Disturbance Cost Calculator
Use this calculator to predict maximum thresholds of disturbance to which birds can be exposed without having a negative effect on their energy reserves.
Overview
This calculator predicts threshold amounts of disturbance that could lead to adverse effects on birds. It does this by taking account of the time and energy costs of each disturbance event combined with the total number of disturbances occuring. It also considers the capacity of birds to compensate for disturbance by feeding for longer. As the total amount of disturbance increases, the overall time and energy costs increase and birds have less time available to compensate. The calculator also accounts for the maximum amounts of energy that birds are able to assimilate. The calculator determines two thresholds beyond which birds cannot compensate, the time threshold and the energy threshold. The time threshold is reached when birds run out of time to compensate for disturbance. The energy threshold is reached when the total amount of energy required exceeds the maximum that can be assimiltated. The calculator considers the time and energy costs over a fixed period of time, for example, a day or the duration of the tidal cycle.
To use the calculator you simply need to select the bird species and the frequency of disturbance. By default, the calculator includes standard estimates of the time and energy costs of disturbance and the rate at which birds are able to assimilate energy from their food. However, ideally these values should be entered for the site and species for which predictions are required. Everything else is calculated automatically. Full details of the calculations, their assumptions and supporting research are given below.
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
| Parameter | Value | Source |
| Daily energy requirements of each bird (E) | Oystercatcher 739 KJ / day | Nagy 1987 |
| Default Ecological Multiplier | Oystercatcher – cockle = 3 Oystercatcher – mussel = 6 |
Goss-Custard et al. 2004 |
| Proportion of energy within shellfish that is assimilated by the birds (A) | Cockles = 0.85 Mussels = 0.85 |
Zwarts et al. 1996 |
| Ratio of ash-free dry mass* of shellfish flesh to wet mass including the shell (R) | Cockles = 0.0372 Mussels = 0.0500 |
Ricciardi & Bourget 1998 |
| Energy content of shellfish flesh (C) | Cockles = 22.5 KJ / g ash-free dry mass Mussels = 22.5 KJ / g ash-free dry mass |
Zwarts et al. 1996 |
References
Goss-Custard, J. D., Stillman, R. A., West, A. D., Caldow, R. W. G., Triplet, P., Durell, S. E. A. Le V. dit & McGrorty, S. (2004) When enough is not enough: shorebirds and shellfishing. Proceedings of the Royal Society, London, Series B, 271: 233-237.
Nagy, K. A. (1987). Field metabolic rate and food requirement scaling in mammals and birds. Ecological Monographs, 57: 111-128.
Ricciardi, A. & Bourget, E. (1998). Weight-to-weight conversion factors for marine benthic macroinvertebrates. Marine Ecology Progress Series, 163: 245-251.
Stillman, R.A. & Wood, K.A. (2013). Towards a simplified approach for assessing bird food requirements on shellfisheries. A report to the Welsh Government. Bournemouth University. 41 pp.
Zwarts, L., Ens, B.J., GossCustard, J.D., Hulscher, J.B. & Durell, S.E.A.l.V.d. (1996). Causes of variation in prey profitability and its consequences for the intake rate of the oystercatcher Haematopus ostralegus. Ardea, 84A: 229-268.