Crop, crap and stir : It's a bird's life in the wetlands

18 August 2003
In his recent article Crop, crap and stir in Bird Numbers, published by the Avian Demography Unit (ADU), researcher Tony Williams examined the impact of water birds on the country's valuable wetlands.

Water birds, he writes, do seem to have a significant effect on wetlands, their actions summarised by the three provocative words of his title.

Picture the scene: It's the rainy season and nutrients are flowing into the wetlands. The sun blazes down and illuminates the shallow bodies of water, enabling micro-algae to flourish and providing food for minute animals.

But without the presence of water birds stomping about, these particles would simply sink to the bottom, depleting nutrient levels. Those animals that develop first feast on these nutrients, further reducing their levels. But the wetland birds such as flamingos and Avocets chomp these up (or crop on, to use the correct terminology).

Initially, one might be inclined to think these birds thus deplete the food levels. But there is an interesting cycle at play here; droppings quickly restore the nutrient value of the wetlands. Secondly, the birds selectively remove the relatively large animals. They generally leave behind smaller eggs and younger larvae. If these bigger animals were not removed, Willams argues, they would dominate the food resources and thus a high proportion of the eggs or small larvae would either fail or grow more slowly.

The most obvious benefit to the wetlands, Williams says, is crap (with apologies, he writes, to Mr Crapper of London, manufacturer of water closets to the gentility and whose name offered this abbreviation). Apparently during one month a flock of 100 Avocets can drop 7kg of fertiliser, adding hugely to the nutrient levels.

Another unappreciated fact is that these birds kick up a lot of bottom sediment. "Wetland birds enhance production at Wadrif (one of the areas being studied) by stirring up the cocktail of nutrients, mainly by walking and feeding," Williams notes.

This redistributes nutrients in the water and fosters increased productivity.

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