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Edwardsiella ictaluri is a causative agent of enteric septicemia of catfish (ESC), a seriously lethal disease in Vietnamese catfish (Pangasius hypophthalmus). A safe and effective vaccine against ESC is currently an urgent demand due to antibiotic overuse in pangasius farms has led to an alarming antimicrobial resistance. In this study, two E. ictaluri wzzE mutants (WzM-L3, deficient in a 1038bp-entire wzzE gene and WzM-S3, a 245bp-partial deletion of wzzE) were developed and their protection efficiacy was evaluated in hatched pangasius against ESC by immersion vaccination. As comparing to the high virulent wild-type strain who caused 73.33% of death on pangasius fingerlings immersed at 7.1 × 106 CFU ml−1, both mutants showed extremely low mortality rates at 3.33% (WzM-S3) and 0% (WzM-L3) on pangasius fingerlings immersed at high concentration of 1.5 × 107 CFU mL−1 and 9.7 × 106 CFU ml−1, respectively. Interestingly, both WzM-S3 and WzM-L3 had a remarkably high protection against ESC, as RPS % were found at 89.29% and 90%, respectively. The mutant WzM-L3 is a potential live attenuated vaccine against ESC in Vietnamese catfish farms with good protection and simple practice.

Essential oils obtained from hydrodistillation of three medicinal plants grown in Vietnam were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC-MS). The monoterpene hydrocarbons, α-pinene (50.2%), β-pinene (23.6%) and limonene (5.3%) were the most abundant constituents of the rhizome oil of Zingiber collinsii Mood & Theilade (Zingiberaceae). The main compounds of the stem oil of Croton kongensis Gagnep., (Euphorbiaceae) were benzyl benzoate (12.7%), β-selinene (9.8%), bulnesol (8.0%) and 5,6,7,8-tetrahydroquinoxaline (7.4%). The leaf oil of Goniothalamus albiflous Ban., consisted mainly of α-pinene (26.2%), caryophyllene oxide (10.6%) and 1,8-cineole (9.7%). The composition of the oils of Zingiber collinsii and Croton kongensis was being reported for the first time.

Agrobacterium tumefaciens-mediated transient transformation has been a useful procedure for characterization of proteins and their functions in plants, including analysis of protein-protein interactions. Agrobacterium-mediated transient transformation of Nicotiana benthamiana by leaf infiltration has been widely used due to its ease and high efficiency. However, in Arabidopsis this procedure has been challenging. Previous studies suggested that this difficulty was caused by plant immune responses triggered by perception of Agrobacterium. Here, we report a simple and robust method for Agrobacterium-mediated transient transformation in Arabidopsis. AvrPto is an effector protein from the bacterial plant pathogen Pseudomonas syringae that suppresses plant immunity by interfering with plant immune receptors. We used transgenic Arabidopsis plants that conditionally express AvrPto under the control of a dexamethasone (DEX)-inducible promoter. When the transgenic plants were pretreated with DEX prior to infection with Agrobacterium carrying a β-glucuronidase (GUS, uidA) gene with an artificial intron and driven by the CaMV 35S promoter, transient GUS expression was dramatically enhanced compared to that in mock-pretreated plants. This transient expression system was successfully applied to analysis of the subcellular localization of a cyan fluorescent protein (CFP) fusion and a protein-protein interaction in Arabidopsis. Our findings enable efficient use of Agrobacterium-mediated transient transformation in Arabidopsis thaliana.

Summary Arabidopsis thaliana calmodulin binding protein 60g (CBP60g) contributes to production of salicylic acid (SA) in response to recognition of microbe-associated molecular patterns (MAMPs) such as flg22, a fragment of bacterial flagellin. Calmodulin binding is required for the function of CBP60g in limiting growth of the bacterial pathogen Pseudomonas syringae pv. maculicola (Pma) ES4326 and activation of SA synthesis. Here, we describe a closely related protein, SARD1. Unlike CBP60g, SARD1 does not bind calmodulin. Growth of Pma ES4326 is enhanced in sard1 mutants. In cbp60g sard1 double mutants, growth of Pma ES4326 is greatly enhanced, and SA levels and expression of PR-1 and SID2 are dramatically reduced. Expression profiling placed the CBP60g/SARD1 node between the PAD4/EDS1 and SA nodes in the defense signaling network, and indicated that CBP60g and SARD1 affect defense responses in addition to SA production. A DNA motif bound by CBP60g and SARD1, GAAATTT, was significantly over-represented in promoters of CBP60g/SARD1-dependent genes, suggesting that expression of these genes is modulated by CBP60g/SARD1 binding. Gene expression patterns showed a stronger effect of cbp60g mutations soon after activation of a defense response, and a stronger effect of sard1 mutations at later times. The results are consistent with a model in which CBP60g and SARD1 comprise a partially redundant protein pair that is required for activation of SA production as well as other defense responses, with CBP60g playing a more important role early during the defense response, and SARD1 to playing a more important role later.

RNA-binding proteins (RBP) can control gene expression at both transcriptional and post-transcriptional levels. Plants respond to pathogen infection with rapid reprogramming of gene expression. However, little is known about how plant RBP function in plant immunity. Here, we describe the involvement of an RBP, Arabidopsis thaliana RNA-binding protein-defense related 1 (AtRBP-DR1; At4g03110), in resistance to the pathogen Pseudomonas syringae pv. tomato DC3000. AtRBP-DR1 loss-of-function mutants showed enhanced susceptibility to P. syringae pv. tomato DC3000. Overexpression of AtRBP-DR1 led to enhanced resistance to P. syringae pv. tomato DC3000 strains and dwarfism. The hypersensitive response triggered by P. syringae pv. tomato DC3000 avrRpt2 was compromised in the Atrbp-dr1 mutant and enhanced in the AtRBP-DR1 overexpression line at early time points. AtRBP-DR1 overexpression lines showed higher mRNA levels of SID2 and PR1, which are salicylic acid (SA) inducible, as well as spontaneous cell death in mature leaves. Consistent with these observations, the SA level was low in the Atrbp-dr1 mutant but high in the overexpression line. The SA-related phenotype in the overexpression line was fully dependent on SID2. Thus, AtRBP-DR1 is a positive regulator of SA-mediated immunity, possibly acting on SA signaling-related genes at a post-transcriptional level.

Numbers of Oystercatchers Haematopus ostralegus overwintering on the Exe estuary, south Devon, increased over the period 1976 to 1991. Mortality of colour-ringed adults also increased over this period. In this study, we tested for density-dependence in adult Oystercatcher annual and winter mortality. We examined mortality in all adults, using estuary-wide bird counts, and in mussel feeders only, using bird densities on the mussel beds. Results for annual mortality were equivocal, but suggested that mortality was density-dependent, both for all adults and for mussel feeders only. No density-dependence was detected in the winter mortality of all adults, but strong density-dependent mortality was found amongst mussel-feeders. By modelling mussel feeder mortality at different times of year, we also predict that strong density-dependent mortality occurs on spring migration.

Population models show that the response of migratory bird populations to a reduction in the area, or change in the quality, of their winter feeding areas depends critically on the shape (linear or non-linear) and strength (slope) of the relationship describing how the proportion dying from starvation changes with population density. Yet the parameters that define such density-dependent functions are extremely difficult to estimate from direct measurements of mortality at different population sizes. Even if they could be estimated, there would be uncertainty as to whether or not the relationship would remain the same, and thus provide reliable predictions, in the new circumstances for which forecasts are required. This paper summarizes studies of the Oystercatcher Haematopus ostralegus which aim to derive the shape and strength of the winter density-dependent mortality and emigration function for one estuary, under existing and new circumstances, from the responses of individual birds to each other and to their spatially and seasonally varying food supply. Based on these studies, an individuals-based, physiologically structured game theoretic distribution model has been built which predicts the carrying capacity and numbers of birds dying at different population sizes. The model also can be applied to Oyster-catchers occupying several estuaries distributed throughout their wintering range and can thus be extended to the entire biogeographical or global population. In addition, it can be used to identify some easily measured behavioural and ecological parameters that identify those species, from a wide range of taxa, that are most likely to be affected by habitat loss and change.

Since 1976, the numbers of Oystercatchers Haematopus ostralegus wintering on the Exeestuary have fluctuated independently of the substantial increase over the same period in the numbers wintering in Britain as a whole. While this might imply that the carrying capacity of the Exe has been reached, closer examination suggests not. Over a sample of the years studied, Oystercatcher numbers on the main Mussel Mytilus edulis feeding areas increased even though the mussel bed quality remained unchanged, food abundance decreased and disturbance on some important beds increased. There was some redistribution of birds between mussel beds, apparently linked to the increased bird numbers and to a relative improvement in the food supply on two preferred mussel beds. The increase in Oystercatcher numbers on the main feeding areas over a period in which the feeding conditions did not improve suggests that carrying capacity was not reached on the mussel beds themselves and, therefore, on the estuary as a whole.

The habitats and foods used by a sample of marked individuals and by the whole population of Oystercatchers Haematopus ostralegus wintering on the Exe estuary, South Devon, are described. In the spring and early summer, only a few hundred immatures were present. Two thousand adults and several hundred juveniles arrived in late summer from the breeding grounds and remained until the following spring. The majority of the birds were then adults. At low water, most adults fed on the mussel Mytilus edulis beds and ate mussels. Most individuals specialised on this prey and ate little else. Some adults fed on mud-flats and sandflats within the estuary and along the coast, and specialised on a mixture of Nereis diversicolor and Scrobicularia plana or on Cerastoderma edule or Littorina spp. Though some juveniles ate mussels from the time they arrived, most did not. In winter, they took Scrobicularia on mudflats, earthworms Lumbricidae and leatherjackets Tipulidae larvae in fields and Spisula and mussels along the coast. At other times of year they mainly ate Nereis. However, as they grew older, more birds began to specialise on mussels, especially in their second, third and fourth summers when the adults were away. At high water, most birds roosted at the mouth of the estuary or along the coast. However, several hundred fed in the fields in winter with more doing so late in the winter on warm days on Neap tides when less time was available for feeding on the estuary in daylight. Birds feeding on Nereis and Scrobicularia at low water were most often seen in the fields, irrespective of age. Birds feeding on Mytilus and Littorina occurred there rarely, but no bird eating Cerastoderma was seen there. The findings are discussed in relation to age differences in feeding skills and ability to compete successfully with other birds.

I explored the fitness implications of individual and sex differences in foraging strategy in the Eurasian oystercatcher Haematopus ostralegus by monitoring the survival of individually colour-ringed birds of known sex and known feeding specialisation. Over the period of this study, adult female annual and overwinter survival was significantly lower than adult male survival. However, contrary to previous findings, no differences in survival were found between birds of different feeding specialisations. Lower female survival was not due to sex differences in feeding specialisation. Possible mechanisms for sex differences in survival and the survival implications of different feeding specialisations are discussed. I conclude that sex differences in survival may be due to differences in social status. I also suggest that worm/clam feeders and mussel-stabbers, feeding specialisations previously associated with lower survival rates, may have benefited more than mussel-hammerers from milder winter temperatures in recent years.

Introductions of non-native species are seen as major threats to ecosystem function and biodiversity. However, invasions of aquatic habitats by non-native species are known to benefit generalist consumers that exhibit dietary switches and prey upon the exotic species in addition to or in preference to native ones. There is, however, little knowledge concerning the population-level implications of such dietary changes. Here, we show that the introduction of the Manila clam Tapes philippinarum into European coastal waters has presented the Eurasian oystercatcher Haematopus ostralegus ostralegus with a new food resource and resulted in a previously unknown predator–prey interaction between these species. We demonstrate, with an individuals-based simulation model, that the presence of this non-native shellfish, even at the current low density, has reduced the predicted over-winter mortality of oystercatchers at one recently invaded site. Further increases in clam population density are predicted to have even more pronounced effects on the density dependence of oystercatcher over-winter mortality. These results suggest that if the Manila clam were to spread around European coastal waters, a process which is likely to be facilitated by global warming, this could have considerable benefits for many shellfish-eating shorebird populations.

Assessments of whether disturbance is having a deleterious effect on populations have often measured behavioural responses to disturbance and assumed that populations with a larger behavioural response are more susceptible to disturbance. However, there is no guarantee that the behavioural response to disturbance is related to the population consequence, measured in terms of decreased reproduction or increased mortality. Individual-based models, consisting of fitness-maximizing individuals, are one means of linking the behavioural responses to disturbance to population consequences. This paper reviews how individual-based models have been used to predict the effect of disturbance on populations of shorebirds and wildfowl at several European sites, and shows how these models could be improved in the future by incorporating a range of alternative responses to disturbance.

Individual feeding specialisation in shorebirds is reviewed, and the possilble mechanisms involved in such specialisations. Any specialisation can he seen as an individual strategy, and the optimum strategy for any given individual will be conditional upon its specific priorities and constraints. Some specialisations are related to social status and some to individual skills. Some are also probably frequency-dependent. However, most shorebird specialisations are constrained to a large extent by individual morphology, particularly bill morphology. For example, larger birds are able to handle larger prey, and birds with longer bills are able to feed on more deeply buried prey. Sex differences in bill length are uncommon in the Charardriidae, which are surface peckers, but are common in the Scolopacidae, which feed by probing in soft substrates. Sex differences in bill morphology are frequently associated with sex differences in feeding specialisation. There is evidence that different feeding specialisations are associated with different payoffs, in which case the probability of failing to reproduce or of dying will not be distributed equally throughout the population. I consider the population consequences of such feeding specialisations, particularly the different risks and benefits associated with different habitats or diets. I also consider the way in which individuals may differ in their response to habitat loss or change. I suggest that population models designed to predict the effect of habitat loss or change on shorebirds should have the ability to investigate the differential response of certain sections of the population, particularly different ages or sexes, that specialise in different diets or feeding methods.

An individuals-based model, MORPH, was used to assess the quality of Poole Harbour, UK, for five overwintering shorebirds: dunlin Calidris alpina, redshank Tringa totanus, black-tailed godwit Limosa limosa, oystercatcher Haematopus ostralegus and curlew Numenius arquata. Site quality, and the effect of environmental change, was measured as predicted overwinter survival. Dunlin had the highest prey biomass densities and were the least likely to be affected by reductions in their food supply, lower temperatures or loss of terrestrial habitats. Black-tailed godwits and curlew had the lowest prey biomass densities and were the most likely to be affected by reductions in their food supply, lower temperatures and loss of terrestrial habitats. All five shorebird species were seriously affected by simulated sea-level rise. Conservation issues identified for the Poole Harbour SPA were the relatively low densities of larger size classes of polychaete worms, the importance of maintaining and managing surrounding terrestrial habitats and the effect of sea-level rise on the length of time for which intertidal food supplies are available.

Sex allocation theory predicts that parents may manipulate the sex ratio of their offspring in response to parental fitness or possible future fitness gains. In birds, family-level brood sex-ratio variation has been related to the external environment, the social environment, parental quality, and the timing of breeding. In this study of sex ratios in overwintering Oystercatchers, Haematopus ostralegus, in the British Isles, I show that neither the female bias in juvenile birds nor the change in sex ratio with age can be explained by differential migration. I also show that adult females have lower survival rates than males. If one sex is less numerous than the other, it is likely to have greater opportunities for breeding success. The results presented here indicate that Oystercatchers may adjust the sex ratio of their offspring in order to produce more of the rarer sex. This is the first evidence that birds may manipulate the sex ratio of their young in response to large-scale population processes.

It is increasingly important to be able to monitor and maintain the quality of estuaries for overwintering shorebirds. Bird numbers alone are not sufficient to indicate quality nor, as recent research shows, can it be assumed that site quality is being maintained simply because there is enough food to meet the birds’ physiological demands; i.e., the amount of food available per bird in autumn needs to be greater than the amount eaten. But how much greater? We used a simple individuals-based and behaviour-based model to explore the factors that affect the relationship between overwinter mortality and the amount of food available per bird in autumn (the food:bird ratio). The aims were to explore how the natural history characteristics of a shorebird species affected the shape of this relationship, in order to identify characteristics of the system that should be included in any monitoring programme of site quality. In all cases there was a minimum threshold food:bird ratio above which mortality did not vary and below which mortality increased steadily. The amount of food per bird required at the threshold varied from one to six times the amount actually consumed by each bird during winter, depending on the amount of realism included in the model. The minimum threshold ratio was most strongly influenced by the shape of the relationship between intake rate and prey density – the functional response – and, when interference competition occurred, by the distribution of the birds’ main food supply. Simulations with realistic spatial distributions of prey predicted that the minimum required amount of food per bird would be between two and five times the amount actually consumed. The threshold approach could provide a simple method of monitoring estuary quality based on food supply and bird numbers. It also provides a potentially clearly-defined and unambiguous measure of the carrying capacity of a site. If applied to a sufficient number of real estuaries, general guidelines for maintaining estuary quality based on estuary characteristics, food supply and bird species might be established.

A behaviour-based model was used to explore the effect of an extension of the port at Le Havre (Port 2000), and the effect of proposed mitigation measures, on the mortality and body condition of the three main shorebird species that overwinter in the estuary of the river Seine, France. In the model, a 20% reduction in the area of mudflats on the north side of the estuary had little effect on curlew Numenius arquata mortality and body condition but significantly increased mortality and decreased body condition in dunlin Calidris alpina and oystercatchers Haematopus ostralegus. Disturbance of feeding birds both day and night had a significant effect on the mortality and body condition of all three shorebird species, as did disturbance of roosting birds. Disturbance of feeding birds in the daytime only had a significant effect on dunlin mortality and body condition, but not that of curlew and oystercatchers. In the model, the creation of a buffer zone to reduce disturbance of feeding birds restored shorebird mortality and body condition to pre-disturbance levels. A new mudflat area was also effective in mitigating the effect of habitat loss on all three shorebirds and in mitigating the effect of roost disturbance on dunlin and curlew. However, a new mudflat area was not effective in mitigating the effect of roost disturbance on oystercatcher mortality and body condition. The effectiveness of the mitigating mudflat depended as much on its size as its quality. We believe that this is the first time that anyone has been able to forecast the efficacy of proposed mitigation measures.

Breeding territoriality in birds means that a certain proportion of the adult population may breed in suboptimal habitats or not breed at all. Non-breeding birds, or floaters, can have a `buffer effect' on breeding population size. Farmland bird declines in Britain are measured and analysed in terms of numbers of territory-holding birds. We use a population dynamics model to illustrate how, because of the buffer effect of floaters, there can be a time lag between the start of total population decline and detectable breeding population decline. Differences in the timing of declines of closely related farmland birds could therefore be due to slight differences in their population dynamics, rather than differences in their response to agricultural change.

Bottom cultivation of mussels on intertidal flats is practiced throughout the world. This often generates conflicts between commercial interests and competing birds such as oystercatchers. At the Menai Strait, United Kingdom, the overwinter consumption of 242 tonnes (1 metric tonne = 1000 kg) of commercially harvestable mussels (>40 mm) by oystercatchers in 1999–2000 was worth £133 000 ($226 000 U.S. dollars). This represents 19% of the value of the landings. We used a behavior-based simulation model to predict the extent to which such losses can be reduced by novel commercial management practices, and to explore the consequences for the oystercatcher population. Simulations of novel lay management practices indicated that the losses of commercially harvestable mussels to oystercatchers can be considerably reduced by altering the shore level and/or extent of the commercial lays. We propose a novel management strategy for the bottom cultivation of mussels in intertidal areas. Seed mussels (15–20 mm) should...

Capsule Distribution varied with age, sex and feeding method. Aims To investigate how shorebirds are differentially distributed on their overwintering grounds. Methods We analysed age and bill measurements of Oystercatchers captured on the Wash in the 1990s. Results Oystercatchers caught on the southwest shores were more likely to be young birds and females. Birds caught on the east shore were more likely to be adults and males. Shellfish-stabbers were more common on the south and east shores and shellfish-hammerers more common on the west shore. Most mudflat-feeding birds were caught on the south shore. Conclusions Phenotypic differences mean that juvenile and female Oystercatchers tend to specialize in different diets and feeding methods than adults and males. This results in age and sex differences in distribution. Understanding differences in distribution can be critical when trying to predict the effect of environmental change on a given population.

In a number of extensive coastal areas in northwest Europe, large numbers of long-lived migrant birds eat shellfish that are also commercially harvested. Competition between birds and people for this resource often leads to conflicts between commercial and conservation interests. One policy to prevent shellfishing from harming birds is to ensure that enough food remains after harvesting to meet most or all of their energy demands. Using simulations with behaviour-based models of five areas, we show here that even leaving enough shellfish to meet 100% of the birds' demands may fail to ensure that birds survive in good condition. Up to almost eight times this amount is needed to protect them from being harmed by the shellfishery, even when the birds can consume other kinds of non-harvested prey.

Forest malaria is a complex but common phenomenon occurring in southeast Asia. We studied its epidemiology through a prospective community-based study in central Vietnam. A total of 585 individuals were followed for two years by active case detection and biannual cross-sectional surveys. The prevalence of antibodies to Plasmodium falciparum was constantly about 20% across surveys and the incidence rate of clinical episodes of P. falciparum malaria was 0.11/person-year. Multivariate analysis showed that regular forest activity was the main risk factor for clinical malaria and malaria infections. Untreated bed nets had a significant protective effect (60%), except for people regularly sleeping in the forest. The population-attributable fraction for regular forest activity was estimated to be 53%. Our results confirm the major role played by forest activity on the malaria burden in this area and provide the basis for targeting control activities to forest workers. New interventions based on insecticide-treated materials need to be urgently evaluated.

Summary 1 The debate over the interaction between shellfishing and shorebirds is long-running. Behaviour-based models predict how animal populations are influenced by environmental change from the behavioural responses of individual animals to this change. These models are a potential tool for addressing shellfishery problems, but to be of value they must produce reliable predictions using data that are readily available or can be collected relatively quickly. 2 We parameterized a behaviour-based model for the oystercatcher population of the Wash, UK, for 1990–99 using data from shellfishery (mussels and cockles), shorebird and climate monitoring schemes. During the 1990s the overwinter mortality rates of Wash oystercatchers varied widely. The model correctly identified the years in which the observed overwinter mortality was either low (1–2%) or high (10–26%) from annual variation in the food supply, oystercatcher population size and temperature. 3 Many oystercatchers were observed and predicted to die when only a fraction of the available food was consumed. Within the model at least, this was because interference competition excluded the least dominant birds from part of the food supply and the least efficient foragers died before the food supply was fully depleted. A simplified model, which excluded interference and individual variation, incorrectly predicted that all birds survived in all years. Models that exclude these two components of behaviour will tend to underestimate the effect of mussel and cockle food shortage on oystercatchers. Shellfishery management based on such predictions may cause high oystercatcher mortality rates even though enough food would appear to be reserved for the birds. 4 Synthesis and applications. This study shows how a behaviour-based model can be parameterized and predict annual variation in oystercatcher mortality using data routinely collected from the Wash. The principle on which the model is based, that animals behave in order to maximize their chances of survival and reproduction, applies to any system, and the shellfishery, bird and climate data used to parameterize the model are widely available. The model can be used to advise how to manage shellfisheries, by predicting the proportion of the stock that needs to remain unfished in order to maintain low oystercatcher mortality rates.

The Burry inlet, South Wales, supports a licensed cockle Cerastoderma edule fishery and occasional mussel Mytilus edulis fishery. It is also an important overwintering ground for oystercatchers Haematopus ostralegus. In recent years mussels have settled over parts of some cockle beds, preventing cockle fishery there and leading to a request by shellfishers to remove this 'mussel crumble'. Conservation managers, however, were concerned that the mussel crumble might be providing a high-quality food source for the oystercatchers, making its removal detrimental to the birds. A behaviour-based model of oystercatcher feeding on cockles and mussels was parameterised for the inlet and its predictions tested against the distribution of birds across the shellfish beds and the amount of time they spent feeding. The model was then used to explore whether the birds were currently food-limited and what would be the effects on their mortality rate and body condition if the mussel crumble were to be removed, thereby re-exposing underlying cockle beds. The model predicted successfully the proportion of birds feeding on the different types of food and the number of hours birds spent feeding on neap tides. It was predicted that, at current bird population sizes, there would have to be a 50% reduction in shellfish stocks and the areas of shellfish beds from 2000-01 levels to cause noticeable extra emigration or mortality. A given area of mussel bed was predicted to be able to support more birds than the same area of cockle bed, but the greater area of the cockle beds meant that they were more important than mussels in determining the number of birds supported by the inlet. The simulated removal of mussel crumble to expose underlying cockles had no effect on predicted bird mortality and body condition at 2000-01 shellfish stock levels. However, there were circumstances under which the mussel crumble was predicted to increase the inlet's capacity to support birds, particularly when the area of existing cockle and mussel beds was substantially reduced.

To assess the long-term effects of human disturbance on birds, ways of predicting its impacts on individual fitness and population size must be found. In this paper we use a behaviour-based model to predict the impact of human disturbance on oystercatchers (Haematopus ostralegus) on their intertidal feeding grounds in the Exe estuary in winter. The model predicted that, for the same overall area disturbed, numerous small disturbances would be more damaging than fewer, larger disturbances. When the time and energy costs arising from disturbance were included, disturbance could be more damaging than permanent habitat loss. Preventing disturbance during late winter, when feeding conditions were harder, practically eliminated its predicted population consequences. Although disturbance can cause increased mortality, it was not predicted to do so at the levels currently occurring in the Exe estuary.

Summary 1 It is often difficult to quantify the separate contribution of exploitation and interference competition to the starvation of poor competitors yet, in vertebrates, it seems widely supposed that depletion is normally the mechanism underlying density-dependent starvation. This paper presents arguments that density-dependent starvation occurs through interference in wintering oystercatchers Haematopus ostralegus eating mussels Mytilus edulis without prey depletion having a significant influence. 2 Exclosure experiments and mussel bed surveys showed that oystercatchers (i) depleted mussel numerical density by up to 25% in their most preferred large prey size classes, but by only 12·1% overall; (ii) reduced mean mussel length by 1·5%; but (iii) had no detectable effect on the thickness of the shells and therefore the availability of mussels to the 60% of oystercatchers that opened mussels by breaking the shell. 3 Oystercatcher intake rate did not fall until prey biomass had decreased to very low densities. Depletion was not nearly large enough for mussel biomass to be reduced to this point. 4 The flesh content of individual mussels decreased from autumn to spring by 40–50% but oystercatchers did not increase their rate of foraging to compensate. Intake rate declined over the winter because of deteriorating mussel condition; very little was because of depletion. 5 Over the current range of population size, the density-dependent functions produced by a behaviour-based individual’s model were the same whether depletion was allowed to occur or not, confirming that depletion played no role in the density-dependent starvation. 6 At current population sizes, intake rate was reduced as population density increased only through increased interference, this leading to starvation late in winter when mussel flesh content was low. It was not just the subdominant birds most susceptible to interference that starved, but the least efficient ones as well, even though the density dependence was not due to exploitation competition.

A modelling approach is used to explore the effect of age and sex differences in oystercatcher (Haematopus ostralegus) winter mortality on population size, population structure and the population response to habitat loss or change. Increasing the mortality of first and second year birds reduced population size, but had very little effect on the proportion of the population that were adults. Increasing female mortality reduced population size and resulted in a male-biased population. A sex bias amongst birds of breeding age meant that there were fewer potential breeding pairs for a given population size, reducing the size of the breeding population and the breeding output. Increasing the mortality of one sex relative to the other reduced population size, even when mean adult mortality rates remained unchanged. Increasing the strength of density-dependent mortality in young birds caused a greater reduction in population size as habitat was lost. Increasing the strength of female density-dependent mortality had the same effect, even though male density-dependent mortality had been correspondingly reduced. Increasing density-independent or density-dependent winter mortality in one sex relative to another also exaggerated the disproportional effect of winter habitat loss on separate breeding subpopulations using the same overwintering area. These results suggest that any study of population dynamics should be aware of both age and sex differences in mortality. Conservationists should be particularly aware of any age or sex differences in diet or habitat use that may result in a differential response to environmental change.

Human interests often conflict with those of wildlife. In the coastal zone humans often exploit shellfish populations that would otherwise provide food for populations of shorebirds (Charadrii). There has been considerable debate on the consequences of shellfishing for the survival of shorebirds, and conversely the effects of shorebird predation on the shellfish stocks remaining for human exploitation. Until now, it has been difficult to determine the impact of current shellfishery practices on birdsor to investigate how possible alternative policies would affect their survival and numbers. One long-running contentious issue has been how to manage mussel Mytilus edulis and cockle Cerastoderma edule shellfisheries in a way that has least effect on a co-dependent shorebird, the oystercatcher Haematopus ostralegus, which also consumes these shellfish. This study used a behaviour-based model to explore the effects that the present-day management regimes of a mussel (Exe estuary, UK) and a cockle (Burryinlet, UK) fishery have on the survival and numbers of overwintering oystercatchers. It also explored how alternative regimes might affect the birds. The model includes depletion and disturbance as two possibly detrimental effects of shellfishing and some of the longer-term effects on shellfish stocks. Importantly, model birds respond to shellfishing in the same ways as real birds. They increase the time spent feeding at low tide and feed in fields and upshore areas at other times. When shellfishing removes the larger prey, birds eat more smaller prey. The results suggest that, currently, neither shellfishery causes oystercatcher mortality to be higher than it would otherwise be in the absence of shellfishing; at present intensities, shellfishing does not significantly affect the birds. However, they also show that changes in management practices, such as increasing fishing effort, reducing the minimum size of shellfish collected or increasing the daily quota, can greatly affect oystercatcher mortality and population size, and that the detrimental effect of shellfishing can be greatly increased by periods of cold weather or when prey are unusually scarce. By providing quantitative predictions of bird survival and numbers of a range of alternative shellfishery management regimes, the model can guide management policy in these and other estuaries.

Parasite loads vary among feeding sites in European oystercatchers, Haematopus ostralegus, hibernating in the Exe Estuary, in Great Britain. This variation might be the result of active or passive selection of non-parasitised mussels (Mytilus edulis) because they are the preferred food of the oystercatcher in the area and are also the intermediate hosts of one of the main parasites of the bird. Parasite loads (Renicola sp. and Psilostomum brevicolle) of the mussels were studied in relation to morphological parameters known to influence the choice of prey by the oystercatcher: length of mussel, meat content, and thickness of shell. Smaller mussels generally carry the lightest parasite loads, but the number of metacercariae of the two parasites potentially consumed in a day varies with feeding site and mussel size. Thus, an oystercatcher will ingest fewer metacercariae of Renicola sp. and P. brevicolle by selecting smaller mussels from bank 1, which is one of the least visited sites in the estuary. However, the birds will have to select mussels of 37 mm or more on banks 30 and 31, which are among the most used sites. We suggest that there is no active selection of non-parasitised mussels, but rather a kind of passive selection as the mussels chosen for their energetic value happen to be also the least parasitised.[Journal translation]

Measurements were made of Oystercatcher corpses to find the most reliable measure of body condition in live birds. Total body weight was as accurate as muscle thickness in assessing the weight of the pectoral muscles, a major store of protein reserves. Total body weight was also a good indicator of fat reserves. Wing-length was the most reliable measure of body size. Body condition in live birds was measured as an individual's residual weight derived from a general linear model of (loge) body weight against age, month, (loge) wing-length and time since capture. On the Exe estuary in autumn, there was no significant difference in body condition between same-age Oystercatchers of different diets and feeding methods. In the winter months, however, body condition differed significantly between mussel feeders using different feeding methods, and between mussel feeders and birds that fed on worms and clams. Differences between diet/feeding methods were not the same for each age group. Birds with the highest mean body condition index in adults were mussel hammerers, in immatures mussel stabbers and in juveniles, worm/clam feeders. No significant difference in body condition was found between the sexes in any age group in autumn. In winter, when birds with the same diet/feeding method were considered, females were significantly heavier than males. However, when birds of all diets and feeding methods were combined, weight differences between the sexes were not apparent. We examine reasons why birds with certain diets and feeding methods may be heavier than others in winter, and why females are sometimes heavier than males. We conclude that, in most cases, it is due to differences between birds in their ability to achieve a common target weight. However, we conclude that adult females try to achieve a higher target weight than males, probably as a premigratory and prebreeding strategy. In the light of this, we consider the consequences for adult females, in terms of reproductive success and survival, of specializing in diets and feeding methods associated with lower body weights in winter.

Summary 1. In order to assess the future impact of a proposed development or evaluate the cost effectiveness of proposed mitigating measures, ecologists must be able to provide accurate predictions under new environmental conditions. The difficulty with predicting to new circumstances is that often there is no way of knowing whether the empirical relationships upon which models are based will hold under the new conditions, and so predictions are of uncertain accuracy. 2. We present a model, based on the optimality approach of behavioural ecology, that is designed to overcome this problem. The model's central assumption is that each individual within a population always behaves in order to maximize its fitness. The model follows the optimal decisions of each individual within a population and predicts population mortality rate from the survival consequences of these decisions. Such behaviour-based models should provide a reliable means of predicting to new circumstances because, even if conditions change greatly, the basis of predictions – fitness maximization – will not. 3. The model was parameterized and tested for a shorebird, the oystercatcher Haematopus ostralegus. Development aimed to minimize the difference between predicted and observed overwinter starvation rates of juveniles, immatures and adults during the model calibration years of 1976–80. The model was tested by comparing its predicted starvation rates with the observed rates for another sample of years during 1980–91, when the oystercatcher population was larger than in the model calibration years. It predicted the observed density-dependent increase in mortality rate in these years, outside the conditions for which it was parameterized. 4. The predicted overwinter mortality rate was based on generally realistic behaviour of oystercatchers within the model population. The two submodels that predicted the interference-free intake rates and the numbers and densities of birds on the different mussel Mytilus edulis beds at low water did so with good precision. The model also predicted reasonably well (i) the stage of the winter at which the birds starved; (ii) the relative mass of birds using different feeding methods; (iii) the number of minutes birds spent feeding on mussels at low water during both the night and day; and (iv) the dates at which birds supplemented their low tide intake of mussels by also feeding on supplementary prey in fields while mussel beds were unavailable over the high water period. 5. A sensitivity analysis showed that the model's predictive ability depended on virtually all of its parameters. However, the importance of different parameters varied considerably. In particular, variation in gross energetic parameters had a greater influence on predictions than variations in behavioural parameters. In accord with this, much of the model's predictive power was retained when a detailed foraging submodel was replaced with a simple functional response relating intake rate to mussel biomass. The behavioural parameters were not irrelevant, however, as these were the basis of predictions. 6. Although we applied the model to oystercatchers, the general principle on which it is based applies widely. We list the key parameters that need to be measured in order to apply the model to other systems, estimate the time scales involved and describe the types of environmental changes that can be modelled. For example, in the case of estuaries, the model can be used to predict the impact of habitat loss, changes in the intensity or method of shellfishing, or changes in the frequency of human disturbance. 7. We conclude that behaviour-based models provide a good basis for predicting how demographic parameters, and thus population size, would be affected by novel environments. The key reason for this is that, by being based on optimal decision rules, animals in these models are likely to respond to environmental changes in the same way as real ones would.

Patellid limpets are very important grazers on NE Atlantic rocky shores, and can occur in clumps or as solitary individuals. When oystercatchers are foraging for limpets on rocky shores, they are often presented with a choice between solitary and clumped limpets. We attempted to establish whether oystercatchers express a preference for limpet prey items from these different distributions. Oystercatchers spent far more time foraging in areas of less clumped limpets than in areas of clumped limpets, despite lower prey density. We found a strong preference for solitary prey items, and increased chances of successful predation of solitary limpets. Possible explanations for this choice are discussed. This preference for attacking solitary limpets may have important implications for rocky shore dynamics by shaping the spatial patterns of limpets. This may then influence the probability of Fucus germlings escaping limpet grazing.

Arabidopsis RPS2 is a typical nucleotide-binding leucine-rich repeat resistance protein, which indirectly recognizes the bacterial effector protein AvrRpt2 and thereby activates effector-triggered immunity (ETI). Previously, we identified two hypersensitive induced reaction (AtHIR) proteins, AtHIR1 (At1g09840) and AtHIR2 (At3g01290), as potential RPS2 complex components. AtHIR proteins contain the stomatin/prohibitin/flotillin/HflK/C domain (also known as the prohibitin domain or band 7 domain). In this study, we confirmed that AtHIR1 and AtHIR2 form complexes with RPS2 in Arabidopsis and Nicotiana benthamiana using a pulldown assay and fluorescence resonance energy transfer (FRET) analysis. Arabidopsis has four HIR family genes (AtHIR1–4). All AtHIR proteins could form homo- and hetero-oligomers in vivo and were enriched in membrane microdomains of the plasma membrane. The mRNA levels of all except AtHIR4 were significantly induced by microbe-associated molecular patterns, such as the bacterial flagellin fragment flg22. Athir2-1 and Athir3-1 mutants allowed more growth of Pto DC3000 AvrRpt2, but not Pto DC3000, indicating that these mutations reduce RPS2-mediated ETI but do not affect basal resistance to the virulent strain. Overexpression of AtHIR1 and AtHIR2 reduced growth of Pto DC3000. Taken together, the results show that the AtHIR proteins are physically associated with RPS2, are localized in membrane microdomains, and quantitatively contribute to RPS2-mediated ETI.

Spatial variations in the availability and quality of the mussel Mytilus edulis food supply of Oystercatchers Haematopus ostralegus on the Exe estuary, England, are described. Oystercatchers open mussels by stabbing into gaping mussels (or prising open closed ones) or by hammering a hole in either the dorsal or ventral shells. Spatial variations in the food supply are considered at four scales. In decreasing order of size, these are (i) whole mussel beds, (ii) zones within a mussel bed, (iii) different places within one zone, and (iv) different places within one clump of mussels. The first two scales are clearly related to exposure time. Both between and within the 12 main mussel beds of the estuary, most upshore mussels are up to 10% less likely than downshore mussels to be hidden under mud. However, upshore mussels of a given length contain less flesh, have thicker ventral shells and, except on high-level beds subject to wave erosion, have thicker dorsal shells than downshore mussels. Mussels at the top of the shore also contain the highest infestations of the helminth parasite of Oystercatchers, Psilostomum brevicolle . At a particular shore level, mussels of a given length have less flesh and thicker shells, though only on the dorsal side, in areas of high mussel density. Within one clump, mussels of a given length with thick dorsal shells have more flesh than those with thin shells. In contrast, flesh content is slightly higher in mussels that are thin on the ventral side. Flesh content and shell thickness on both sides are unaffected by whether a mussel is visible at the edge of a clump or hidden inside. Simulations with a model of foraging Oystercatchers suggested that variations in mussel availability (visibility and shell thickness) and flesh content at all four scales could sometimes have an important influence on intake rate. Most published values of intake rates of Oystercatchers eating surface-dwelling mussels may be biassed (and often considerably over-estimated) by the erroneous assumption that prey of average flesh content are taken.

The impact of potentially toxic chemicals on wildlife is commonly assessed by comparing the intake of the contaminant with the “no observable effects level” (NOAEL) of intake. It is known, however, that there are considerable uncertainties inherent in this method. This study presents a Monte-Carlo based model to assess the degree of risk posed to birds (dunlin, Calidris alpina) from important estuarine habitats, and to show the limitations of such risk assessments, particularly with regard to data availability. The model was applied to predict the uptake of metals (Hg, Pb) in this shorebird species in Poole Harbour and the Severn Estuary/Bristol Channel, UK, two internationally important shorebird habitats. The results show that in both areas, Pb and Hg concentrations may pose an ecologically relevant toxic risk to wading birds. For Pb, uncertainty in NOAEL values dominates the overall uncertainty. Use of lethal toxicity data (LD50/100) was investigated as a method for assessing sub-lethal impacts from Hg. It was found that this method led to a significant under-estimate of the potential impact of Hg contamination, compared with direct estimation of NOAEL.

Conservation managers need to be able to assess and prioritize issues that may affect their target habitats and species. In the Baie de Somme, France, conservation issues affecting overwintering shorebirds include hunting pressure, cockle fishing, recreational disturbance, Spartina encroachment, and changing sediment levels. We used an individual-based model to predict the effect of these issues on the survival of three shorebird species: dunlin Calidris alpina, oystercatcher Haematopus ostralegus and curlew Numenius arquata. In the model, removing hunting from the mudflats in the eastern part of the estuary had the greatest positive effect on shorebird survival. Oystercatcher survival decreased markedly when stocks of large cockles were reduced to ,250 m -2 or numbers of fishermen per day were doubled. Short-term disturbance events, such as walkers, had more effect on shorebird survival than long- term events, such as fishermen. Dunlin, as a protected species, were able to feed outside the Reserve Naturelle and were unaffected by disturbance within the Reserve. Oys- tercatcher survival decreased when the number of distur- bance events within the Reserve exceeded one h -1 , and curlew survival when disturbance events exceeded six h -1 . Spartina encroachment caused dunlin survival to decline steadily as feeding habitat was lost. Dunlin were also the species most affected by changes in sediment levels, likely to occur through either sedimentation or sea level rise.

Droppings of Dunlin Calidris alpina and Grey Plover Pluvialis squatarola were collected on the Wash, south-east England, and examined for remains of prey species taken. Dropping analysis was found to be useful in describing the diet of these two waders. In addition to confirming known prey species, previously unrecorded prey were discovered: in the case of Dunlin, worms -of the families Spionidae and Phyllodocidae and in the case of Grey Plover, the amphipod crustacean Corophium volutator. Both Dunlin and Grey Plover were found to be taking the opisthobranch mollusc Retusa obtusa¸, a prey species which has not been recorded in the diet of waders before.

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type 11 ('disc equation') formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching. A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (< 150/m(-2)). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote. A multivariate analysis of 468 'spot' estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81 % of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3 %), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93 % of cases. We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested. As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.

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.

In a previous paper (Goss-Custard et al. 1995), a demographic model suggested that different breeding subpopulations of a migrating shorebird, the oystercatcher Haematopus ostralegus, that shared the same wintering area might be affected in different ways by the progressive reduction of their winter habitat. Initially, as winter habitat was removed in simulations, all subpopulations decreased in parallel. However, as habitat loss continued, the subpopulations with lower fledgling production began to be disproportionately affected. This paper explores what caused this difference in response between subpopulations. A simplified version of the model was used in which two or four subpopulations wintered in one area. Two simple density-dependent functions were included, one for breeding territoriality and one for winter mortality, with density dependence starting only above a certain threshold density. Each pair that acquired a breeding territory reared a constant number of fledged young each year. As winter habitat was reduced, the reduction in population size of the subpopulations with lower reproductive rates began to accelerate as soon as the point was reached when all adults were breeding. Before this point, breeding birds that had died in the winter had been replaced by non-breeders. After this point, the subpopulation reproductive output was not sufficient to replace birds dying in the winter. At the same time, winter mortality rates remained high as winter densities were sustained by birds from the other subpopulations. The results suggest that, in order to detect the first effects of winter habitat loss, conservationists should pay special attention to monitoring breeding numbers in local subpopulations with below average rates of reproduction. The results also suggest that the cause of a decline in some local breeding subpopulations, when others remain stable, may need to be sought on the wintering grounds and not in the breeding localities themselves.

1. We provide evidence that interference between overwintering oystercatchers Haematopus ostralegus feeding on mussels Mytilus edulis is absent or has only a negligible effect on intake rate at low competitor densities, and only reduces intake rate above a threshold bird density. The threshold for interference did not differ significantly between individual birds and showed no association with their local dominance. The threshold value was c. 50 birds ha -1 for birds that opened prey by hammering and 150 birds ha -1 for those that stabbed into mussels. 2. The interference-free intake rate at densities below the threshold for interference differed significantly between individual birds but was not associated with local dominance; nor did it vary either during the tidal exposure period or between autumn and winter. 3. At competitor densities above the threshold for interference, birds of lower local dominance were more susceptible to interference than those of higher dominance, the magnitude of the difference being smaller in hammering birds than in stabbers. For birds of a given local dominance, interference was most intense towards the end of winter and, in hammerers, at the beginning of the exposure period. 4. We conclude that theoretical models in which interference operates as soon as competitor density starts to increase provide a poor description of this system. Rather, our data support those models in which interference begins only after density has reached a particular, and constant, level.

1. The basic structure is described of a game theory model of the distribution of a wintering shorebird, the oystercatcher Haematopus ostralegus, foraging on 12 mussel Mytilus edulis beds whose quality as feeding areas for the birds differed. Model parameters were derived from field studies made over 15 years. A companion paper tests model predictions and illustrates how the model can be used to derive survival curves and predict estuary carrying capacity. 2. Mussel-bed quality was measured as the intake rate achieved by an oystercatcher of average competance foraging at random over the bed, and varied threefold across beds. Each individual in the model was given its own foraging efficiency, drawn at random from a normal distribution whose mean and variance depended on the bird's age and feeding method. Its susceptibility to interference from other foraging birds was calculated from its local dominance score, the percentage of wins it had over the other individuals currently on the same bed. A bird's local dominance score on a particular mussel bed was calculated from the proportion of birds on that bed having a lower global dominance rank, a measure of a bird's competitive ability relative to those of all other birds on the estuary. Across individuals, foraging efficiency and susceptibility to interference were unrelated. 3. In every iteration, the model calculated a bird's putative intake rate on each bed by subtracting the reduction in intake rate due to interference from the intake rate the bird could achieve there in the absence of interference. Each individual could then move to the mussel bed which, at that time, gave it the highest intake rate, provided the differences were sufficiently large (3%) for it to discriminate. 4. The main model limitations were the absence of: (i) a real-time base, so time-lags due to learning were not included; (ii) the energy costs associated with moving between beds; (iii) the opportunity for birds to increase their dominance score or foraging efficiency through familiarity with a mussel bed; (iv) the opportunity for individuals to control the amount of competition to which they are exposed by selecting particular times in the tidal cycle at which to feed; and (v) factors known to affect bed attractiveness to oystercatchers, such as the consistency of the substrate. None the less, the model was regarded as a suitable starting point for exploring how the proportion failing to acquire enough food, and either emigrating or starving, is affected by population size

1. Previous papers have related individual differences in diet and feeding method in oystercatchers on the Exe estuary to age. In this paper they are also related to sex. 2. Oystercatchers caught and ringed on the Exe estuary were sexed by discriminant analysis. 3. Bill tip shape was measured and related to diet and feeding method. Birds with pointed bill tips were mudflat feeders, birds with chisel-shaped bill tips were mussel stabbers and birds with blunt bill tips were mussel hammerers. 4. Whilst in birds with chisel-shaped bill tips the sex ratio was 50:50, over 70% of birds with pointed bill tips were females and nearly 90% of birds with blunt bill tips were males

(1) A winter survey of seven species of wading birds (Charadrii) at forty intertidal sites in six estuaries in south-west England was made to identify the variables that determined the variation in bird densities between the sites and to develop a method for predicting bird densities should a tidal power barrage be built on the Severn estuary. (2) The densities of the two smallest waders, the dunlin (Calidris alpina) and redshank (Tringa totanus), were comparable on the twelve sites in the Severn to those in the other estuaries. In contrast, the densities of the larger species, the grey plover (Pluvialis squatarola), bar-tailed and black-tailed godwits (Limosa lapponica and L. limosa), curlew (Numenius arquata) and oystercatcher (Haematopus ostragleus), were comparatively low on sites in the Severn. (3) Within most estuaries, bird densities correlated with the densities of one to three widely taken prey species. When data from all estuaries were combined, bird densities correlated with the densities of one or two prey species, the polychaete worm Nereis diversicolor providing the best correlation in five cases. The densities of the larger birds were correlated with the densities of their larger-sized prey. Allowing for the effects of prey density, sediment parameters correlated additionally with the density of bar-tailed godwit and redshank and exposure-time correlated with the density of oystercatcher. (4) When the effects of these variables were taken into account, bird densities on sites on the Severn were only significantly different from those on other estuaries for one or two species. Therefore, the low densities of larger wader species on the Severn can be explained by the low densities there of the larger-sized polychaete worms and bivalve molluscs upon which they feed: there was no reason to invoke a special unidentified factor in the Severn to account for the low densities of these species. It was concluded that other estuaries provide an analogue with which to predict post-barrage bird densities on the Severn from predicted densities of their prey. (5) By holding back the ebbing tide, a barrage would substantially reduce the area of intertidal flats available at low water for the birds to feed. On the other hand, the productivity of the estuary, and thus the invertebrate density, could increase in the generally more benign post-barrage environmental conditions. Wader densities would have to increase approximately twofold to allow the same overall numbers of birds to remain post-barrage as occur on the Severn at present. Provisional estimates are given of the increases in prey density required to allow bird densities to increase by this amount. With the -exception of the prey of dunlin, the required values fall well within the ranges of densities found in other estuaries, and so could in principle be attained in the post-barrage Severn. (6) An attempt was made to derive equations with which to predict post-barrage densities of invertebrates from easily measured, static environmental variables whose post-barrage values could themselves be predicted. Although the densities of six of the ten most important prey categories correlated with static environmental variables, such as the particle-size and organic content of the sediment, the fact that a site was in the Severn had a significant additional effect on invertebrate density in seven cases. This suggests that there is a special feature of the Severn, probably one associated with its highly dynamic nature, whose effect was not captured by measuring static variables. This factor must be identified, and the effect of a barrage upon it evaluated, if the post-barrage densities of invertebrates, and thus of the birds, are to be successfully predicted.

By comparing the shells of those mussels Mytilus edulis that had been opened by oystercatchers Haematopus ostralegus with mussels of similar size that had not been opened, it was shown that oystercatchers that break into their prey by hammering a hole in the shell selected between prey within a size-class. Ventral hammerers selected mussels that were relatively thin on the ventral surface, were brown in colour and carried few barnacles. Dorsal hammerers selected eroded mussels with thin dorsal shells. Stabbing oystercatchers did not select for thin-shelled prey. In conjunction with the great individual variation in flesh content between mussels of the same length, prey size in this case can be only a poor predictor of prey profitability.

Oystercatchers feeding on mussels foraged in small, overlapping areas and aggressive encounters were common amongst all birds irrespective of their method of opening mussels. Individuals varied consistently in aggressiveness. The more aggressive birds were more frequently present on the mussel bed, fed in those parts where food was most abundant, and were more successful in stealing mussels from other birds. Only the more aggressive birds stole more mussels than they had stolen from them, and stealing increased their rate of food intake.