Does sun glare increase antipredator behaviour in prey?

As the sun gradually lowers over the horizon, prey species with more sun in their eyes should have more difficulty in visually monitoring their surroundings for threats and thus experience a higher predation risk. In a unique setting, I could examine changes in antipredator behaviour in a prey species, the semipalmated sandpiper Calidris pusilla, facing attacks by peregrine falcons Falco peregrinus, which originated from the general direction of the lowering sun. I predicted gradual changes in antipredator behaviour as sun glare becomes more problematic later in the day. As the day progressed, sandpipers occurred in sparser groups when the sun glared but not when clouds obscured the sun, suggesting that fewer individuals engaged in risky foraging. Pecking rate and foraging success decreased later in the day when the sun glared but not otherwise implying an increase in vigilance at the expense of foraging. When more sun hit their eyes, sandpipers also moved faster suggesting increased skittishness. The sun glare effect might be relevant to any species foraging in open areas not only when the sun sets but also when it rises especially if predators can target prey species at these vulnerable times. The temporal gradient in predation risk that the sun glare effect creates might thus apply broadly and have important consequences for antipredator vigilance, foraging efficiency, and habitat use.     

Competition in foraging flocks of migrating semipalmated sandpipers

I examined the effect of competitor density on foraging success in staging semipalmated sandpipers (Calidris pusilla) foraging on a burrowing amphipod (Corophium volutator) in each of two study years. Little is known about the effect of competitor density when predation attempts disturb prey, causing a temporary decrease in food availability. Controlling for Corophium density and other potentially confounding factors such as temperature, pecking rate and capture rate increased linearly with sandpiper density. Success rate, the ratio of captures to pecks, was not influenced by sandpiper density. The effect of sandpiper density was similar in each of the two study years and was documented early and late in the low tide period. The increase in foraging rate is argued to be a response to increased competition for rapidly depleting prey at the temporal scale of exploitation by a flock. Potential fitness costs associated with higher foraging costs may include decreased ability to distinguish between the profitability of different prey and reduced vigilance against predators.     

Functional response of staging semipalmated sandpipers feeding on burrowing amphipods

Despite its fundamental relevance to many ecological processes in predator–prey relationships, the functional response, which relates predator intake rate to prey density, remains difficult to document in the field. Here, I document the functional response of semipalmated sandpipers (Calidris pusilla) foraging on a burrowing amphipod Corophium volutator during three field seasons at the peak of fall migration in the upper Bay of Fundy (New Brunswick, Canada). I gathered data during the ebbing tide when all sandpipers are highly motivated to feed after a lengthy hide-tide fast. As birds follow the receding tideline, foragers encounter prey at different densities and do not aggregate in the richest food patches. Results show that intake rate increased at a decreasing rate with Corophium density, yielding a type II functional response typical of many shorebird species. Intake rate decreased in the later stages of migration stopover at a time where preferred prey items have been shown to occur at lower densities due to prior depletion. At this period of lower prey availability, intake rate also decreased with sandpiper density providing evidence for interference at low prey density. The results illustrate the fact that the functional response may not be unique but instead vary as a function of the type of competitive relationship among foragers.     

Environmental heterogeneity amplifies behavioural response to a temporal cycle

Resource acquisition is integral to maximise fitness, however in many ecosystems this requires adaptation to resource abundance and distributions that seldom stay constant. For predators, prey availability can vary at fine spatial and temporal scales as a result of changes in the physical environment, and therefore selection should favour individuals that can adapt their foraging behaviour accordingly. The tidal cycle is a short, yet predictable, temporal cycle, which can influence prey availability at temporal scales relevant to movement decisions. Here, we ask whether black‐legged kittiwakes Rissa tridactyla can adjust their foraging habitat selection according to the tidal cycle using GPS tracking studies at three sites of differing environmental heterogeneity. We used a hidden Markov model to classify kittiwake behaviour, and analysed habitat selection during foraging. As expected for a central‐place forager, we found that kittiwakes preferred to forage nearer to the breeding colony. However, we also show that habitat selection changed over the 12.4‐h tidal cycle, most likely because of changes in resource availability. Furthermore, we observed that environmental heterogeneity was associated with amplified changes in kittiwake habitat selection over the tidal cycle, potentially because environmental heterogeneity drives greater resource variation. Both predictable cycles and environmental heterogeneity are ubiquitous. Our results therefore suggest that, together, predictable cycles and environmental heterogeneity may shape predator behaviour across ecosystems.     

Adaptive variation in the foraging behaviour of Grey Plover Pluvialis squatarola and Whimbrel Numenius phaeopus

Between-site variation in the diet and foraging behaviour of a fixed-method forager, the Grey Plover Pluvialis squatarola , and a versatile forager, the Whimbrel Numenius phaeopus , was examined and compared at nine tropical and south temperate sites. Grey Plover always foraged in a run-stop-search manner but changed the emphasis of components of this behaviour in response to prey type. Whimbrel foraged tactilely or visually and readily changed foraging speed and habitat. Both species ate a variety of prey species depending on their availability. The diet of Grey Plover was dominated by small prey, polychaetes and crabs, and Whimbrel ate mostly crabs, with Whimbrel being the more specialized in prey choice. The broader diversity of prey types consumed by Grey Plover was attributed to limitations imposed by obligate visual foraging, whereas the flexible foraging behaviour of Whimbrel allowed the latter to concentrate their efforts on the most profitable prey. Grey Plover appeared to have a density upper limit, determined by their stereotyped foraging behaviour. Whimbrel densities varied greatly in response to prey type and foraging method. Foraging effort could not be predicted from measurements of instantaneous daytime energy intake rates, and reasons for the lack of this relationship are discussed.     

Low Foraging Success of Semipalmated Sandpipers at the Edges of Groups

Variation in foraging success in relation to spatial position in a group is little known in species that feed on mobile prey that can hide or flee upon disturbance by foragers. I examined the foraging success of individuals located either at the edge or at the centre of flocks of semipalmated sandpipers (Calidris pusilla) feeding on a burrowing amphipod (Corophium volutator) during migration stopover in the Bay of Fundy, Canada. The rates of pecking, prey capture and success were lower for individuals foraging at the edge than at the centre of flocks. Edge birds spent more time running and more time flying than centre birds. Edge birds moved away from the centre of the flock and made frequent short flights towards the centre. In contrast, centre birds rarely moved in a specific direction and flew mostly to relocate elsewhere with the whole flock. Sandpiper flocks foraged over a large area in a relatively short amount of time. In addition, amphipod density is high in this habitat. It thus appears unlikely that prey depletion or low food availability at the edges of groups could explain the spatial variation in foraging success. Low foraging success at the edges of flocks thus arose mainly because of time costs related to flock expansion and retraction. The effect of mutual interference among foragers and of predation risk by falcons is discussed with respect to flock expansion and retraction.     

Small-scale spatial variability in the diet of pampas foxes (<Emphasis Type="Italic">Pseudalopex gymnocercus</Emphasis>) and human-induced changes in prey base

Predators display functional responses to variation in prey availability that result in dietary distinction at different spatial scales. Such differentiation can be enhanced when human-induced changes in land use increase spatial heterogeneity in prey availability. This has been recorded for canids, but available information is skewed to regional scales. Further, though human activities have strongly altered neotropical biomes, knowledge about small-scale dietary differentiation of canids come mainly from holartic species. Thus, we quantified variability in the diet of a ubiquitous South American canid species (i.e., the pampas fox, Pseudalopex gymnocercus) over a relatively small area of grasslands in Argentina, comparing its diet at three close sampling sites varying in human disturbance. We found that small-scale dietary differentiation was influenced by human-induced habitat modifications, both directly by local subsidizing of introduced prey (e.g., livestock carrion and hares) and indirectly by altering the availability of native prey. Overall, pampas foxes seemed to be highly flexible in their trophic ecology, being able to cope with marked habitat alterations and likely benefiting from food cross-subsidizing from agricultural landscape to protected grassland relicts. This has implications for management and conservation purposes, and highlights the importance of quantifying variability in foraging behavior of carnivores over small spatial scales.     

Local feeding specialization by badgers (Meles meles) in a mediterranean environment

A case of local feeding specialization in the European badger (Meles meles), a carnivore species with morphological, physiological and behavioural traits proper to a trophic generalist, is described. For the first time, we report a mammalian species, the European rabbit (Oryctolagus cuniculus), as the preferred prey of badgers. Secondary prey are consumed according to their availability, compensating for temporal fluctuations in the abundance of rabbit kittens. We discuss how both predator (little ability to hunt) and prey (profitability and predictability) features, may favour the observed specialization, as predicted by foraging theory. Badgers show a trend to specialize on different prey in different areas throughout the species range. It is suggested that changes in prey features can reverse the badger feeding strategy at the population level. Such dynamic behavioural responses make difficult to label badgers as generalists or specialists at the species level.     

The effects of human presence, flock size and prey density on shorebird foraging rates

Animals may alter their foraging behaviour in the presence of humans because they perceive humans as potential predators. In this study I determined whether people caused shorebirds to reduce feeding rates at a stopover site in coastal British Columbia, Canada. I controlled for prey density and flock size because these variables may influence both the foraging rates as well as the effect of human disturbance on feeding efficiency. Semipalmated plovers decreased feeding rates when there were more people on the beach (multiple regression: F_1,15=5.86, b=0.59, P=0.029, R²=37.6%). For least sandpipers, the effect of human densities on feeding rates depended on flock size (F_1,21=5.97, P=0.023) and amphipod availability (F_1,21=4.98, P=0.037). This study demonstrated the importance of measuring subtle behavioural changes in foraging rates along with key ecological variables in order to assess the true impact of human disturbance on migratory shorebirds.     

Behavioral Response of Corophium volutator to Shorebird Predation in the Upper Bay of Fundy,Canada

Predator avoidance is an important component of predator-prey relationships and can affect prey availability for foraging animals. Each summer, the burrow-dwelling amphipod Corophium volutator is heavily preyed upon by Semipalmated Sandpipers (Calidris pusilla) on mudflats in the upper Bay of Fundy, Canada. We conducted three complementary studies to determine if adult C. volutator exhibit predator avoidance behavior in the presence of sandpipers. In a field experiment, we monitored vertical distribution of C. volutator adults in bird exclosures and adjacent control plots before sandpipers arrived and during their stopover. We also made polymer resin casts of C. volutator burrows in the field throughout the summer. Finally, we simulated shorebird pecking in a lab experiment and observed C. volutator behavior in their burrows. C. volutator adults were generally distributed deeper in the sediment later in the summer (after sandpipers arrived). In August, this response was detectably stronger in areas exposed to bird predation than in bird exclosures. During peak predator abundance, many C. volutator adults were beyond the reach of feeding sandpipers (>1.5 cm deep). However, burrow depth did not change significantly throughout the summer. Detailed behavioral observations indicated that C. volutator spent more time at the bottom of their burrow when exposed to a simulated predator compared to controls. This observed redistribution suggests that C. volutator adults move deeper into their burrows as an anti-predator response to the presence of sandpipers. This work has implications for predators that feed on burrow-dwelling invertebrates in soft-sediment ecosystems, as density may not accurately estimate prey availability.     

Landscape structure influences the use of social information in an insectivorous bat

In anthropogenic landscapes, aerial insectivores are often confronted with variable habitat complexity, which may influence the distribution of prey. Yet, high mobility may allow aerial insectivores to adjust their foraging strategy to different prey distributions. We investigated whether aerial-hunting common noctules Nyctalus noctula adjust their foraging strategy to landscapes with different habitat complexity and assumingly different prey distribution. We hypothesized that the movement behaviour of hunting common noctules and changes of movement behaviour in reaction towards conspecifics would depend on whether they hunt in a structurally poor cropland dominated landscape or a structurally rich forest dominated landscape. We tracked flight paths of common noctules in northeastern Germany using GPS loggers equipped with an ultrasonic microphone that recorded foraging events and presence of conspecifics. Above cropland, common noctules hunted mainly during bouts of highly tortuous and area restricted movements (ARM). Bats switched from straight flight to ARM after encountering conspecifics. In the forested landscape, common noctules hunted both during ARM and during straight flights. The onset of ARM did not correlate with the presence of conspecifics. Common noctules showed a lower feeding rate and encountered more conspecifics above the forested than above the cropland dominated landscape. We conjecture that prey distribution above cropland was patchy and unpredictable, thus making eavesdropping on hunting conspecifics crucial for bats during search for prey patches. In contrast, small scale structural diversity of the forested landscape possibly led to a more homogeneous prey distribution at the landscape scale, thus enabling bats to find sufficient food independent of conspecific presence. This suggests that predators depending on ephemeral prey can increase their foraging success in structurally poor landscapes by using social information provided by conspecifics. Hence, a minimum population density might be obligatory to enable successful foraging in simplified landscapes.     

Foraging speed in staging flocks of semipalmated sandpipers: evidence for scramble competition

Foraging speed is a key determinant of fitness affecting both foraging success and predator attack survival. In a scramble for food, for instance, evolutionary stable strategy models predict that speed should increase with competitor density and decrease when the risk of attack by predators increases. Foraging speed should also decrease in richer food patches where the level of competition is reduced. I tested these predictions in fall staging flocks of semipalmated sandpipers (Calidris pusilla) foraging for an evasive prey. Capture rate of these prey decreased with sandpiper density as the presence of competitors reduced the availability of resources for those behind. Foraging speed was evaluated indirectly by measuring the time needed to cross fixed boundaries on mudflats over 6 years. As predicted, foraging speed increased with sandpiper density and decreased with food density, but, unexpectedly, increased closer to obstructive cover where predation risk was deemed higher. When foraging closer to cover, from where predators launch surprise attacks, the increase in foraging speed may compensate for an increase in false alarms that interrupted foraging. While foraging in denser flocks decreases foraging success, joining such flocks may also increase safety against predators. In semipalmated sandpipers that occupy an intermediate position in the food chain, foraging behavior is influenced simultaneously by the evasive responses of their prey and by the risk of attack from their own predators.     

Despite prolonged association in closed populations,an intertidal predator does not prefer abundant local prey to novel prey

The diets of predators should reflect interactions between their behavioural and anatomical constraints and the availability and accessibility of prey, although feeding preferences may also reflect adaptation to locally abundant prey, particularly in closed populations. On the south‐east coast of Australia, the whelk Haustrum vinosum (Lamarck, 1822) and its prey communities provide a model system in which to test the effect of variation in prey availability on diet and dietary preferences. Haustrum vinosum is a direct developing species, forming effectively closed populations, with the potential for local adaptation at local and regional scales. Here we show that populations of whelks east and west of a biogeographical barrier encounter different prey assemblages, and have different feeding patterns and apparent prey preferences. We then use a prey choice experiment to test for evidence that H. vinosum from three populations west of the barrier display an inherent preference for its most frequently encountered western prey species, the mussel Brachidontes rostratus (Dunker, 1857), over a novel prey, the barnacle Tesseropora rosea (Krauss, 1848). We detected no prey preference within any population, suggesting past association with B. rostratus did not influence prey selection. Our data support the hypothesis that predators with limited dispersal and high population differentiation are able to maintain flexible generalist foraging patterns, even when they encounter novel prey. © 2013 The Linnean Society of London     

Foraging modes of stream benthic fishes in relation to their predation effects on local prey density

Habitat use and foraging behavior of two benthic insectivorous gobies, Rhinogobius sp. CO (cobalt type) and Rhinogobius sp. DA (dark type), were examined in relation to their predation effects on local prey density in a small coastal stream in southwestern Shikoku, Japan. Correlations among the foraging range, frequency of foraging attempts and current velocity indicated that individuals using fast-current habitats had small foraging ranges and infrequently made foraging attempts while those in slow currents frequently foraged over large areas. The former and the latter were recognized as ambush and wandering foragers, respectively. Interspecific comparisons of habitat use, foraging behavior and prey preference suggested that Rhinogobius sp. CO selectively forage mobile prey by ambushing in fast currents, whereas Rhinogobius sp. DA randomly forage available prey by wandering in slow-current habitats. A cage experiment was conducted to assess prey immigration rate and the degree of predation effects on local prey density in relation to current velocity. The results of the experiment support, at least in part, our initial predictions: (1) prey immigration rates increase with current velocity and (2) the effects of fish predation on local prey density are reduced as current velocity increases. Overall results illustrated a link between the foraging modes of the stream gobies and their predation effects on local prey density: fish adopt ambush foraging in fast currents, where the decrease in prey density tends to be less, whereas fish actively forage over large areas in slow currents, where the decrease in prey is relatively large.     

Web building flexibility of an orb‐web spider in a heterogeneous agricultural landscape

Intensification of land‐use in agricultural landscapes is responsible for a decline of biodiversity which provide important ecosystem services like pest‐control. Changes in landscape composition may also induce behavioural changes of predators in response to variation in the biotic or abiotic environment. By controlling for environmentally confounding factors, we here demonstrate that the orb web spider Araneus diadematus alters its web building behaviour in response to changes in the composition of agricultural landscapes. Thereby, the species increases its foraging efficiency (i.e. investments in silk and web asymmetry) with an increase of agricultural land‐use at intermediate spatial scales. This intensification is also related to a decrease in the abundance of larger prey. A negative effect of landscape properties at similar spatial scales on spider fitness was recorded when controlling for relative investments in capture thread length. This study consequently documents the web building flexibility in response to changes in landscape composition, possibly due to changes in prey availability.     

Parasitized Salamanders are Inferior Competitors for Territories and Food Resources

Parasites have been shown to impair the behaviour of their hosts, compromising the host's ability to exploit and compete for resources. We conducted two experiments to determine whether infestation with an ectoparasitic mite (Hannemania eltoni) was associated with changes in aggressive and foraging behaviour in the Ozark zigzag salamander, Plethodon angusticlavius. In a first experiment, male salamanders with high parasite loads were less aggressive overall than males with low parasite loads during territorial disputes. In addition, males with high parasite loads were more aggressive toward opponents with high parasite loads (symmetric contests) than toward opponents with low parasite loads (asymmetric contests). In contrast, males with low parasite loads did not adjust their level of aggression according to the parasite load of the opponent. In a second experiment, foraging behaviour of females was tested in response to ‘familiar’ (Drosophila) prey and ‘novel’ (termite) prey. Latency to first capture was significantly longer for parasitized than non‐parasitized females when tested with ‘familiar’ prey, but not for ‘novel’ prey. Our results suggest that parasite‐mediated effects may have profound influences on individual fitness in nature.     

Influence of Predator Presence and Prey Density on Behavior and Growth of Damselfly Larvae (Ischnura elegans) (Odonata: Zygoptera)

Foraging behavior is often determined by the conflicting benefits of energy gain and the risk of mortality from predation or other causes. Theory predicts that animals should have lower activity levels when either the risk of predation or the availability of resources in the environment is high. We investigated the adjustment of the behavior of I. elegans larvae to predator presence (Anax imperator) and prey density (Daphnia sp.) and their interaction in a completely crossed factorial experiment in the lab and the effect of behavior on growth. The foraging activity of the I. elegans larvae was significantly reduced in the presence of a free-swimming predator but not a caged predator. Abdominal movements were significantly reduced at a low prey density. Growth was significantly reduced by the presence of a free swimming predator and low prey densities. These results provide evidence that these damselfly larvae adjust their behavior to the presence of predators to increase their survival at the expense of reduced growth and development.     

Seasonal Variations in the Diet and Foraging Behaviour of Dunlins Calidris alpina in a South European Estuary: Improved Feeding Conditions for Northward Migrants

During the annual cycle, migratory waders may face strikingly different feeding conditions as they move between breeding areas and wintering grounds. Thus, it is of crucial importance that they rapidly adjust their behaviour and diet to benefit from peaks of prey abundance, in particular during migration, when they need to accumulate energy at a fast pace. In this study, we compared foraging behaviour and diet of wintering and northward migrating dunlins in the Tagus estuary, Portugal, by video-recording foraging birds and analysing their droppings. We also estimated energy intake rates and analysed variations in prey availability, including those that were active at the sediment surface. Wintering and northward migrating dunlins showed clearly different foraging behaviour and diet. In winter, birds predominantly adopted a tactile foraging technique (probing), mainly used to search for small buried bivalves, with some visual surface pecking to collect gastropods and crop bivalve siphons. Contrastingly, in spring dunlins generally used a visual foraging strategy, mostly to consume worms, but also bivalve siphons and shrimps. From winter to spring, we found a marked increase both in the biomass of invertebrate prey in the sediment and in the surface activity of worms and siphons. The combination of these two factors, together with the availability of shrimps in spring, most likely explains the changes in the diet and foraging behaviour of dunlins. Northward migrating birds took advantage from the improved feeding conditions in spring, achieving 65% higher energy intake rates as compared with wintering birds. Building on these results and on known daily activity budgets for this species, our results suggest that Tagus estuary provides high-quality feeding conditions for birds during their stopovers, enabling high fattening rates. These findings show that this large wetland plays a key role as a stopover site for migratory waders within the East Atlantic Flyway.     

What you need is what you eat? Prey selection by the bat Myotis daubentonii

Optimal foraging theory predicts that predators are selective when faced with abundant prey, but become less picky when prey gets sparse. Insectivorous bats in temperate regions are faced with the challenge of building up fat reserves vital for hibernation during a period of decreasing arthropod abundances. According to optimal foraging theory, prehibernating bats should adopt a less selective feeding behaviour – yet empirical studies have revealed many apparently generalized species to be composed of specialist individuals. Targeting the diet of the bat Myotis daubentonii, we used a combination of molecular techniques to test for seasonal changes in prey selectivity and individual‐level variation in prey preferences. DNA metabarcoding was used to characterize both the prey contents of bat droppings and the insect community available as prey. To test for dietary differences among M. daubentonii individuals, we used ten microsatellite loci to assign droppings to individual bats. The comparison between consumed and available prey revealed a preference for certain prey items regardless of availability. Nonbiting midges (Chironomidae) remained the most highly consumed prey at all times, despite a significant increase in the availability of black flies (Simuliidae) towards the end of the season. The bats sampled showed no evidence of individual specialization in dietary preferences. Overall, our approach offers little support for optimal foraging theory. Thus, it shows how novel combinations of genetic markers can be used to test general theory, targeting patterns at both the level of prey communities and individual predators.     

Movements of wintering surf scoters: predator responses to different prey landscapes

The distribution of predators is widely recognized to be intimately linked to the distribution of their prey. Foraging theory suggests that predators will modify their behaviors, including movements, to optimize net energy intake when faced with variation in prey attributes or abundance. While many studies have documented changes in movement patterns of animals in response to temporal changes in food, very few have contrasted movements of a single predator species naturally occurring in dramatically different prey landscapes. We documented variation in the winter movements, foraging range size, site fidelity, and distribution patterns of a molluscivorous sea duck, the surf scoter (Melanitta perspicillata), in two areas of coastal British Columbia with very different shellfish prey features. Baynes Sound has extensive tidal flats with abundant clams, which are high-quality and temporally stable prey for scoters. Malaspina Inlet is a rocky fjord-like inlet where scoters consume mussels that are superabundant and easily accessible in some patches but are heavily depleted over the course of winter. We used radio telemetry to track surf scoter movements in both areas and found that in the clam habitats of Baynes Sound, surf scoters exhibited limited movement, small winter ranges, strong foraging site fidelity, and very consistent distribution patterns. By contrast, in mussel habitats in the Malaspina Inlet, surf scoters displayed more movement, larger ranges, little fidelity to specific foraging sites, and more variable distribution patterns. We conclude that features associated with the different prey types, particularly the higher depletion rates of mussels, strongly influenced seasonal space use patterns. These findings are consistent with foraging theory and confirm that predator behavior, specifically movements, is environmentally mediated.