John D. Goss-Custard

Modelling interference from basic foraging behaviour

1. We develop an individuals-based model that predicts the strength of interference between foraging animals from basic elements of their behaviour. The model is based on the same principles as previous behaviour-based interference models, but extends and adds further realism to these models. One key difference is that in our model the responses of animals to competitors are not fixed, as is assumed in previous models. Instead, animals use optimal decision rules to determine responses which maximize their intake rate. 2. The general shape of interference function generated by the model is similar to that predicted by previous behaviour-based models. Interference is insignificant at low competitor densities, but steadily increases in intensity as density rises. However, comparison with the observed level of interference between oystercatchers, Haematopus ostralegus, feeding on mussels, Mytilus edulis, shows that the models predictive power is substantially increased through the addition of optimal decision rules. When animals have a fixed response to encounters, too much interference occurs because dominant animals waste time avoiding subdominants and subdominants waste time attempting, but failing, to steal prey from dominants. When animals use optimal decision rules, only subdominants avoid, and only dominants initiate attacks. Interference is therefore reduced and is much closer to that observed. 3. The conditions under which optimal decision rules will lead to interference are described in terms of basic elements of foraging behaviour. Interference is predicted to occur when handling time and the probability of winning fights are high, and when prey encounter rate and the duration of fights are low. These parameters are used to predict successfully the presence or absence of interference in a range of shorebird-prey systems. 4. We suggest that behaviour-based interference models will need to incorporate optimal decision rules if they are to predict accurately the strength of interference observed in real predator-prey systems.

Sediment characteristics, invertebrate densities and shorebird densities on the inner banks of the Wash

This paper tests the possibility that, primarily through their effect on invertebrate prey densities, sediment characteristics can be used to predict the densities of shorebirds (Charadrii), on the inner banks of the Wash, east England. 192 quadrats were established with, on most shores, a transect of nine quadrats from the marsh edge to the Low Water Mark (LWM). Shorebirds were counted in winter in each quadrat. The densities of the main invertebrate prey species were sampled in early winter/late autumn at the mid-point of each quadrat, together with the sediment particle size and organic content. The time for which each quadrat was inundated over high water spring tides was also measured

Deriving population parameters from individual variations in foraging behaviour. I: Empirical game theory distribution model of oystercatchers Haematopus ostralegus feeding on mussels Mytilus edulis

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 birds 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 birds 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 birds 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 birds 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

Predicting the impacts of disturbance on shorebird mortality using a behaviour-based model

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.

Carrying capacity in overwintering migratory birds

The carrying capacity of the food supply in a non-breeding site for migrating birds can be measured either as the maximum number of bird-days that it can support or as the maximum numbers that can survive the non-breeding season. We identify the circumstances when it is appropriate to measure carrying capacity in either way, but we then show that birds may often emigrate or starve well before carrying capacity is reached, however it is defined. It is therefore unsafe to argue that, because the carrying capacity would not be reached, a change in site management would not affect birds. The key question in environmental impact studies of this kind is whether a change in site management would decrease the survival rate or the proportion of birds accumulating the body reserves required to migrate to the breeding grounds.

Individual‐based ecology of coastal birds

Conservation objectives for non‐breeding coastal birds (shorebirds and wildfowl) are determined from their population size at coastal sites. To advise coastal managers, models must predict quantitatively the effects of environmental change on population size or the demographic rates (mortality and reproduction) that determine it. As habitat association models and depletion models are not able to do this, we developed an approach that has produced such predictions thereby enabling policy makers to make evidence‐based decisions. Our conceptual framework is individual‐based ecology, in which populations are viewed as having properties (e.g. size) that arise from the traits (e.g. behaviour, physiology) and interactions of their constituent individuals. The link between individuals and populations is made through individual‐based models (IBMs) that follow the fitness‐maximising decisions of individuals and predict population‐level consequences (e.g. mortality rate) from the fates of these individuals. Our first IBM was for oystercatchers Haematopus ostralegus and accurately predicted their density‐dependent mortality. Subsequently, IBMs were developed for several shorebird and wildfowl species at several European sites, and were shown to predict accurately overwinter mortality, and the foraging behaviour from which predictions are derived. They have been used to predict the effect on survival in coastal birds of sea level rise, habitat loss, wind farm development, shellfishing and human disturbance. This review emphasises the wider applicability of the approach, and identifies other systems to which it could be applied. We view the IBM approach as a very useful contribution to the general problem of how to advance ecology to the point where we can routinely make meaningful predictions of how populations respond to environmental change.

Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

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 II (‘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.

Shape of the interference function in a foraging vertebrate

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.

Towards predicting wading bird densities from predicted prey densities in a post-barrage Severn Estuary

(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.

Sex-related differences in diet and feeding method in the oxystercatcher Haematopus ostralegus

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