over the entire range of aggregation scales observed in juvenile pollock. Our results indicate that, despite having a varied diet, fur seal foraging paths were defined by juvenile pollock aggregations. The presence of hierarchical, scale-dependent aggregation in both predator and prey provides new insights into fur seal behavior and a means to predict the dynamics of their interactions with prey. KEY WORDS: Patchiness · Spatial scale · Predator?prey · Foraging behavior · Hierarchical · Northern fur seal · Juvenile walleye pollock Full text in pdf format PreviousCite this article as: Benoit-Bird KJ, Battaile BC, Nordstrom CA, Trites AW (2013) Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey. Mar Ecol Prog Ser 479:283-302. https://doi.org/10.3354/meps10209 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 479. Online publication date: April 08, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Predation on Dovekies by Goosefish Over Deep Water in the Northwest Atlantic Ocean

Fourteen Alle alle (Dovekie) were recovered from the stomachs of 14 Lophius americanus (Goosefish) caught during winter and spring 2007–2010. All fish were caught in gill nets set at depths of 85–151 m (276–491 ft) 104–150 km (65–94 mi) south of Chatham, MA. Dovekies showed few signs of digestion by the fish, indicating recent capture. Post mortem revealed no cause of mortality. Capture of birds by fish so far from shore and in deep water leads to speculation that the birds were preyed on by Goosefish at or near the surface. Evidence from electronic tagging of Goosefish suggests that Goosefish vertical migrations could bring them into contact with Dovekies feeding offshore. If Goosefish are concentrated during onshore-offshore migrations and Dovekies are concentrated for feeding on prey patches, predation by Goosefish on Dovekies could be episodically important.

Cannibalism, optimal egg size and vulnerable developmental stages in insect predators

Cannibalism in insect predators is widely reported and commented on. However, the reasons for the high levels of cannibalism observed, the vulnerability of certain developmental stages, and its adaptive significance is poorly understood. Here we show that unlike in parasitoids there could be advantages for a predator in laying many small eggs, rather than fewer large eggs. The temporal incidence of cannibalism within a patch is possibly a consequence of changes in the ratio of predator to prey numbers and/or changes in the vulnerability of the developmental stages of the predator. Cannibalism is advantageous for cannibals because conspecifics are a rich source of food for larval development or egg production, and it reduces intraspecific competition. Adults reduce the cannibalism of their offspring by not ovipositing in patches of prey where conspecific larvae are present.

Underwater acrobatics by the world's largest predator: 360° rolling manoeuvres by lunge-feeding blue whales

The extreme body size of blue whales requires a high energy intake and therefore demands efficient foraging strategies. As an obligate lunge feeder on aggregations of small zooplankton, blue whales engulf a large volume of prey-laden water in a single, rapid gulp. The efficiency of this feeding mechanism is strongly dependent on the amount of prey that can be captured during each lunge, yet food resources tend to be patchily distributed in both space and time. Here, we measured the three-dimensional kinematics and foraging behaviour of blue whales feeding on krill, using suction-cup attached multi-sensor tags. Our analyses revealed 360° rolling lunge-feeding manoeuvres that reorient the body and position the lower jaws so that a krill patch can be engulfed with the whale's body inverted. We also recorded these rolling behaviours when whales were in a searching mode in between lunges, suggesting that this behaviour also enables the whale to visually process the prey field and maximize foraging efficiency by surveying for the densest prey aggregations. These results reveal the complex manoeuvrability that is required for large rorqual whales to exploit prey patches and highlight the need to fully understand the three-dimensional interactions between predator and prey in the natural environment.

Can Foraging Ecology Drive the Evolution of Body Size in a Diving Endotherm?

Within a single animal species, different morphs can allow for differential exploitation of foraging niches between populations, while sexual size dimorphism can provide each sex with access to different resources. Despite being potentially important agents of evolution, resource polymorphisms, and the way they operate in wild populations, remain poorly understood. In this study, we examine how trophic factors can select for different body sizes between populations and sexes in a diving endotherm. Dive depth and duration are positively related to body size in diving birds and mammals, a relationship explained by a lower mass-specific metabolic rate and greater oxygen stores in larger individuals. Based on this allometry, we predict that selection for exploiting resources situated at different depths can drive the evolution of body size in species of diving endotherms at the population and sexual level. To test this prediction, we studied the foraging ecology of Blue-eyed Shags, a group of cormorants with male-biased sexual size dimorphism from across the Southern Ocean. We found that mean body mass and relative difference in body mass between sexes varied by up to 77% and 107% between neighbouring colonies, respectively. Birds from colonies with larger individuals dived deeper than birds from colonies with smaller individuals, when accounting for sex. In parallel, males dived further offshore and deeper than females and the sexual difference in dive depth reflected the level of sexual size dimorphism at each colony. We argue that body size in this group of birds is under intense selection for diving to depths of profitable benthic prey patches and that, locally, sexual niche divergence selection can exaggerate the sexual size dimorphism of Blue-eyed Shags initially set up by sexual selection. Our findings suggest that trophic resources can select for important geographic micro-variability in body size between populations and sexes.

Environmental Complexity Influences Association Network Structure and Network-Based Diffusion of Foraging Information in Fish Shoals

Socially transmitted information can significantly affect the ways in which animals interact with their environments. We used network-based diffusion analysis, a novel and powerful tool for exploring information transmission, to model the rate at which sticklebacks (Gasterosteus aculeatus) discovered prey patches, comparing shoals foraging in open and structured environments. We found that for groups in the open environment, individuals tended to recruit to both the prey patch and empty comparison patches at similar times, suggesting that patch discovery was not greatly affected by direct social transmission. In contrast, in structured environments we found strong evidence that information about prey patch location was socially transmitted and moreover that the pathway of information transmission followed the shoals' association network structures. Our findings highlight the importance of considering habitat structure when investigating the diffusion of information through populations and imply that association networks take on greater ecological significance in structured than open environments.

Prey Patch Patterns Predict Habitat Use by Top Marine Predators with Diverse Foraging Strategies

Spatial coherence between predators and prey has rarely been observed in pelagic marine ecosystems. We used measures of the environment, prey abundance, prey quality, and prey distribution to explain the observed distributions of three co-occurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus). Predictions of statistical models were tested using movement patterns obtained from satellite-tracked individual animals. With the most commonly used measures to quantify prey distributions - areal biomass, density, and numerical abundance - we were unable to find a spatial relationship between predators and their prey. We instead found that habitat use by all three predators was predicted most strongly by prey patch characteristics such as depth and local density within spatial aggregations. Additional prey patch characteristics and physical habitat also contributed significantly to characterizing predator patterns. Our results indicate that the small-scale prey patch characteristics are critical to how predators perceive the quality of their food supply and the mechanisms they use to exploit it, regardless of time of day, sampling year, or source colony. The three focal predator species had different constraints and employed different foraging strategies – a shallow diver that makes trips of moderate distance (kittiwakes), a deep diver that makes trip of short distances (murres), and a deep diver that makes extensive trips (fur seals). However, all three were similarly linked by patchiness of prey rather than by the distribution of overall biomass. This supports the hypothesis that patchiness may be critical for understanding predator-prey relationships in pelagic marine systems more generally.

Feeding associations between Grey Phalaropes Phalaropus fulicarius and Basking Sharks Cetorhinus maximus over a tidal-topographic front off southwest Cornwall

This paper documents a previously unreported feeding association between Grey Phalaropes Phalaropus fulicarius and Basking Sharks Cetorhinus maximus. The observations were made during effort-based visual monitoring from Gwennap Head in southwest Cornwall, in October 2008. Flocks of up to 50 Grey Phalaropes were seen to follow and aggregate around surface-feeding Basking Sharks, mostly in an area characterised by a prominent tidal-topographic front. It is proposed that Grey Phalaropes were initially attracted to visual manifestations of upwellings and convergence zones over and adjacent to this front (with enhanced surficial prey availability), and then opportunistically used the tall dorsal fins of foraging Basking Sharks as a visual cue to home in on particularly dense prey patches.