Overview

This calculator predicts the shellfish food requirements of birds in two steps. First, the amount of food that will be consumed by the birds is calculated. This is called the physiological requirement and depends on the number of birds, the time for which they need to be supported, the daily energy requirements of the birds and the energy content of the shellfish the birds are consuming. Second, the amount of food required in the environment to support the bird population is calculated. This is called the ecological requirement and is generally larger than the physiological requirement as birds cannot find all of the food, some birds are excluded from some of the food through competition and food is lost due to sources other than the birds. The ecological multiplier measures how much larger the ecological requirement is than the physiological requirement.

To use the calculator you simply need to select the bird and shellfish species, enter the number of birds, number of days for which the birds need to be supported, and select the proportion of energy requirements that the birds obtain from shellfish. Everything else is calculated automatically. Full details of the calculations, their assumptions and supporting research are given below.

At present the Food Requirement Calculator is limited to oystercatcher consuming cockles or mussels. 

Details of calculations

Ecological Requirement

The ecological requirement is the total amount of shellfish that needs to be reserved in the environment to support the bird population. It is calculated using the following formula.

Ecological Requirement (tonnes fresh mass) = Physiological Requirement (tonnes fresh mass) x Ecological Multiplier

Physiological Requirement

The physiological requirement is the total amount of shellfish that the birds will need to consume in order to meet their energy requirements. It is calculated using the following formula.

Physiological Requirement (tonnes fresh mass) = (P x N x D x E) / (A x R x C) / 1000000

where P = proportion of energy obtained from shellfish, N = number of birds, D = number of days for which birds need to be supported, E = daily energy requirements of each bird (KJ), A = proportion of energy within shellfish that is assimilated by the birds, R = ratio of ash-free dry mass* of shellfish flesh to wet mass including the shell and C = energy content of shellfish flesh (KJ / gram ash-free dry mass). The result is divided by 1000000 to convert the requirement from grams to tonnes.

The values of P, N and D are required for the particular study site. P may be difficult to estimate and so a precautionary approach would be to leave this at the default value of 1, assuming that the birds need to obtain all of their energy from shellfish. It is important that N is the mean (average) number of birds present on the shellfish beds over the time for which predictions are required, rather than a peak count. The values of E, A, R and C are automatically calculated for the selected bird and shellfish species (see parameters).

* Ash-free dry mass is the standard way in which the organic mass of shellfish is measured. It is calculated by initially extracting the shellfish flesh from the shell. The dry mass of this flesh is then measured after the flesh has been kept within a drying oven. The ash mass is then measured after the flesh has been kept in a furnace that burns off all of the organic matter. The ash-free dry mass is then the dry mass of flesh minus the mass of the ash.

Ecological Multiplier

The ecological multiplier measures how many times greater the ecological requirement is than the physiological requirement. This has been calculated from the predictions of individual-based models (Goss-Custard et al. 2004) and depends on the shellfish species being consumed by the birds (see parameters).

Parameters