Harsh winters lucrative for picky feeders

The specialized feeders among wading birds – those birds that have specialized in feeding on particular kinds of prey – have better chances of survival and produce more young after harsh winters than birds with less specialized feeding habits, the generalists. After mild winters, it is the generalists who are the better performers. As Dutch winters are getting warmer and warmer, wading birds will need to change from specialized feeders into generalists to adapt to the change in climate.

These are the results of long-term research conducted by the University of Groningen, the SOVON Dutch Centre for Field Ornithology and the Australian National University that were published in the March issue of the scientific journal Evolution.

Diet specialization is a common phenomenon in the animal kingdom. However, it is often unclear why specialists and generalists do not outcompete each other. Particular groups within a species can also follow differing strategies. For example, among oystercatchers, the most familiar Dutch wading bird, there are two kinds of specialists: worm eaters and shellfish eaters. Other groups of oystercatchers are generalists and feed on both worms and shellfish.

Best strategy?

Earlier research had already demonstrated that shellfish specialists are generally more capable of satisfying their daily nutritional requirements and are in better physical condition. But does this also increase their chances of survival? In order to understand why not all oystercatchers become shellfish specialists, the biologists studied their long-term survival and reproduction success in relation to feeding specialization. It was believed that the different strategies would be determined by the availability of food and the climatic conditions and thus would vary over the years.

Harsh and mild winters

By studying the diet, survival rate and reproduction rate of oystercatchers on the island of Schiermonnikoog over a period of 26 years, the researchers have learnt that the feeding generalists are slightly more successful than the specialists. However, during the rare, extremely harsh winters (cold enough to hold the famous ‘Eleven Cities Tour’ ice-skating race), the specialists are the better performers. These fluctuating pressures on selective feeding behaviour may well explain why specialists and generalists do not outcompete each other: when averaged over many years, the different strategies prove to have a similar level of success.

Adapting to climate change

Dutch winters are getting warmer and warmer because of climate change. Biologist Dr Martijn van de Pol expects that this could disturb the balance between specialist feeders and generalists:  “Because specialists perform poorly after mild winters, in the future, all oystercatchers will be under pressure to feed on a broader diet of both worms and shellfish. However, the question is how quickly the oystercatchers will be able to adapt. Learning to find and open various kinds of prey is a process that takes years, and the choice of diet may even have a genetical component. For this reason, we expect that the process of adapting to climate change will go very slowly, taking several generations to complete."

Article: van de Pol, M. Brouwer, L. Ens, B.J., Oosterbeek, K. & Tinbergen, J.M.
(2010) Fluctuating selection and the maintenance of individual and sex-specific diet specialization in free-living Oystercatchers. Evolution 64: 836-851.

Note for the press

More information:
- Martijn van de Pol, Australian National University, +61 2 6125 3078,    
  m.van.de.pol myscience.eu
- Bruno Ens, SOVON Dutch Centre for Field Ornithology, +31 6 10664469, bruno.ens sovon.nl
- Joost Tinbergen, University of Groningen, +31 50 3632065 (office), +31 50 4062928 (home), j.m.tinbergen rug.nl