Individual variation in winter foraging of black-capped chickadees

Summary Wintering black-capped chickadees (Paridae: Parus atricapillus) in northwestern Massachusetts showed a high degree of individual variation in foraging behavior. After accounting for the effects of different habitats and weather conditions, individual differences comprised 6–17% of the total observed variation in four measures of foraging location and rate of feeding. Differences between age and sex groups were not significant and explained comparatively little variation (0.0–1.4%). The chickadees did not fall into a few distinct behavioral categories but instead showed continuous variation on all measures of foraging behavior. It appeared that some variation among individuals was a consequence of behavioral convergence within social groups, since birds that were observed together were more similar in their foraging than expected by chance, after taking habitat differences into account. Our results therefore do not support the interpretation that individual variation in feeding behavior serves to reduce exploitation competition within social groups.     

Autotomy shapes the trade-off between seeking cover and foraging in larval damselflies

Animals commonly choose between microhabitats that differ in foraging return and mortality hazard. I studied the influence of autotomy, the amputation of a body part, on the way larvae of the damselfly Lestes sponsa deal with the trade-off between foraging or seeking cover. Survival of Lestes larvae when confronted with the odonate predator Aeshna cyanea was higher in a complex than in a simple microhabitat, indicating that this more complex microhabitat was safer. Within the simple microhabitat, larvae without lamellae had a higher risk for mortality by predation than larvae with lamellae, showing a long-term cost of autotomy. When varying the foraging value (food present or absent) and predation risk (encaged predator or no predator) in the simple microhabitat, larvae with and without lamellae responded differentially to the imposed trade-off. All larvae spent more time in the simple microhabitat when food was present than when food was absent. Larvae without lamellae, however, only sporadically left the safe microhabitat, irrespective of the presence of the predator. In contrast, larvae with lamellae shifted more frequently towards the risky microhabitat than those without lamellae, and more often in the absence than in the presence of the predator. These decisions affected the foraging rates of the animals. I show for the first time that refuge use is higher after autotomy and that this is associated with the cost of reduced foraging success. The different microhabitat preferences for larvae with and without lamellae are consistent with their different vulnerabilities to predation and demonstrate the importance of intrinsic factors in establishing trade-offs. Received: 4 June 1999 / Received in revised form: 18 August 1999/ Accepted: 18 August 1999     

The impact of parasite manipulation and predator foraging behavior on predator-prey communities

Parasites are known to directly affect their hosts at both the individual and population level. However, little is known about their more subtle, indirect effects and how these may affect population and community dynamics. In particular, trophically transmitted parasites may manipulate the behavior of intermediate hosts, fundamentally altering the pattern of contact between these individuals and their predators. Here, we develop a suite of population dynamic models to explore the impact of such behavioral modifications on the dynamics and structure of the predator-prey community. We show that, although such manipulations do not directly affect the persistence of the predator and prey populations, they can greatly alter the quantitative dynamics of the community, potentially resulting in high amplitude oscillations in abundance. We show that the precise impact of host manipulation depends greatly on the predator's functional response, which describes the predator's foraging efficiency under changing prey availabilities. Even if the parasite is rarely observed within the prey population, such manipulations extend beyond the direct impact on the intermediate host to affect the foraging success of the predator, with profound implications for the structure and stability of the predator-prey community.     

The interplay between foraging mode, habitat structure, and predator presence in antlions

Antlion larvae are sand-dwelling insect predators, which ambush small arthropod prey while buried in the sand. In some species, the larvae construct conical pits and are considered as sit-and-wait predators which seldom relocate while in other species, they ambush prey without a pit but change their ambush site much more frequently (i.e., sit-and-pursue predators). The ability of antlion larvae to evade some of their predators which hunt them on the sand surface is strongly constrained by the degree of sand stabilization or by sand depth. We studied the effect of predator presence, predator type (active predatory beetle vs. sit-and-pursue wolf spider), and sand depth (shallow vs. deep sand) on the behavioral response of the pit building Myrmeleon hyalinus larvae and the sit-and-pursue Lopezus fedtschenkoi larvae. Predator presence had a negative effect on both antlion species activity. The sit-and-wait M. hyalinus larvae showed reduced pit-building activity, whereas the sit-and-pursue L. fedtschenkoi larvae decreased relocation activity. The proportion of relocating M. hyalinus was negatively affected by sand depth, whereas L. fedtschenkoi was negatively affected also by the predator type. Specifically, the proportion of individual L. fedtschenkoi that relocated in deeper sand was lower when facing the active predator rather than the sit-and-pursue predator. The proportion of M. hyalinus which constructed pits decreased in the presence of a predator, but this pattern was stronger when exposed to the active predator. We suggest that these differences between the two antlion species are strongly linked to their distinct foraging modes and to the foraging mode of their predators. Reut Loria and Inon Scharf contributed equally to the paper.     

Changing priorities: the effect of pre-migratory fattening on the trade-off between foraging and vigilance

Summary Many birds accumulate fat reserves prior to departure on long distance migration. Since there will be an increased food requirement during the pre-migratory period, it is to be expected that more time will be invested in foraging, at the expense of other activities. The allocation of time to anti-predatory behavior in migratory ruddy turnstones (adults) was found to decrease prior to migration (Fig. 1); non-migratory individuals (juveniles) showed no decrease over the same time period (Fig. 2). This is interpreted as a change in the optimal adult behavior, the cost of a reduced rate of resource accumulation outweighing the additional risk of predation which results from the decrease in vigilance.     

Individual differences in foraging behavior and cortisol levels in recently emerged brook charr (Salvelinus fontinalis)

Recently emerged brook charr (Salvelinus fontinalis) foraging in still-water pools along the sides of streams tend to be either sedentary, feeding from the lower portion of the water column (a sit-and-wait tactic), or very active, feeding from the upper portion of the water column (an active search tactic). We tested whether the individual differences in foraging behavior were associated with baseline concentrations and responses of cortisol, a steroid hormone linked to personality differences in a variety of animals including fishes. We quantified the proportion of time spent on moving by focal charr in the field and then capturing them. Captured individuals were either (i) sacrificed immediately to quantify baseline cortisol concentrations, (ii) held in an unfamiliar field environment for 15 min and then sacrificed to quantify cortisol concentrations in response to handling and holding in a novel field environment, or (iii) held in an unfamiliar field environment with a white Plexiglas base (stressor) for 15 min to quantify cortisol concentrations in response to a novel object. Eleven statistical models relating cortisol concentrations to the proportion of time individuals spent on moving while searching for prey were compared using multi-model inferencing. Cortisol concentrations were higher for charr that spent a lower proportion of time on moving in the field than for charr that spent a higher proportion of time on moving. For a given proportion of time spent on moving, mean cortisol concentrations between baseline and experimental treatments, our measure of cortisol response, did not differ markedly. Our findings suggest that the foraging tactics displayed by wild brook charr in the field could reflect differences in how individuals perceive their environment.     

Linking movement, diving, and habitat to foraging success in a large marine predator

Establishing where and when predators forage is essential to understanding trophic interactions, yet foraging behavior remains poorly understood in large marine carnivores. We investigated the factors leading to foraging success in gray seals (Halichoerus grypus) in the Northwest Atlantic in the first study to use simultaneous deployments of satellite transmitters, time depth recorders, and stomach-temperature loggers on a free-ranging marine mammal. Thirty-two seals were each fitted with the three types of instrumentation; however, complete records from all three instruments were obtained from only 13 individuals, underscoring the difficulty of such a multi-instrument approach. Our goal was to determine the characteristics of diving, habitat, and movement that predict feeding. We linked diving behavior to foraging success at two temporal scales: trips (days) and bouts (hours) to test models of optimal diving, which indicate that feeding can be predicted by time spent at the bottom of a dive. Using an information-theoretic approach, a Generalized Linear Mixed Model with trip duration and accumulated bottom time per day best explained the number of feeding events per trip, whereas the best predictor of the number of feeding events per bout was accumulated bottom time. We then tested whether characteristics of movement were predictive of feeding. Significant predictors of the number of feeding events per trip were angular variance (i.e., path tortuosity) and distance traveled per day. Finally, we integrated measures of diving, movement, and habitat at four temporal scales to determine overall predictors of feeding. At the 3-h scale, mean bottom time and distance traveled were the most important predictors of feeding frequency, whereas at the 6-h and 24-h time scales, distance traveled alone was most important. Bathymetry was the most significant predictor of feeding at the 12-h interval, with feeding more likely to occur at deeper depths. Our findings indicate that several factors predict feeding in gray seals, but predictor variables differ across temporal scales such that environmental variation becomes important at some scales and not others. Overall, our results illustrate the value of simultaneously recording and integrating multiple types of information to better understand the circumstances leading to foraging success.     

Feeding behavior of the sea star Astropecten articulatus (Echinodermata: Asteroidea): an evaluation of energy-efficient foraging in a soft-bottom predator

The energy efficiency of the foraging behavior of Astropecten articulatus (Say) was evaluated in the laboratory. Individuals utilized in the study were collected in the northern Gulf of Mexico from 1990 to 1992. Sea stars presented with equal numbers and weights of low-quality and high-quality prey consistently selected prey of the higher quality. Choice of prey appeared to be mediated by contact chemoreception. Individuals presented with equal weights or equal numbers of different-sized prey models demonstrated a significant preference for smaller prey. Size-selective feeding may be attributable to the ease of manipulation and ingestion of smaller prey, which mazimizes food intake per unit time. In the absence of prey, A. articulatus exhibited a directional pattern of movement. However, as prey were encountered, both the frequency and magnitude of angular deviations in the foraging path increased, resulting in increased foraging in areas of higher prey density. This response to prey availability may increase foraging efficiency by maximizing the rate of prey encounter. Like four other species of the genus Astropecten, A. articulatus exhibited two peaks of activity corresponding with dawn and dusk. Diurnal activities with periods of increased prey availability or periods during which predators are diminished or absent. The sea star A. articulatus exhibits foraging behaviors consistent with the maximization of net energy intake per unit foraging time.     

Brood neglect and contingent foraging behavior in a pelagic seabird

Among species where there is a risk to leaving offspring unattended, parents usually take alternating shifts guarding their young. However, they may occasionally exhibit brood neglect by leaving their offspring unattended at the nest. To investigate this phenomenon further, we examined the foraging behavior of the northern gannet (Morus bassanus) during chick-rearing. This species has a prolonged nestling period (13 weeks) and the single chick is usually guarded by one or other of its parents, because unattended chicks are frequently attacked by conspecifics. We tested the prediction that the foraging behavior of adults when they left their offspring alone at the nest (unattended trips) would differ in character to when adults left offspring with their partner (attended trips). Brood neglect typically occurred after a longer-than-average attendance period at the nest. Unattended trips were, on average, about half the duration of attended trips, and therefore much closer to the colony. There was also a difference in departure direction between attended and unattended trips, with unattended trips tending to be northeast of the colony. Chicks were fed by parents after both attended and unattended trips, but the frequency and the duration of unattended trips increased as chicks grew older whereas the duration of attended trips decreased as chicks grew. These results indicate that parents may be making a trade-off between risk of attack to their offspring and food-intake rate, and that the solution to this trade-off is dependent on chick age.Communicated by C. Brown     

Intraspecific variation in a predator affects community structure and cascading trophic interactions

Intraspecific phenotypic variation in ecologically important traits is widespread and important for evolutionary processes, but its effects on community and ecosystem processes are poorly understood. We use life history differences among populations of alewives, Alosa pseudoharengus, to test the effects of intraspecific phenotypic variation in a predator on pelagic zooplankton community structure and the strength of cascading trophic interactions. We focus on the effects of differences in (1) the duration of residence in fresh water (either seasonal or year-round) and (2) differences in foraging morphology, both of which may strongly influence interactions between alewives and their prey. We measured zooplankton community structure, algal biomass, and spring total phosphorus in lakes that contained landlocked, anadromous, or no alewives. Both the duration of residence and the intraspecific variation in foraging morphology strongly influenced zooplankton community structure. Lakes with landlocked alewives had small-bodied zooplankton year-round, and lakes with no alewives had large-bodied zooplankton year-round. In contrast, zooplankton communities in lakes with anadromous alewives cycled between large-bodied zooplankton in the winter and spring and small-bodied zooplankton in the summer. In summer, differences in feeding morphology of alewives caused zooplankton biomass to be lower and body size to be smaller in lakes with anadromous alewives than in lakes with landlocked alewives. Furthermore, intraspecific variation altered the strength of the trophic cascade caused by alewives. Our results demonstrate that intraspecific phenotypic variation of predators can regulate community structure and ecosystem processes by modifying the form and strength of complex trophic interactions.     

Diel foraging patterns of the sea urchin Centrostephanus coronatus as a predator avoidance strategy

During the day, the diadematid sea urchin Centrostephanus coronatus occupies holes and crevices in shallow subtidal rocky substrata. Individuals emerge from these after sunset and forage on organisms attached to the surrounding rock surface. Each urchin travels <1 m from its shelter and returns to the same one before sunrise. The sheephead wrasse Pimelometopon pulchrum does not remove urchins from their shelters, but will attack and consume urchins placed in normal feeding locations during the daytime. The active periods of the sheephead and the urchin do not overlap; urchins begin foraging about 20 min after the diurnal sheephead retire in the evening and return to their shelters 1 to 2 h before sheephead resume feeding in the morning. We infer that the urchin's daytime crevice-dwelling and nocturnal foraging habits have evolved as a response to sheephead predation. Moreover, because shelters are limited in supply, shelter fidelity may have evolved to insure refuge from sheephead.     

Individual- and population-level responses of a keystone predator to geographic variation in prey

Investigating how food supply regulates the behavior and population structure of predators remains a central focus of population and community ecology. These responses will determine the strength of bottom-up processes through the food web, which can potentially lead to coupled top-down regulation of local communities. However, characterizing the bottom-up effects of prey is difficult in the case of generalist predators and particularly with predators that have large dispersal scales, attributes that characterize most marine top predators. Here we use long-term data on mussel, barnacle, limpet, and other adult prey abundance and recruitment at sites spread over 970 km to investigate individual- and population-level responses of the keystone intertidal sunstar Heliaster helianthus on the coast of Chile. Our results show that this generalist predator responds to changes in the supply of an apparently preferred prey, the competitively dominant mussel Perumytilus purpuratus. Individual-level parameters (diet composition, per capita prey consumption, predator size) positively responded to increased mussel abundance and recruitment, whereas population-level parameters (density, biomass, size structure) did not respond to bottom-up prey variation among sites separated by a few kilometers. No other intertidal prey elicited positive individual predator responses in this species, even though a large number of other prey species was always included in the diet. Moreover, examining predator-prey correlations at approximately 80, 160, and 200 km did not change this pattern, suggesting that positive prey feedback could occur over even larger spatial scales or as a geographically unstructured process. Thus individual-level responses were not transferred to population changes over the range of spatial scales examined here, highlighting the need to examine community regulation processes over multiple spatial scales.     

Calanoid copepod escape behavior in response to a visual predator

Calanoid copepods typically exhibit escape reactions to hydrodynamic stimuli such as those generated by the approach of a predator. During the summers of 2000, 2001 and 2004, two small calanoid species, Temora turbinata Dana, 1849 and Paracalanus parvus Claus, 1863 were exposed to a visual predatory fish, the blenny Acanthemblemaria spinosa Metzelaar, 1919, and their predator–prey interactions were recorded using both high-speed and standard videographic techniques. Copepod escape reaction components, including swimming pattern, reactive distance, turning rate, and jump kinetics, were quantified from individual predation events using motion analysis techniques. Among the observed escape reaction components, differences were noted between the species’ swimming patterns prior to attack and their response latencies. Temora turbinata was a continuous cruiser and P. parvus exhibited a hop-and-sink swimming pattern. During periods of sinking, P. parvus stopped beating its appendages, which presumably reduced any self-generated hydrodynamic signals and increased perceptual abilities to detect an approaching predator. Response latency was determined for each copepod species using a hydrodynamic stimulus produced by a 1 ms acoustic signal. Response latencies of T. turbinata were significantly longer than those of P. parvus. Despite some apparent perceptual advantages of P. parvus, the blenny successfully captured both species by modifying its attack behavior for the targeted prey.     

The community effects of phenotypic and genetic variation within a predator population

Natural populations are heterogeneous mixtures of individuals differing in physiology, morphology, and behavior. Despite the ubiquity of phenotypic variation within natural populations, its effects on the dynamics of ecological communities are not well understood. Here, we use a quantitative genetics framework to examine how phenotypic variation in a predator affects the outcome of apparent competition between its two prey species. Classical apparent competition theory predicts that prey have reciprocally negative effects on each other. The addition of phenotypic trait variation in predation can marginalize these negative effects, mediate coexistence, or generate positive indirect effects between the prey species. Long-term coexistence or facilitation, however, can be preceded by long transients of extinction risk whenever the heritability of phenotypic variation is low. Greater heritability can circumvent these ecological transients but also can generate oscillatory and chaotic dynamics. These dramatic changes in ecological outcomes, in the sign of indirect effects, and in stability suggest that studies which ignore intraspecific trait variation may reach fundamentally incorrect conclusions regarding ecological dynamics.