Foraging Ability Research Articles

Effect of Delayed Toxicity of Chemical Barriers to Control Argentine Ants (Hymenoptera: Formicidae)

Brief exposures of Argentine ants to four different insecticide treatments, bifenthrin, beta-cyfluthrin, bifenthrin + beta-cyfluthrin, and fipronil, were conducted to determine Kaplan-Meier product limit survivorship percentiles (SPs) at 21-23 and 27-29 degrees C. Bifenthrin, beta-cyfluthrin, and bifenthrin + beta-cyfluthrin provided rapid kill at 21-23 degrees C with SP10 values ranging from 11.2 to 33.7 min. Fipronil provided delayed toxicity at 21-23 degrees C with SP10 values ranging from 270 to 960 min. At 27-29 degrees C, all of the SP10 values significantly decreased. Field tests in which Argentine ants were induced to forage across insecticide-treated surfaces were used to determine the effect that speed of action has on foraging and recruitment ability, and whether these insecticides are repellent. The slower-acting fipronil allowed a greater amount of foraging and consequently a greater fraction of the colony to be exposed, whereas fast-acting bifenthrin, beta-cyfluthrin, and bifenthrin + beta-cyfluthrin inhibited recruitment, resulting in fewer ants being exposed and killed. Implications for controlling ants by using perimeter barrier treatments are discussed.

Influence of perennial forages on subsoil organic carbon in a long-term rotation study in Uruguay

Abstract Interest is increasing in Uruguay in management practices that can store carbon in soils, such as the use of perennial forage crops. Previous studies on agricultural soils have focused on organic carbon in surface soil layers. However, due to the ability of perennial forages to develop extensive root systems, the potential exists for these crops to add carbon in the subsoil. A 38-year rotation experiment on a silty clay loam soil in southwestern Uruguay was used to examine the effect of a crop rotation including periods of pasture on total organic carbon (TOC) and particulate (>53 μm) organic carbon (POC) at soil depths of 20–40 and 40–60 cm. Analysis showed the pasture rotation had higher POC concentrations at both depths than an annual crop rotation. This may indicate increased addition of organic carbon in the pasture rotation and signal a future change in TOC and mineral-associated organic carbon levels. Total organic carbon was higher in the rotation including pasture at a P level of 0.14. These results present preliminary observations on the management effects of including perennial forages in rotations on subsoil carbon levels, using a long-term experiment resource to assess very slow changes.

Successional status and root foraging for phosphorus in seven tropical tree species

In this study, the impact of localized nutrient patches on above- and belowground growth of tropical trees was examined. Seedlings of seven tree species (Albizia guachapele Kunth (Dugand), Cedrela odorata L., Ceiba pentandra (L.) Gaertn., Cordia alliodora (Ruiz & Pavón) Oken, Dalbergia retusa Hemsl., Gliricidia sepium (Jacq.) Kunth ex Walp., and Swietenia macrophylla King) of varying successional status were used in a greenhouse container experiment. Treatments of homogeneous and heterogeneous P fertilization were used to determine each species' ability to forage morphologically for P. In each pot, root length density, average root diameter, and root length to shoot ratio were measured. It was hypothesized that early successional species would be most effective at taking advantage of the heterogeneous soil environment. This prediction was based on studies showing a high degree of soil heterogeneity in early successional forest environments and the need for plants to take advantage of this variability. The study found great variability in the foraging capacity between species, but the hypothesis that successional status and root foraging ability are related was not supported in the species tested. Overall, five of the seven species examined showed some indication that they were taking advantage of local nutrient patches. In four of these species, increased root length density was detected in P patches (A. guachapele, C. odorata, G. sepium, and S. macrophylla) and one species had decreased root width in the P patches (G. sepium). When comparisons were made between homogeneously and heterogeneously fertilized pots, G. sepium and C. alliodora appeared to take advantage of the heterogeneous resource. Both of these species increased aboveground growth in the heterogeneous pots and C. alliodora also had a decreased root to shoot ratio.

Trade-offs in skillacquisition and time allocation among juvenile chacma baboons.

We hypothesize that juvenile baboons are less efficient foragers than adult baboons owing to their small size, lower level of knowledge and skill, and/or lesser ability to maintain access to resources. We predict that as resources are more difficult to extract, juvenile baboons will demonstrate lower efficiency than adults will because of their lower levels of experience. In addition, we hypothesize that juvenile baboons will be more likely to allocate foraging time to easier-to-extract resources owing to their greater efficiency in acquiring those resources.We use feeding efficiency and time allocation data collected on a wild, free-ranging, non-provisioned population of chacma baboons (Papio hamadryas ursinus) in the Moremi Wildlife Reserve, Okavango Delta, Botswana to test these hypotheses. The major findings of this study are:1. Juvenile baboons are significantly less efficient foragers than adult baboons primarily for difficult-to-extract resources.We propose that this age-dependent variation in efficiency is due to differences in memory and other cognitive functions related to locating food resources, as is indicated by the greater amount of time juvenile baboons spend searching for food. There is no evidence that smaller body size or competitive disruption influences the differences in return rates found between adult and juvenile baboons in this study.2. An individual baboon's feeding efficiency for a given resource can be used to predict the duration of its foraging bouts for that resource.These results contribute both to our understanding of the ontogeny of behavioral development in nonhuman primates, especially regarding foraging ability, and to current debate within the field of human behavioral ecology regarding the evolution of the juvenile period in primates and humans.

Dichromatic and Trichromatic Callithrix geoffroyi Differ in Relative Foraging Ability for Red-Green Color-Camouflaged and Non-camouflaged food

An advantage for trichromatic color vision in primates is shown by its presence in many lineages, but little attention has been paid to the potential disadvantages of trichromacy. Most New World monkey species are polymorphic for color vision, with both dichromats and trichromats present within a single population. We tested the foraging ability of trichromatic and dichromatic Geoffroy's marmosets (Callithrix geoffroyi) for colored cereal balls (Kix®) under conditions of red-green color camouflage (orange/green Kix® against an orange/green background) or lack of camouflage (Kix® same color as background) in a naturalized captive setting. In separate experiments designed to test foraging ability at long distances (<6 m) and short distances (<0.5 m), trichromats found significantly fewer Kix® under the camouflage condition than in the non-camouflage condition. In contrast, there is no difference in the ability of dichromats to detect color-camouflaged versus non-camouflaged Kix®. There is no significant difference between dichromats and trichromats for either camouflaged or non-camouflaged Kix®, though the power in the tests is low because of high individual variation. The results have clear implications for the foraging strategies of trichromatic marmosets. Differences in intensity of competition between trichromats and dichromats for items of food of different colors in relation to background may also have consequences for the foraging behavior of dichromats.

Utilisation by ruminants of nitrogen compounds in silage-based diets

This paper considers the various complex changes that occur to nitrogen (N) containing compounds in forages through the processes of ensiling, rumen degradation and microbial synthesis, post-ruminal digestion and absorption and synthesis into milk protein. Particular emphasis is placed on reviewing recent data on the efficiency of utilisation of N-containing compounds in silages by rumen microbes, since low efficiency here is believed to be a major cause of large N losses to the environment on some silage-based diets. Data are reviewed which show that although rumen degradation of N compounds in silage is rapid and extensive, up to 10% of the soluble N can escape the rumen by being associated with the liquid phase. There is now firm evidence that the composition of the amino acids (AAs) absorbed is heavily dependent on the process of ensiling and that wilting or use of certain silage additives conserve the initial amino acid profile of the forage. This provides an opportunity to manipulate the amino acid supply to better match demand thus potentially enhancing utilisation. This review confirms that utilisation of the N fractions in grass and legume silages in particular, is poor and the efficiency of microbial protein synthesis (EMPS) is consistently higher on maize silage-based diets. It is concluded that the way in which grass and legume silages in particular are produced and used in the future needs a radical rethink. New research needs to be aimed at enhancing the utilisation of N in the rumen through a better understanding of N/carbohydrate relationships and the ability of forages to supply degraded carbohydrate. Also more emphasis is needed on understanding of the potentially different role of the different N fractions that exist in silages.

Effects of Novel Pesticides on Bumble Bee (Hymenoptera: Apidae) Colony Health and Foraging Ability

Abstract Two experiments were conducted testing for lethal and sublethal effects of the transgenic proteins Cry1Ac and chitinase, and the chemical seed and soil treatment imidacloprid on bumble bees (Bombus occidentalis Greene and B. impatiens Cresson, Hymenoptera: Apidae). In the first experiment, B. occidentalis colonies were exposed to realistic residue levels of Cry1Ac, chitinase, and imidacloprid found in pollen. There were no effects on pollen consumption, bumble bee worker weights, colony size, amount of brood, or the number of queens and males produced. In the second experiment, using B. impatiens, we tested the effects of Cry1Ac and two levels of imidacloprid. Similar colony health measures were collected as in the first experiment, but in addition foraging ability of individual bees was tested on complex artificial flowers. There were no differences in colony characteristics among treatments. However, bees in the high-imidacloprid treatment had longer handling times on the complex flowers than b...

The effect of mercury on the feeding behavior of fathead minnows ( Pimephales promelas)

Fathead minnows ( Pimephales promelas) were exposed to mercury (1.69, 6.79, and 13. 57 μg/L HgCl 2, 10 days exposure), and afterward their foraging ability was tested in a vegetated habitat for 7 days. Among the foraging metrics used were foraging efficiency, capture speed, and the ability to learn and retain information regarding habitat characteristics. In addition to behavioral tests, muscle tissue acetylcholinesterase activity and brain levels of several neurotransmitters were investigated. Comparisons with control fish and fish from the two highest exposure groups revealed consistent performance deficits in foraging efficiency and capture speed. However, no treatment effects on learning were detected, nor were differences in neurotransmitter levels detected. In determining the underlying proximate cause of the foraging deficits, it is believed that the greater pause time exhibited by treatment fish while foraging was the main cause of treatment differences. In the future, behavioral studies will continue to allow toxicity testing of environmentally relevant variables such as those used by behavioral ecologists.

Feeding methods, visual fields and vigilance in dabbling ducks (Anatidae)

Summary 1. Visual fields were determined in two species of dabbling ducks (Anatini): Shoveler Anas clypeata L. (planktivore whose foraging is guided mainly by tactile cues) and Wigeon A. penelope L. (herbivore whose foraging is guided mainly by visual cues). 2. The binocular fields of Shoveler and Wigeon are of similar maximum width (20°), but they differ in their position and vertical extent. The bill of the Shoveler lies in the very periphery of its frontal binocular field, which extends through 220° thus providing comprehensive visual coverage about the head. In Wigeon the bill is positioned more centrally in the frontal binocular field, which extends through 150° and results in the birds having a narrow blind area behind the head. 3. The vigilance behaviour of Shoveler and Wigeon when foraging simultaneously was studied using a focal observation procedure at sites where the two species winter in sympatry. Focal Wigeon almost only fed by grazing. Only Shoveler feeding by dabbling (filtering the first centimetres of water) were used in the analyses. Wigeon spent significantly more time in head‐up vigilance than Shoveler (F1,75 = 14·70, P = 0·0003). 4. It is proposed that this interspecific difference in the proportion of time spent in vigilance behaviour may be an adaptive response to differences in the visual field topography of these species, particularly with respect to the presence/absence of a blind area to the rear of the head. 5. The ability of foragers to combine part of their vigilance behaviour with head‐down feeding has recently been recognized as influencing the trade‐offs related to vigilance while foraging. This study shows that this ability may vary significantly between species, even within the same genus, and that these variations are likely to be due to contrasted visual fields, themselves related to the type of feeding techniques employed by the different duck species.

Total cover and cover quality: predicted and actual effects on a predator's foraging success

I tested the importance of both total cover and cover quality to the foraging ability of large Fundulus heteroclitus fish in tanks. Total cover was measured as the combined areas of all the indi- vidual structures, viewed as shadows. I divided total cover by the bottom area of the tank to form Ct/At, an index that measures the total amount of structure available in a given area for prey to hide behind. Cover quality was measured as the width of each individual structure divided by the width of the indi- vidual prey organism (Cw/Pw). This index measures how visible a prey organism is when hiding be- hind an individual structure. Both of these complexity indices are dimensionless and potentially ap- plicable to any habitat type and habitat scale. I predicted that prey survivorship should increase hyperbolically with increasing Ct/At, because increasing the amount of structure within a habitat in- creases the amount of the habitat blocked off from detection, but with diminishing returns. Past ex- periments demonstrated that increased Ct/At does increase prey survivorship, but the shape of the re- lationship was unclear. I also predicted that prey survivorship should increase hyperbolically with increasing Cw/Pw. I predicted this by simulating a prey organism hiding behind a flat structure being viewed by a predator from all potential angles. In this simple simulation, the 'average amount of the prey hidden from view' increased hyperbolically with increasing Cw/Pw to an asymptote of 0.5. I per- formed 2 experiments to test these ideas. In the first, I kept Ct/At constant between treatments and var- ied the width of the individual structures relative to the width of shrimp prey Paleomonetes spp. In the second experiment, I kept the inter-structural space widths nearly constant between treatments and varied Ct/At. The prey for the second experiment were mobile amphipods. I tested the hypothesis that survivorship and treatment were independent using log-linear models for both experiments. I found that there was no significant difference in survivorship between treatments for the Cw/Pw experiment, and there was a significant difference in survivorship between treatments for the Ct/At experiment. The relationship between Ct/At and prey survivorship appeared to increase hyperbolically, as pre- dicted. Finally, I used the results of the Ct/At experiment, and results from a past study, to form a mod- eled regression equation of the survivorship surface for large F. heteroclitus feeding on amphipods for various levels of Ct/At and Sp/Pr (inter-structural space size/predator width). This equation was highly significant, and fit the data well. In this regression, amphipod survivorship increased hyperbolically with increasing Ct/At, and decreased sigmoidally with increasing Sp/Pr.

Towards a cognitive niche: divergent foraging strategies resulting from limited cognitive ability of foraging herbivores in a spatially complex environment

A model was developed to explain one mechanism whereby differential optimal foraging strategies can occur between species as a result of selection for competition avoidance. This is the primary requirement for niche differentiation to evolve without a difference in the underlying foraging ability or morphology. The model used an individual-based patch choice mechanism, whereby herbivores move from patch to patch seeking food with the highest nutrient intake characteristics. The choice of patch was governed by a parameter, μ, which determined to what extent information in the landscape at different distances from the herbivore was used by it to make foraging decisions. A genetic algorithm was used to optimise the value, μ, in a complex landscape. The value of μ quickly converged to a single value with stabilising selection occurring when there was only a single species foraging. When there was a competing species with a fixed value of μ, the value of μ evolved to be above or below the mean for the single species mean depending on whether the value of μ for the competitor was below, or above the single-species mean, respectively. This was indicative of niche segregation. However μ tended to vary unstably over time when allowed to vary simultaneously in both species, although there was evidence for interaction between the two values. These results indicate that there can be a competitive advantage in choosing a cognitive strategy that is complementary to that used by other species.

The influence of age on reproductive performance in the Brown Thornbill

I examined age effects on reproduction in the Brown Thornbill Acanthiza pusilla in Canberra, Australia. I found that the reproductive performance of both males and females improved with age, although only age‐related improvement in male performance had a significant effect on annual reproductive success. Reproductive success improved with male age as a result of improved performance during two stages of the breeding cycle: first‐year males were less likely to fledge young than those aged two or more, while both first and second‐year males were less successful at raising fledglings to independence than males of three or more. Male performance appears to improve over three years as they gain experience at provisioning nestlings and caring for fledglings without attracting predators, rather than as a direct result of improved foraging skills. In contrast, reproductive success only improved slightly with female age, although females of two or more years initiated their first clutch earlier in the season than one‐year‐old females, and tended to be more likely to re‐nest if a breeding attempt failed. The poor performance of young females appears unlikely to be related to their foraging ability but may be associated with costs imposed by dispersing to a breeding vacancy earlier in the year. Although the reproductive performance of Brown Thornbills improves considerably with age I found no evidence that performance improved as a result of repeated breeding attempts with the same partner.

The influence of body size on dive duration of underyearling southern elephant seals (Mirounga leonina)

AbstractThe dive duration of 16 underyearling (6–12 months old) southern elephant seals Mirounga leonina during their second trip to sea was investigated using geolocating time–depth recorders. Underyearling seals had a lesser diving ability, with respect to duration and depth, than adult southern elephant seals. Individual underyearlings dived for average durations of up to 20.3 min and depths up to 416 m compared to durations and depths of 36.9 min and 589 m, respectively for adults. Dive duration was positively related to their body mass at departure, indicating that smaller seals were limited to shorter dive durations, perhaps as a result of their lesser aerobic capacity. All seals often exceeded their theoretical aerobic dive limit (average of 22.1 ± 18.1%). The number of dives exceeding the theoretical aerobic dive limit was not related to mass, suggesting that factors other than mass, such as foraging location or prey availability, may have been responsible for differences in diving effort. Foraging ability, indicated by the ability of the seals to follow vertically moving prey, was positively related to seal mass, indicating that small mass restricted foraging ability. The shorter dive durations of the smaller seals inferred that they had shallower dive depths in which to search for prey, thus restricting foraging ability. Although foraging ability was restricted by size, foraging success was found to be inversely related to mass, the smaller seals gaining a higher proportion of blubber than larger seals during their foraging trips. Thus, despite smaller seals being restricted to shallower depths and shorter durations, their foraging success was not affected.

The influence of body size on dive duration of underyearling southern elephant seals (Mirounga leonina)

The dive duration of 16 underyearling (6–12 months old) southern elephant seals Mirounga leonina during their second trip to sea was investigated using geolocating time–depth recorders. Underyearling seals had a lesser diving ability, with respect to duration and depth, than adult southern elephant seals. Individual underyearlings dived for average durations of up to 20.3 min and depths up to 416 m compared to durations and depths of 36.9 min and 589 m, respectively for adults. Dive duration was positively related to their body mass at departure, indicating that smaller seals were limited to shorter dive durations, perhaps as a result of their lesser aerobic capacity. All seals often exceeded their theoretical aerobic dive limit (average of 22.1 ± 18.1%). The number of dives exceeding the theoretical aerobic dive limit was not related to mass, suggesting that factors other than mass, such as foraging location or prey availability, may have been responsible for differences in diving effort. Foraging ability, indicated by the ability of the seals to follow vertically moving prey, was positively related to seal mass, indicating that small mass restricted foraging ability. The shorter dive durations of the smaller seals inferred that they had shallower dive depths in which to search for prey, thus restricting foraging ability. Although foraging ability was restricted by size, foraging success was found to be inversely related to mass, the smaller seals gaining a higher proportion of blubber than larger seals during their foraging trips. Thus, despite smaller seals being restricted to shallower depths and shorter durations, their foraging success was not affected.

Determination of Factors Limiting Prey Size Swallowed by Larval and Small Juvenile Largemouth Bass

Abstract Mouth diameter limits prey size for adult and subadult largemouth bass. In this study we determined the relationships of mouth diameter and cleithrum orifice size to total length of larval and juvenile largemouth bass Micropterus salmoides. Largemouth bass (6–49 mm total length, TL) collected from Centerton State Hatchery and Lake Wedington (Washington County, Arkansas) were preserved in 10% buffered formalin and stored in 70% ethanol. Total length was significantly related (P < 0.001) to mouth size (gape = 0.117TL − 0.129) and cleithrum orifice size (width: CW = 0.079TL + 0.712; height: CH = 0.082TL + 1.24). These relationships differed significantly (P < 0.05) between late postlarvae and juveniles, late postlarvae having a disproportionately larger mouth and cleithrum orifice for their body size. Late larvae disperse from male largemouth bass parental protection, and the change in proportions of feeding structures to body size could affect their foraging ability during this period of vulnerability.

Influence of Insect Larvae and Seedling Roots on the Host-Finding Ability of Steinernema feltiae (Nematoda: Steinernematidae)

The ability of nematode foragers to locate appropriate insect hosts is essential to their performance as successful biological control agents. We investigated the host-finding ability and chemotaxis of Steinernema feltiae in the presence of cues from Galleria mellonella larvae and tomato and radish seedling roots, given individually and in combination, over 120 min. In agar arena tests, infective juveniles of S. feltiae responded positively to unsterilized and sterilized larvae and tomato seedling roots and negatively to unsterilized radish seedling roots. This negative response changed following surface sterilization of these seeds. The response of the infective juveniles to a combination of larva and seedling roots depended on the nature of the individual cues and their proximity to each other. For example, the response of the nematodes to a combination of cues from a sterilized larva and an unsterilized tomato seedling root placed adjacent to each other was intermediate to the separate responses to cues from a sterilized larva and an unsterilized tomato seedling root given individually. However, the response of the nematodes to a combination of adjacent cues from a sterilized larva and an unsterilized radish seedling root was not significantly different from that to cues from an unsterilized radish seedling root given individually. When the cues from the larva and seedling roots were separated by a distance, the response of the nematodes favored the larva. However, this positive effect was lessened when the larva was surface sterilized as compared with the response to the unsterilized larva. The altered responses of the infective juveniles to target cues following surface sterilization suggest that cues from the larval cuticle and seedling roots, such as those associated with their surface microflora, may significantly influence their host-finding ability. The use of entomopathogenic nematodes as biological control agents under field applications can be improved by careful consideration of the application protocols and by the recognition that chemical alterations of the soil rhizosphere may influence their host-finding ability.

Chick provisioning by the Yellow‐nosed Albatross Diomedea chlororhynchos: response of foraging effort to experimentally increased costs and demands

We examined the provisioning strategy of a long-lived seabird to test the prediction from life-history theory that adults should preferentially allocate resources towards their own needs rather than towards their offspring, and to test the abilities of adults to regulate provisioning according to the chick needs. The individual provisioning behaviour of Yellow-nosed Albatrosses Diomedea chlororhynchos was studied, costs of flight being increased by adding a weight handicap to foraging parents, and needs of the chicks being increased by induced regurgitation (underfed chicks) or decreased by food supplementation (overfed chicks). Control birds were found to regulate provisioning in relation to the nutritional status of the chick and to the mass of the adult. As a result of increased foraging costs, and possibly of reduced foraging ability, handicapped adults spent more time foraging and delivered smaller meals. Unlike control birds, they were unable to regulate provisioning and lost slightly more mass than control birds, but they had similar survival to the next breeding season. The behaviour of parents rearing underfed chicks was similar to that of control birds, but adults rearing overfed chicks delivered smaller meals, at the same frequency as control birds. Thus, although parent Yellow-nosed Albatrosses can perceive the nutritional status of the chick and regulate provisioning accordingly, they are unable to increase the provisioning rate significantly. They primarily keep resources for themselves, their own body condition determining the level of investment in the chick. Most albatrosses and petrels behave so that primarily adult body condition is protected often at the expense of chick provisioning. Regulation abilities differ between species probably according to several factors such as foraging strategies, distance to feeding zones or body size.

Foraging under Multiple Costs: The Importance of Predation, Energetic, and Assessment Error Costs to a Desert Forager

We investigated spatial and temporal variation in the use of resource patches by Chaetodipus penicillatus by documenting changes in foraging activity over several seasons, under two moon phases, and in two resource patch types. In particular, we examined the influence of nocturnal illumination (a surrogate for predation risk) and ambient temperature on: (1) foraging effort (which is inversely proportional to the giving-up density, GUD), and (2) GUD equality, i.e. the ability of foragers to accurately assess patch quality by equalizing heterogeneous resource patches to the same GUD. There were significant positive relationships between foraging effort and relative GUD equality, ambient temperature and foraging effort, and ambient temperature and GUD equality. In contrast, despite having a strong effect at the beginning of our study, moonlight did not have a consistent or significant impact on foraging effort and GUD equality when analyzed over several seasons. Our results indicate that as energetic or predation costs decline, foraging effort rises, which in turn, leads to an increase in GUD equality and a reduction in assessment error costs. This increased harvesting equality is due to both enhanced functional harvest rates and increased accuracy of patch assessment by foragers as more time is spent within a patch. In addition, predation risk and energetic costs appeared to switch in relative importance in C. penicillatus over the course of several seasons. These results emphasize the importance of considering multiple foraging costs and their additive effects in studies of foraging and patch use.

The Influence of Adult Age on Territorial Attendance by Breeding Great Skuas Catharacta skua: An Experimental Study

Territorial or nest attendance by adult breeding seabirds is often essential to prevent egg or chick mortality. As a result, parent birds, when not engaged in foraging trips, should stay on their territories whenever there is a risk from leaving their eggs and chicks unattended. Differences in territorial attendance and time spent foraging by birds of different ages have been interpreted as resulting from differences in adult foraging ability or in motivation to invest in the current reproductive attempt. However, most studies have failed to take into account seasonal variations in food availability, which could affect these patterns, because timing of breeding is often dependent on adult age. We designed an experiment to investigate the levels of territorial attendance displayed by Great Skuas of known age facing the same environmental conditions and with similar food requirements. This was achieved by manipulating clutches so that all study pairs hatched their chicks at the same time. Pairs with one chick had a higher attendance (mean number of adults per territory) than pairs with two chicks. This suggests that the need to increase the time foraging in response to the nutritional demands of a larger brood overrides any effect due to differential motivations in the amount of parental effort to be invested in brood protection. Pairs with older males had a higher attendance than pairs with younger males. About 61% of the estimated foraging time of the pairs fell on the males. Attendance and adult age were not correlated with chick body condition. We conclude that pairs with two chicks that had high attendance achieved this by being more efficient foragers, and not by provisioning their chicks less well. Therefore, our results suggest that older males are better foragers than younger birds. Female age did not correlate with attendance levels. However, pairs with older females were less likely to leave their chicks unattended, which suggests that those pairs had a better co-ordination of their foraging and guarding activities. Breeding success in Great Skuas improves with adult age. Thus, our study identifies a mechanism by which older birds can achieve better performance than younger individuals. Old birds are less likely to leave their chicks exposed to predation by conspecifics, which is the main cause of pre-fledging mortality in this species.

Individual variation in the competitive ability of interference‐prone foragers: the relative importance of foraging efficiency and susceptibility to interference

Summary 1. Individual variation in the competitive ability of foraging animals arises from variation in their intrinsic foraging efficiency and in their susceptibility to interference from competitors. Empirical and theoretical studies have concentrated on quantifying the latter and examining its role in determining the distribution and dynamics of animal populations, but have seldom considered the role of variation in foraging efficiency. Using the frequency of occurrence of oystercatchers in supplementary feeding habitats as an index of their competitive ability, we assessed the relative importance of foraging efficiency and susceptibility to interference in determining the overall competitive ability of an individual. 2. Individual mussel‐feeding oystercatchers varied in their tendency to supplement their low‐tide intake by feeding on other prey on upshore tidal flats and in fields. Individuals that opened mussels by stabbing occurred on upshore flats more often than birds which hammered mussels, whereas young birds were more likely to visit the fields than older birds. This difference between habitats emphasized the need to understand the individual differences underlying these class effects. 3. Foraging efficiency increased with age and differed between feeding methods. Dominance also increased with age, but did not differ between feeding methods. An individual's foraging efficiency was not related to its dominance. 4. Individual variation in the usage of either upshore flats or fields, and of each habitat separately, was related to individual variation in foraging efficiency, but not to variation in dominance. Individuals of poor intrinsic foraging ability made greater use of supplementary feeding habitats than did more efficient foragers. 5. Our results show that, even in an interference‐prone system and across a wide range of circumstances, individual variation in foraging efficiency is the major determinant of overall competitive ability. We believe, therefore, that this source of individual variation is of greater importance in determining variation in mortality risk within a population than the effort invested in its study hitherto would suggest. We suggest that a modelling approach is necessary to establish the competitive conditions under which susceptibility to interference might become the more important determinant of competitive ability and, hence, whether such conditions are ever likely to occur in natural populations. We argue that greater emphasis needs to be placed on identifying the determinants of foraging efficiency and its variation between individuals.