Eurasian Oystercatchers, blue mussels and common cockles: an invitation to contribute data to a graph and to share authorship of a paper

This is a call by John Goss-Custard & Richard Stillman to add your observation of feeding oystercatchers and your data about the site to their project.

Fig. 1 shows data for estuaries and coastal flats in Britain where most of the Eurasian Oystercatchers Haematopus ostralegus during the non-breeding season obtain most of their food from the shellfish, common cockles Cerastoderma edule and blue mussels Mytilus edulis. It shows the proportion of oystercatchers foraging on mussel beds as a function of the proportion of the shellfish beds (i.e. beds of cockles and mussels combined) that is comprised of mussels. If this function is widely applicable, we could estimate the numbers of oystercatchers in a site that forage on either cockles or mussels without actually having to count them. All that would be needed would be estimates of (i) the total number of oystercatchers; (ii) the total surface area of mussel beds, and (iii) the total surface area of the cockle beds. This could be useful for estimating, for example, the contribution made by each shellfish species to the total consumption of oystercatchers over the entire non-breeding season. It could also be used to demonstrate the special importance of mussel beds to oystercatchers when Environmental Impact Assessments are made.

Fig. 1.  The proportion of oystercatchers foraging on mussel beds in relation to the proportion of the shellfish beds that consists of mussels. The large dots show the data for particular, named sites. The small dots show the expected hypothetical trend if the density of oystercatchers on the beds averages 30/ha.

Inevitably, the two points at the extremities – the Ythan and Dee estuaries – coincide with expectation as only one shellfish species is present! What matters is the region in between where there is a mixture of cockle and mussel beds and, at present, there are only three estimates. A larger sample size is required to test the expectation. Hence this request!

If you think you might be able to contribute a data point or two, please read on!!

Before describing how you might contribute, an explanation of how the expected curve in Fig. 1 was calculated. It was made for a series of hypothetical sites with approximately the same surface area and oystercatcher numbers as the Exe estuary but with the relative proportion of cockle:mussel bed areas varying from 0.9:0.1 to 0.1:0.9. The first assumption in calculating the expectation is that oystercatchers generally occupy mussel beds preferentially because of the large size and energy content of mussels compared with cockles. The second assumption is that, when the mussel beds are fully occupied, the remaining birds feed on cockle beds. The third assumption is that mussel beds are fully occupied when the average density of oystercatchers on them is 30/ha. This value is based on data from the three sites shown in Fig. 1: using densities of 20/ha or 40/ha to calculate the expected curve does not make much difference anyway.

Here is an example of the simple calculations involved. The mussel beds cover a combined area of 20ha out of the 100ha where shellfish of either or both species are present. The shellfish beds (cockles and mussels combined) support 1,500 oystercatchers. The proportion of oystercatchers on the mussel beds is therefore 0.4 ((30×20)/1,500) and the proportion of the site covered by mussels is 0.2 (20/100). So in this case, 0.4 of the shellfish-eating oystercatchers occur on the 0.2 of the combined shellfish beds that consists of mussels.

This is what is required for one site/occasion (year and season) for you to contribute a data point to the graph:

1. Data from a site where, during part or the whole of a given non-breeding season, most of the oystercatchers over the low water period obtain most of their food from cockles and mussels. The site could be a whole estuary or coastal flat or a large enough part of a huge site to be considered a ‘unit’ within which most of the birds are likely to spend most of their time: g. the east side of the Wash or south side of the Solway Firth.

2. Numbers of oystercatchers that typically forage on the mussel beds over the low water period. [If this is not available, it can be approximated by subtracting the number of oystercatchers on cockle beds (item 3) from the total numbers foraging on the mussel and cockle beds combined.]

3. The numbers of oystercatchers that typically forage on the cockle beds over the low water period. [If this is not available, it can be approximated by subtracting the number of oystercatchers on mussel beds (item 2) from the total numbers foraging on the mussel and cockle beds combined.]

4. The fully-exposed surface area of all the mussel beds combined.

5. The fully-exposed surface area of all the cockle beds combined.

6. The fully-exposed surface area of the entire intertidal area of mudflat and sand flat (excluding saltmarsh).

Item 6 is included because the area occupied by cockles (item 5) may not always be known. The current indications are that expressing the area of the mussel beds as a proportion of the entire intertidal area of mud and sand – including those parts without cockles – may still yield a usable function (Fig. 2).

Fig. 2.  The proportion of oystercatchers foraging on mussel beds plotted against the proportion of the total surface area of intertidal mud-and sand-flats that are covered by mussels.

When we have acquired the data, we will prepare a manuscript and add your name and address to the list of authors, and circulate the manuscript for feed-back. If Figs. 1 and 2 are confirmed, they will prove to be very useful in a range of circumstances. So please do make the time to contribute if you have the data to do so!

John D. Goss-Custard* & Richard A. Stillman

Faculty of Science and Technology, University of Bournemouth, Poole, UK *Corresponding author: johngc66@gmail.com

Please address all correspondence to John Goss-Custard at: johngc66@gmail.com.