Red knot survives thanks to poisonous preys

05-06-2013   It may seem wise to avoid eating poisonous food. But what if there is no other food available? The red knot, a small shorebird feeding on molluscs, faces this problem when it winters on the Mauritanian mudflats off the African west coast.

Red knots feed on small molluscs that they swallow whole and that are crushed by the stomach. The red knots can only swallow the shells if they are not too big; and the thinner the shell, the easier it is to crush. An ideal prey for the red knots in Mauritania seems to be Loripes: it is the right size and its shell is thin as well.

There is, however, one big disadvantage: because of certain bacteria living in its gills, Loripes is full of sulphur compounds. This is poisonous to the red knot: if it should only eat Loripes, it would get diarrhoea and become dehydrated.

Fortunately, there is also another species of mollusc: Dosinia. This prey may have a thicker shell, but it does not cause diarrhoea. The more Dosinia the red knot eats, the less diarrhoea. In most years, however, Dosinia is not available in sufficient amounts, which means that the red knot has to supplement its diet with Loripes. In years with Dosinia in large amounts, the chances of survival for the red knots are higher than in years when they mainly have to make do with Loripes. In years with few Dosinia, red knots would starve if it were not for unwholesome Loripes. This is how red knots survive thanks to poisonous molluscs.

The research was carried out in the period 2002-2010 by a research team from NIOZ, the Royal Netherlands Institute for Sea Research. In Mauritania, approximately 200 red knots were ringed annually with individually identifiable coloured rings. The percentage of ringed birds that were seen again was used to estimate the annual survival rate. Loripes and Dosinia densities were measured by taking soil samples, approximately 130 annually. The red knots' diets were reconstructed by identifying remains of shells in excrement that was collected.

These results will be published in an article in the journal Proceedings of the Royal Society B that will appear online on 5 June 2013.

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Article:
Van Gils JA, van der Geest M, Leyrer J, Oudman T, Lok T, Onrust J, de Fouw J, van der Heide T, van den Hout PJ, Spaans B, Dekinga A, Brugge M, Piersma T. 2013 ‘Toxin Constraint Explains Diet Choice, Survival and Population Dynamics in a Molluscivore Shorebird’. Proc R Soc B 20130861.

More information:
Jan van Gils, NIOZ, +31 222 369 573, +31 6 13 17 42 34
Nienke Bloksma, NIOZ Communication: +31 222 369 460 or +31 6 53 49 47 14

High-resolution photographs of the molluscs are available on request from Nienke Bloksma. High-resolution photographs of the red knots are available on request from Jan van de Kam, Griendtsveen, +31 493 529 306.

Red knot eating a mollusc in Mauritania. ©Jan van de Kam.
Loripes. ©Jeroen Onrust
Dosinia. ©Jeroen Onrust