Kevin Wood
As part of WWT’s Conservation Evidence team, I’m involved in research that provides the evidence needed to inform the conservation actions of WWT and our partners. My current areas of research include improving our understanding of the benefits of wetlands for nature and people, including their storage of carbon, and their provision of socio-cultural services. I have worked extensively on diagnosing the demographic and environmental causes of population declines in threated wetland species.
If you would like pdfs of my publications please see the 'Papers' tab to the left or email me at [email protected]
Address: Wildfowl & Wetlands Trust
Slimbridge
Gloucestershire
GL2 7BT
United Kingdom
If you would like pdfs of my publications please see the 'Papers' tab to the left or email me at [email protected]
Address: Wildfowl & Wetlands Trust
Slimbridge
Gloucestershire
GL2 7BT
United Kingdom
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Papers by Kevin Wood
2. The database contains estimates of the following vital rates: first-year survival; second-year survival; adult annual survival; first breeding (both age-specific recruitment probability, and breeding propensity across potential recruitment ages); breeding propensity of established female breeders; clutch size; hatching success; and fledging success. These estimates are drawn from 134 studies, across the scientific and grey literature, including three previously inaccessible datasets on clutch size that were contributed in response to a call for data through the IUCN Species Survival Commission's Duck Specialist Group.
3. Although clutch size has been much studied, the contributed datasets have enhanced coverage of studies reported in non-English languages, which were otherwise only represented when cited in English-language publications. Breeding propensity has been little studied, perhaps because adult females are often assumed to attempt breeding every year; we obtained a mean breeding propensity of 0.72. Our synthesis highlights the following gaps in data availability: juvenile and male survival; population change; and studies from Russia (at least accessible in English).
4. The database is intended to serve population modellers and scientists involved in the policy and practice of seaduck conservation and management.
A more equitable publishing system is needed. Platinum and diamond open access (see L. Barnes Open Book Publishers https://doi.org/g3tb; 2018), financed by a third party such as a scientific society, avoid article-processing charges (APCs) for authors and paywalls for readers, and can offer the lowest-cost option for eNGOs. Alternatively, journals could offer APC waivers for authors at eNGOs.
Discussions at this year’s United Nations biodiversity conference (COP15) and climate-change conference (COP26) are informed by eNGO research. Mandatory APCs risk pricing eNGOs out of scientific publishing at a time when their research output is most urgently needed.
Methods: In this study, we used data from individual birds captured over a 57-year period to assess the extent, and temporal variability in male bias in nine populations of ducks wintering in the United Kingdom: Gadwall (Mareca strepera), Northern Mallard (Anas platyrhynchos), Northern Pintail (Anas acuta), Common Pochard (Aythya ferina), Common Shelduck (Tadorna tadorna), Northern Shoveler (Spatula clypeata), Eurasian Teal (Anas crecca), Tufted Duck (Aythya fuligula), and Eurasian Wigeon (Mareca penelope).
Results: Overall, eight of these populations were significantly male-biased and adults were more male-biased than first-winter juveniles for all nine populations. The increased male bias among adults is consistent with the hypothesis that factors such as higher mortality of reproductive-age females during the breeding season is a major cause of male bias in duck populations. However, such predation cannot explain the male bias detected in first-winter juveniles in four of the populations. The temporal trends in male bias differed between adults and first-winter juveniles in Northern Mallard, Northern Pintail, Common Pochard, Common Shelduck, Eurasian Teal, Tufted Duck, and Eurasian Wigeon. Over the study period we found increased male bias among adult Northern Mallard, Northern Pintail, Common Pochard, Common Shelduck, and Tufted Duck as well as both adult and first-winter juvenile Northern Shoveler.
Conclusions: We provide evidence that among wintering duck populations, sex ratios are typically male-biased, with adults exhibiting stronger male-biased sex ratios than first-winter juveniles. Improved monitoring of sex ratios of wintering waterbirds would help to increase our understanding of changes in waterbird demography, population structure, and observed population trends; our study shows that birds caught during ringing projects can be a valuable source of such data.
anser marked at Windermere, Cumbria, in summers 2013–16 are used to describe the patterns
of their moult migration. Results show that birds moulting at Windermere are migrating mainly
from the nearby counties of Lancashire, North Yorkshire and West Yorkshire, and from within
Cumbria itself. Resightings at Windermere showed that the number of individuals returning to
moult decreased during the study: possible reasons for this are given. We also provide new and
updated information on the survival and mean dispersal distance for non-breeding British
Greylag Geese. The mean dispersal distance away from Windermere for all marked individuals
was 83.3 km (95% CI 73.4–93.2). Annual mean survival rates ranged between 0.568 and 0.872
over the study period, with a mean of 0.680 (95% CI 0.584–0.775). Results from this study
contribute to improving our knowledge of the demography of the British Greylag Goose
population.
2. The database contains estimates of the following vital rates: first-year survival; second-year survival; adult annual survival; first breeding (both age-specific recruitment probability, and breeding propensity across potential recruitment ages); breeding propensity of established female breeders; clutch size; hatching success; and fledging success. These estimates are drawn from 134 studies, across the scientific and grey literature, including three previously inaccessible datasets on clutch size that were contributed in response to a call for data through the IUCN Species Survival Commission's Duck Specialist Group.
3. Although clutch size has been much studied, the contributed datasets have enhanced coverage of studies reported in non-English languages, which were otherwise only represented when cited in English-language publications. Breeding propensity has been little studied, perhaps because adult females are often assumed to attempt breeding every year; we obtained a mean breeding propensity of 0.72. Our synthesis highlights the following gaps in data availability: juvenile and male survival; population change; and studies from Russia (at least accessible in English).
4. The database is intended to serve population modellers and scientists involved in the policy and practice of seaduck conservation and management.
A more equitable publishing system is needed. Platinum and diamond open access (see L. Barnes Open Book Publishers https://doi.org/g3tb; 2018), financed by a third party such as a scientific society, avoid article-processing charges (APCs) for authors and paywalls for readers, and can offer the lowest-cost option for eNGOs. Alternatively, journals could offer APC waivers for authors at eNGOs.
Discussions at this year’s United Nations biodiversity conference (COP15) and climate-change conference (COP26) are informed by eNGO research. Mandatory APCs risk pricing eNGOs out of scientific publishing at a time when their research output is most urgently needed.
Methods: In this study, we used data from individual birds captured over a 57-year period to assess the extent, and temporal variability in male bias in nine populations of ducks wintering in the United Kingdom: Gadwall (Mareca strepera), Northern Mallard (Anas platyrhynchos), Northern Pintail (Anas acuta), Common Pochard (Aythya ferina), Common Shelduck (Tadorna tadorna), Northern Shoveler (Spatula clypeata), Eurasian Teal (Anas crecca), Tufted Duck (Aythya fuligula), and Eurasian Wigeon (Mareca penelope).
Results: Overall, eight of these populations were significantly male-biased and adults were more male-biased than first-winter juveniles for all nine populations. The increased male bias among adults is consistent with the hypothesis that factors such as higher mortality of reproductive-age females during the breeding season is a major cause of male bias in duck populations. However, such predation cannot explain the male bias detected in first-winter juveniles in four of the populations. The temporal trends in male bias differed between adults and first-winter juveniles in Northern Mallard, Northern Pintail, Common Pochard, Common Shelduck, Eurasian Teal, Tufted Duck, and Eurasian Wigeon. Over the study period we found increased male bias among adult Northern Mallard, Northern Pintail, Common Pochard, Common Shelduck, and Tufted Duck as well as both adult and first-winter juvenile Northern Shoveler.
Conclusions: We provide evidence that among wintering duck populations, sex ratios are typically male-biased, with adults exhibiting stronger male-biased sex ratios than first-winter juveniles. Improved monitoring of sex ratios of wintering waterbirds would help to increase our understanding of changes in waterbird demography, population structure, and observed population trends; our study shows that birds caught during ringing projects can be a valuable source of such data.
anser marked at Windermere, Cumbria, in summers 2013–16 are used to describe the patterns
of their moult migration. Results show that birds moulting at Windermere are migrating mainly
from the nearby counties of Lancashire, North Yorkshire and West Yorkshire, and from within
Cumbria itself. Resightings at Windermere showed that the number of individuals returning to
moult decreased during the study: possible reasons for this are given. We also provide new and
updated information on the survival and mean dispersal distance for non-breeding British
Greylag Geese. The mean dispersal distance away from Windermere for all marked individuals
was 83.3 km (95% CI 73.4–93.2). Annual mean survival rates ranged between 0.568 and 0.872
over the study period, with a mean of 0.680 (95% CI 0.584–0.775). Results from this study
contribute to improving our knowledge of the demography of the British Greylag Goose
population.
Based on a meta-analysis of previous waterfowl grazing studies I show that waterfowl biomass density (kg ha-1) rather than individual density (ind. ha-1) is a better predictor of reductions in plant standing crop. Most studies to date have analysed such reductions using only individual densities, despite large between-taxa variation in waterfowl body mass, diet and intake rates.
I quantified the abundance, species richness, evenness, flowering and dominance of the chalk river aquatic plant community in relation to biotic and abiotic factors during the growth-, peak-, and recession-phases of the growth cycle. The relative importance of herbivory, riparian shading, water temperature and distance downstream varied between different phases of the plant growth cycle, highlighting the importance of seasonal patterns in regulation of plant community structure.
The River Frome swan population varied seasonally, being highest in the winter. The population was dominated by non-breeding adults and juveniles that lived in flocks. These flocks exhibited strong seasonal habitat switches between terrestrial pasture in winter and spring, and river in summer and autumn. I provided evidence that this switch was linked to the seasonal decrease in water velocity between spring and summer, which reduced the metabolic costs of river feeding and increased the relative profitability of aquatic food resources.
I used a mathematical population model and an individual-based behavioural model respectively to explore two management options for the alleviation of the swan grazing conflict in chalk rivers: population control and habitat alterations. Population control measures, such as clutch manipulations, fertility control, culling or translocations, were predicted to be unsuccessful except at impractically high levels of management effort, due to the effects of immigration and high survival rates in offsetting removed eggs or individuals. Habitat alterations, in particular the narrowing of river channels to cause a local increase in water velocity and thus swan foraging costs, are more promising management options as they require lower management effort, are less ethically controversial, and address the fundamental reason why swans select their food resources, the rate of net energy gain (‘profitability’).
The calibrated version of our model predicted that a mean (±95% CI) of 874 (± 10) individual Bewick’s Swans would be killed over the course of the simulation (i.e. between September and the following June) in collisions with energy infrastructure across the winter and staging grounds in Europe. This mean predicted mortality represented 4.3% of the total wintering northwest European Bewick’s Swan population. The predicted mortality caused by wind farms and power lines therefore represents approximately 19% of the total annual mortality of the northwest European Bewick’s Swan population, which is in line with the findings of earlier studies that collisions with energy infrastructure represent one of the major causes of death among swans.
In the absence of differing avoidance rate estimates for swans encountering wind turbines compared with power lines, we did not separate avoidance rates for these structures in the models. On this basis, the total predicted mortality was comprised of 376 (± 7) individuals killed in collisions with onshore turbines, 193 (± 6) with offshore turbines, and 305 (± 5) with power lines. The model also predicted considerable variation in the mortality associated with each country. In particular, the Netherlands and Germany accounted for 43% and 33%, respectively, of the total deaths due to onshore turbines, whilst the waters around Germany were also associated with 69% of the deaths caused by offshore turbines. In contrast, the mortality associated with power lines was more broadly distributed, with no single country accounting for more than 24% (Russia) of the total number killed by power lines.
The mortality predictions of our model were highly sensitive to variations in the avoidance rate used, as even a small increase in the avoidance rate from 0.998 to 0.999 decreased the predicted mean mortality from 874 to 445 individuals. The high sensitivity of collision estimates to the avoidance rate highlights the need for accurate, precise measurements of this parameter for collision risk assessments.
Our report illustrates how avian telemetry data can be combined with a simulation-modelling approach to assess the cumulative collision mortality of an entire population at a large spatial scale.
Here, in a study funded by Ørsted, we developed an individual-based model (IBM) of the UK’s overwintering population of Pink-footed Geese (Anser brachyrhynchus) in order to predict the cumulative mortality each winter due to collisions with onshore and offshore wind turbines and overhead power lines. IBMs are spatially- and temporally-explicit models that simulate the interactions between individuals and their environment, informed by the behaviour of their real-world counterparts. Our model was informed by information on the movements and flight heights of 73 geese fitted with GPS-GSM tags, together with census data on the total numbers and key regions used by the birds, as well as some key parameter values from the extensive literature on collision risk. We tested our model against real-world data on goose distributions across the UK; model fit was improved via calibration.
Our calibrated IBM predicted that a mean ± 95% CI of 99 ± 10 Pink-footed Geese would be killed in collisions with all wind turbines (considering onshore and offshore together) and 674 ± 33 geese would be killed in collisions with power lines each winter across the UK. Given the total population size of 479,361 that was considered in our study (mean of the three winter counts from 2016 – 2018), these mean mortality estimates associated with wind turbines and power lines account for just 0.02% and 0.14%, respectively, of the total UK wintering population. Only 1.1% of the total predicted mortality (1 bird) was associated with the offshore wind farms in the NE Irish Sea, an area crossed only during a relatively low number of long-distance flights (and not during the more numerous short-distance daily feeding flights). These mortality estimates for the UK wintering population are lower than suggested previously. For comparison, it is estimated that up to c.50,000 Pink-footed Geese are shot each winter in the UK during the recreational shooting season.
The collision mortality estimates were obtained from simulations using an avoidance rate of 99.8%, as recommended by Scottish Natural Heritage for collision risk studies of geese. However, no single avoidance rate estimate is accepted universally by all stakeholders; therefore, we also ran simulations with alternative avoidance rate values, covering the range commonly suggested for collision risk studies, for comparison. As expected, simulations that were run with lower avoidance rates resulted in higher estimates of collision mortality, i.e. for an avoidance rate of 95% our IBM predicted that a mean ± 95% CI of 2,363 ± 63 Pink-footed Geese would be killed in collisions with wind turbines and 16,664 ± 147 geese would be killed in collisions with power lines each winter. However, an important caveat is that the model was parameterized for typical weather conditions, as we do not currently have sufficient information to model collision risk during atypical conditions (e.g. high density fog). Collision risk during such atypical weather events could be higher than indicated by our simulations.
Future scenarios (under the 99.8% avoidance rate) in which the numbers of turbines and power lines encountered during flights were increased indicated that even a simultaneous doubling of the numbers of all turbines and power lines encountered during flights (relative to the baseline scenarios informed by the tagged geese), which would represent a substantial expansion of the existing network, would have a relatively small effect on the predicted cumulative mortality. The careful siting of any such new energy infrastructure outside of known flight paths and migration routes would reduce these impacts further.
A widely-used approach to quantifying uncertainty associated with an estimate is to calculate a confidence interval, which indicates the likely range in which the mean estimate would be found if the sampling exercise was repeated. More specifically, if the same population was surveyed on multiple occasions and the 95% confidence intervals were estimated for each occasion, the resulting confidence intervals would contain the true population parameter in approximately 95% of the cases. To date, the estimation of confidence intervals for GSMP data has proven difficult, as for many of the monitored populations only a single survey of each site can be undertaken each winter, and the deployment of additional survey effort to repeat the surveys (which could be used to estimate the variance between surveys) is not practical. Even where multiple surveys are currently undertaken, the use of a consistent approach to the estimation of confidence intervals would facilitate meaningful comparisons among different populations. Approaches that would allow the estimation of comparable confidence intervals for all of the populations would therefore be beneficial.
In this report, we compare two methods of estimating confidence intervals for the breeding success or abundance produced by GSMP. These two methods were (i) simple analytical expressions (based on binomial and Poisson distributions), and (ii) alternative approaches based on simulation (bootstrap resampling or Monte Carlo simulations). Both methods could be used for the data that have been routinely collected by GSMP and affiliated schemes. Comparison of the confidence intervals produced indicated broad similarity between the two methods, for juvenile proportion and abundance estimates. Indeed, the confidence intervals estimated by the two methods for the proportions of juveniles within populations were identical in 7 of the 12 populations considered, given the precision with which such estimates have been typically reported (i.e. a percentage given to one decimal place or a proportion given to three decimal places), with only minor deviations of ≤0.009 in the remaining 5 populations.
Similarly, for annual estimates of total abundance, in all populations we found close matches between the size of the confidence intervals derived by Poisson tests and simulation. The mean difference between the sizes of the 95% CI values produced by the two methods did not exceed 7 individuals for any of the populations considered.
As expected, smaller 95% confidence intervals for the proportion of juveniles within a population were found where greater numbers of birds were aged, indicating a trade-off between sampling effort and uncertainty. Moreover, samples containing higher proportions of juveniles had larger confidence intervals for a given total number of aged birds; hence, for populations with higher breeding success greater sample sizes would be required to achieve more precise confidence intervals. For abundance, the absolute size of the confidence interval increased with population size (i.e. higher population sizes have larger confidence intervals). However, when confidence intervals were expressed as a percentage of the population size, their size decreased as total abundance increased.
Based on the findings in our report, we make a series of recommendations for the future development of GSMP and affiliated monitoring schemes. In particular, we recommend the use of binomial and Poisson 95% confidence intervals for age assessment and abundance data, respectively. These analytical methods can be implemented rapidly and require little prior knowledge of statistics or programming to implement. Furthermore, we recommend that consideration should be given to the trade-off between sampling effort and the size of confidence intervals, based on the information presented in this report, in order to optimise the deployment of survey effort as part of GSMP.
The study comprised:
• The collection of new data on the area of mussel beds, the density and size distribution of mussels on these beds, and the numbers and behaviour of oystercatcher on these beds;
• The collation of existing data on the food supply of oystercatchers in the Exe Estuary;
• The development of models to predict the food requirements of oystercatcher;
• Running simulations of the models to predict whether there is / could be any effect on oystercatcher survival of the current / potential future ways of managing the mussel fishery on the Exe Estuary.
The current mussel fishery on the Exe provides a feeding resource for oystercatcher on intertidal lays that are exposed on spring tides. Two potential management options that could be effective at improving the feeding conditions of oystercatcher would be to increase the number and area of intertidal mussel lays, and / or to place mussel discards at a relatively high shore level close to the oystercatcher roost.
This project documented a number of changes that have occurred to the Exe Estuary mussel and oystercatcher populations including:
• The number and size of mussel beds have decreased since traditional methods of maintaining mussel beds in the estuary have ceased.
• The density of mussels within the size range consumed by the birds has generally decreased, but the density of the larger mussels within this size range, which are more profitable to oystercatcher, has generally increased.
• Oystercatcher lose a higher proportion of mussels to attacks by carrion crows and herring gulls than they have in the past.
• The number of oystercatcher wintering in the estuary has declined, but the number of birds feeding on the mussel beds has been relatively stable.
The models developed in the project predict that the present day mussel population is sufficient to support the number of oystercatcher that were observed to feed on mussels.
The presence of mussel lays provides extra food for oystercatcher when these lays are exposed on spring tides. The present area, or increases in the area of mussel lays could increase the survival rate of oystercatcher if the number of birds feeding on mussels was over 2000. Below this threshold, starvation was predicted to affect 0 % of the population and so additional food resources cannot further reduce the starvation. The effect over 2000 birds is relatively small because the lays are only exposed for a short time, and so oystercatcher will obtain the majority of their food from mussel beds that are higher on the shore, and hence exposed for longer. Simulations were not run in which lays were positioned higher on the shore because this would not be commercially viable from a fishery perspective; the growth rate of mussels declines as they are positioned further up the shore because they are inundated with water for less time and so have less time to feed.
Factors that would affect the beneficial effect of discards include the size of the discards, the size of the discard bed and the date from which discards are replenished. Our simulations predicted that larger discards spread at lower density over a larger bed increased oystercatcher survival by the greatest amount. This happened because interference competition excluded some birds from smaller patches, and oystercatcher can maintain high intake rate down to low mussel densities. It is unlikely that the size of discards could be increased, but the simulations suggest that the greatest benefit to oystercatchers could be achieved by spreading discards over a larger area. Our simulations predicted that making discards available from January increased oystercatcher survival by the same amount as making them available from September. This was because the feeding conditions of birds deteriorate through winter as, for example, the ash-free dry mass of prey declines, interference competition intensifies and day length shortens. The intake rate of birds feeding on discards was not measured during the study, but we recommend that this is done to between understand the potential benefit of discards. We recommend that the best place for the discard bed would be along the top of the shore on an area of gravel (and hence of relatively low food value to the birds), to the south of Cockwood. This is south of an area where discards have been laid and exploited by oystercatcher in the past, but would experience lower levels of disturbance from human activity.
Knot were assumed to consume 5-14mm cockles (Cerastoderma edule L.), 5-24mm mussels (Mytilus edulis L.) and 8-16 mm tellin (Macoma balthica L.). Oystercatcher were assumed to consume >15mm cockles, 30-60mm mussels and >12mm tellin. The biomasses of invertebrate prey were derived from intertidal surveys of the site. The population sizes of the bird species were derived from Wetland Bird Survey (WeBS) core counts. Predictions were for the winter of 2013-2014. Shellfishing was assumed to exploit >28mm cockles.
The food requirements of oystercatcher and knot were predicted for different combinations of food supply. All scenarios assumed that the birds could consume cockles, mussels and tellin. Alternative scenarios assumed that knot and oystercatcher could consume other food from upshore areas, or that oystercatcher could consume food from terrestrial habitats. Cockle and tellin biomasses were estimated within Solway Firth, and at Wigtown Bay, a site outside the area in which bird population sizes were estimated. Further scenarios therefore assumed that birds either could, or could not, consume food from Wigtown Bay.
In each scenario the model initially predicted the amount of shellfish biomass not required by the birds. This was then converted into the biomass potentially available for fishing, accounting for the fact that the size range exploited by fishing did not overlap completely with that consumed by the birds. In the case of knot there was no overlap, and so the amount available to fishing was only calculated from the biomass of shellfish not required by oystercatcher.
The model predicted that approximately 700 tonnes of >28mm cockles could potentially be exploited by shellfishing during the winter of 2013-2014, after taking into account the food requirements of the birds, excluding cockle and tellin biomass in Wigtown Bay, and assuming that oystercatcher consumed cockles, mussels, tellin and prey from upshore areas and terrestrial habitats. This was considered to be the most realistic scenario given that oystercatcher can potentially feed on terrestrial and upshore habitats, and given the distance between Wigtown and the area in which oystercatcher population size was estimated. The cockle, mussel and tellin surveys did not cover the entire extent of the Solway Firth, not recording cockles or tellin in English waters or mussels or the Scottish side, and so it is likely that a higher biomass of shellfish food is available to the birds in reality. However, without a more extensive survey it is not possible to quantify this.
The spreadsheet model’s predictions for the winter of 2007-2008 were also compared with those of a more complex individual-based model that was developed for oystercatcher and knot in the Solway Firth based on shellfish biomass during 2005 to 2007. The individual-based model predicted that knot survival was 100% in all simulations for the winter of 2007-2008, consistent with the prediction of the spreadsheet model that 18038 tonnes of shellfish were not required by the birds during this winter. The spreadsheet model predicted that the oystercatcher population required all of the shellfish food available during the winter of 2007-2008. Similarly, the individual-based model predicted that oystercatcher were relatively sensitive to the amount of biomass removed by fishing during this winter. With a shellfishing Total Allowable Catch (TAC) set at 1000 tonnes there was a predicted reduction in survival and TACs set at 500, 750 and 1000 tonnes were predicted to reduce body mass. The spreadsheet model predicted that birds required all of the food during 2007-2008 and hence that any TAC would reduce survival. This demonstrates that the spreadsheet model is capable of producing broadly similar predictions to the more complex individual model, although the latter is more sensitive when stock levels are more critical.
The models are based on the energy requirements of the birds and the energy value of their shellfish food. The spreadsheet model predicts the amount of shellfish required to maintain high survival rates within the oystercatcher population. The individual-based model predicts how the survival rate within the oystercatcher population is related to the amount of shellfish food and the amount removed by shellfishing. Although more complicated, the individual-based model represents the system in a more realistic way and can simulate specific shellfishing scenarios.
The models produced relatively similar predictions, especially when it was assumed that birds fed on upshore and terrestrial food in addition to cockles. As the biomass of cockles has declined since 2008, the models predicted that the amount required by the birds became close to the total available in 2012. The cockle biomass during 2013 was lower than that during 2012 and the spreadsheet model predicted that the birds required virtually all of the cockle stocks available.
We review oystercatcher diet and prey selection in order to quantify the dependence of this species on shellfish, and determine the size ranges of shellfish which the birds consume. We also review the food requirements of oystercatchers, based on their energetic needs and the nutritional quality of shellfish. In general the data agree well with those used in previous oystercatcher modelling studies. However, there is a possibility that the daily energy requirements, calculated from an all bird allometric equation, may yield an underestimate of oystercatcher food requirements. A comparison of the physiological food requirements, i.e. the quantity directly consumed, and the ecological food requirements, i.e. the quantity required to avoid high mortality, indicated that the ecological food requirement was between 2.0 and 7.8 times greater, with the value depending on the proportion of cockles Cerastoderma edule and mussels Mytilus edulis in a site. These ratios are calculated from empirical data on oystercatcher survival and the predictions of individual-based models predicting the relationship between mortality rate and the abundance of the food supply. Data from the Burry Inlet indicated that the mean ecological food requirement was 3.3 times greater at this site.
We describe a simplified spreadsheet model, which we used to predict the food requirements of the oystercatcher population of the Burry Inlet, and thus the quantity of shellfish which must be left unharvested in order to maintain low mortality rate. The model is based on parameter values derived from the literature reviews in this study, including the energy requirements of the birds, the energy content of shellfish, the minimum size of cockles and mussels consumed, and the ratio of the ecological and physiological requirements. We describe the assumptions and limitations of the model, and compare the model with more detailed individual-based models that can be used to predict the mortality rate of shorebirds in relation to the amount of food available.