Eurasian oystercatchers-Do they catch oysters?

by Matthew Norton

For a long, long time we have been naming and classifying every living thing around us (animals, plants etc) in an attempt understand the natural world. We often derive these names from the creature’s appearance, behaviour, where it is found as who it is related to. However, sometimes a name sticks even when later discoveries would justify the name being changed.

The Eurasian oystercatcher (Haematopus ostralegus) is one such species with a name that is not entirely suitable. This seabird, found on seashores, estuaries and some inland areas, does have a wide range that includes Northwest Europe, the Mediterranean and parts of Asia. This justifies the Eurasian part of the name, but oysters are only a small part of a varied diet which includes mussels, cockles, limpets, sand gapers, lugworms and earthworms.

Eurasian oystercatcher image 1
Eurasian oystercatchers are pretty seabirds with bills that can be modified for different prey types. The map on the right shows their winter range (blue), summer range (yellow) and year long range (green).

Many Eurasian oystercatchers may be unable to catch oysters because they have modified their bills to the shape, size and thickness that suits a completely different diet. These modifications are achieved by simply allowing the bill to be scraped and moulded into a particular shape by the prey they eat and the habitat they live in.

Naturally, different bills for different diets would also require different feeding behaviours. Shell hammerers use blunt beaks to break open the shells of their prey, shell stabbers use chisel shaped beaks to prise open mussels (and other bivalve molluscs) and probers dig their pointed bills into mud and soil to search for buried prey. These are the three main categories of bill shape and feeding behaviour, but in some cases these birds may develop bills that combine features from two categories, allowing the bird to use two feeding behaviours. For example some birds may use pointed chisel bills to probe in soil and stab mussel shells on the shore.

eurasian oystercatcher image 2
Shell stabbing oystercatchers target this space that mussels (and other bivalve molluscs) have between their shell valves and cut the muscles that keeps the shell closed.

You may be wondering how and why oystercatchers ‘decide’ to specialise on a particular prey type. In some cases this is simply a matter of what prey is available in the habitat. Oystercatchers on rocky shores will find mostly mussels and other shellfish while in estuaries and inland soil they will have better luck finding buried prey such as sand gapers, lugworms and earthworms. In other cases there is a clear preference in feeding strategy, even when all kinds of prey are available. This is based on what prey gets them the most food with the least cost from the prey’s defences and from competition with other birds.

For example the shell hammering strategy is typically used by the large adults who can break open the larger mussel shells, whereas younger (and smaller) birds may adopt the stabbing technique to handle smaller mussels and/or probe mud/soil for worms. These prey are less nutritious than the larger mussels, but at least these meagre meals won’t attract competition from the larger, more dominant adults. Similar differences in feeding strategy can be seen between male and female oystercatchers with the latter being smaller and more likely to adopt stabbing and probing feeding behaviours.

In summary Eurasian oystercatchers are remarkably flexible to their individual circumstances and can readily adopt the most suitable feeding strategy for where they live, what prey is available to them and what competition they have to contend with. This is a valuable skill to have in an unpredictable world that presents its inhabitants with all kinds of challenges and opportunities.

From a human perspective

Birds have a special place in human culture that spans hundreds, if not thousands of years and the Eurasian oystercatcher is no exception. In particular, this bird is the national bird of the Faroe Islands, a small archipelago located in the North Atlantic between Norway, Iceland and are symbolised as a defender of the Faroese people from oppressive Danish colonists. This is probably linked to the tendency for oystercatchers to allow smaller birds (i.e. The Faroese people) to nest close to their own nests for protection from larger birds of prey (i.e. The Danish colonists). Also, their arrival to the islands each year, almost always on the 12th March, is celebrated as part of their St Gregory’s Day celebrations and as a sign that summer is on its way.

eurasian oystercatcher image 3
The Faroese Islands have embraced the Eurasian oystercatcher as a national symbol and celebrate their return each year to breed.

Unfortunately, our interactions with oystercatchers are not always so positive. Sometimes there are conflicts between oystercatchers and fishermen who are after the same shellfish in the same area. These conflicts get particularly intense when supplies are low and proper fisheries management is not in place. For example, during the 1980s there were massive declines in mussel and cockle stocks in the Dutch Wadden Sea, but fishermen continued harvesting them , which is probably what led to 10,000’s of oystercatchers dying from starvation.

We also cause harm to oystercatchers by polluting their habitats, destroying their breeding and wintering grounds and even hunting them for food and feathers in some regions. However, these ‘headline threats’ make it easy to forget that even our mere presence on the seashore can be harmful if we are not careful. This is because when they see us, and they will, they see a possible predator and will fly away if we get too close. This might sound like nothing to worry about, but these birds cannot eat and fly away from predators at the same time and with repeated disturbances they lose valuable feeding time. This is especially true during the winter, when food is harder to find, and breeding season, when they have eggs to look after and chicks to feed.

Eurasian oystercatcher image 4
Both oystercatcher eggs and chicks are indirectly vulnerable to human disturbance as it threatens to change the behaviour of the adults.

I am not suggesting that we should never visit the seashore, the activity improves our health and reminds us of our connection with the sea. Fortunately, Eurasian oystercatchers are a low-medium risk species according to the RSPB and IUCN, but we should keep a respectful distance when we visit their habitat and try to keep a harmony between us and the wildlife we admire.

eurasian oystercatcher image 5
Thanks for reading

Sources

Wikipedia. 2018. Eurasian oystercatcher. https://en.wikipedia.org/wiki/Eurasian_oystercatcher. Last accessed 14/01/2019

Arkive. 2018. https://www.arkive.org/oystercatcher/haematopus-ostralegus/. Last accessed 14/01/2019

Pol et al. 2009. Oystercatchers’ bill shapes as a proxy for diet specialization: more differentiation than meets the eye

Durell et al. 1993. Sex-related differences in diet and feeding method in the oystercatcher Haematopus ostralegus

Cayford and Goss-Custard. 1990. Seasonal changes in the size selection of mussels, Mytilus edulis, by oystercatchers, Haematopus ostralegus: an optimality approach

Boates and Goss-Custard. 1992. Foraging behaviour of oystercatchers Haematopus ostralegus specializing on different species of prey

Safriel. 1985. Diet dimorphism’ within an Oystercatcher Haematopus ostralegus population– adaptive significance and effects on recent distribution dynamics

Goss-Custard and Durell. 1988. The effect of dominance and feeding method on the intake rates of oystercatchers, Haematopus ostralegus, feeding on mussels

Hulscher and Ens. 1992. Is the bill of the male oystercatcher a better tool for attacking mussels than the bill of the female?

Faroeislands.Fo. 2018. The Oystercatcher. https://www.faroeislands.fo/the-big-picture/national-symbols/the-oystercatcher/. Last accessed 14/01/2019

Rove.me. 2018. Grækarismessa: The Arrival of the Oyster-Catchers 2021. https://rove.me/to/faroe-islands/graekarismessa-the-arrival-of-the-oyster-catchers. Last accessed 14/01/2019

Ellis. 2018. Oystercatcher.  http://www.birdsofbritain.co.uk/bird-guide/oystercatcher.asp. Last accessed 14/01/2019

Wood. 2015. Using evidence to manage shellfisheries and their wading birds. https://marinescience.blog.gov.uk/2015/07/08/evidence-manage-shellfisheries-wading-birds/. Last accessed 14/01/2019

Atkinson et al. 2010. Impacts of shellfisheries and nutrient inputs on waterbird communities in the Wash, England

BirdLife International. 2018. Eurasian Oystercatcher Haematopus ostralegus. http://datazone.birdlife.org/species/factsheet/eurasian-oystercatcher-haematopus-ostralegus/text. Last accessed 14/01/2019

Stillman and Goss-Custard. 2002. Seasonal changes in the response of oystercatchers Haematopus ostralegus to human disturbance

Coleman et al. 2003.Sub-dispersive human disturbance of foraging oystercatchers Haematopus ostralegus

Verhulst et al. 2001. Experimental evidence for effects of human disturbance on foraging and parental care in oystercatchers

IUCN. 2017. Eurasian Oystercatcher. https://www.iucnredlist.org/species/22733462/117739875. Last accessed 14/01/2019

RSPB. 2018. Oystercatcher. https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/oystercatcher/. Last accessed 14/01/2019

Image sources

Trepte. 2005. [CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)%5D. https://commons.wikimedia.org/w/index.php?curid=347217

Bjoertvedt. 2008. [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D. https://commons.wikimedia.org/w/index.php?curid=16934100

All other images are public domain and do not require attribution

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s