Wader magic

Southern Africa is blessed with nearly 10% of the world`s birds. Of the 10 800-odd bird species, 971 have been recorded in Southern Africa.  Waders only constitute 8% of our birds and apart from an obvious few like oystercatchers, are difficult to identify, confusingly variable and often distant.  There are 251 wader species worldwide, including 7 recently extinct species.  In Southern Africa 80 wader species have been recorded. 

The West Coast National park is arguably South Africa`s top wader-watching spot.  It protects Langebaan Lagoon (and seabird breeding islands in Saldanha Bay), and is one of the country`s and indeed the world`s most important wader refuges.  It supports 10% of South Africa`s coastal wader population, with summer counts exceeding 16,500 birds.

Only 30 of our 80 waders are not migratory.  That being said, even the “resident” species are prone to nomadic wanderings, local movements or altitudinal shifts, depending on conditions.  Most of the migratory species that visit the southern African sub-region do not breed here but have their breeding grounds in high Arctic and sub-Arctic latitudes.  Many of them undertake annual migrations totaling more than 25 000 kilometers between the northern and southern limits of the Old World.  During our summers the diversity and abundance of waders in southern Africa increases as the Arctic breeders move south to escape the rigours of the northern winter.  These long-distance migrations are among the most spectacular journeys undertake by any animals.  The Bar-tailed Godwit undertakes the longest known migration of any animal, and may fly for a week without pause – without food, drink or sleep. 

Equally impressive is migratory wader`s adaptability to climatic extremes.  One month it might be enjoying a sun-drenched tropical beach in Mozambique, and a few weeks later it is in the Arctic Circle, waiting for the snow and ice to melt enough so it can lay its eggs. 

The long-distance migrations of waders use up a great deal of energy.  The amount of food needed to fuel a flight of many thousands of kilometers is very much larger than that which is needed for day-to-day existence.  Reserves have to be laid down for the journey at the right time, for the timing of migrations is important, especially for those species that breed at very high latitudes in the short Arctic summer and therefore have only three months in which to complete their breeding cycle.  Before departing from Southern Africa, waders increase their weight to store fuel – a mixture of fat and protein – for the flight.  The more fuel a bird takes on board, however, the heavier it becomes and the more energy it must expend to fly.  This means that there is a limit to how much fuel a bird can carry on one flight.  None of the Palearctic-breeding waders leaving the southern African sub-region can store enough energy to reach the breeding grounds in one flight; they must all stop to refuel on the way.  In general, small waders put on proportionally more weight than large waders.  Turnstones and Sanderlings increase their weight by some 80 to 90%, whereas the much larger Whimbrels put on only 50%.  A Whimbrel leaving the Cape coast has a potential flight range of nearly 6000 km, but the ranges of Sanderlings, Turnstones and Curlew sandpipers are between 3000 and 4500 km.  The speed of waders in normal flight is about 75km/h. 

Invertebrates predominate in the diets of most waders.  Fish are eaten by some species including Greenshanks, Grey Plovers, Sanderlings and Curlews.  Turnstones and Sanderlings occasionally scavenge at beached carcasses and Ruffs will eat plant material.  There is a far greater diversity of invertebrate species on rocky shores than on sandy beaches and this is generally reflected in a greater diversity of wader species on rocky shores.  Although waders are found in a wide variety of habitats, they do not assemble anywhere to feed in more spectacular numbers than on estuarine mud flats.  On the Berg River, for example, foraging densities of migrant waders alone sometimes exceed an average of 100 birds to the hectare.  High densities of waders on estuaries therefore indicate that these environments are a good source of food.  On some mud flats in the Berg River estuary the average density of invertebrates is 90 000 to the square meter;  this is equivalent to a staggering 900 million invertebrates to the hectare.  The density of one species of snail alone can sometimes exceed 180 000 to the square meter.

The habitats in which a species forages and the prey it can catch and eat depends largely on the bird`s size and the shape of its bill and the length of its legs.  There is considerable variation in the size and shape of waders` bills, from the Curlew`s decurved probe, three times the length of it`s head, to the stubby little bill of the Chestnutbanded Plover.  All short-billed species have essentially stout, straight bills but as the lengthy increases, the variety of forms it may take does so too.  These differences are reflected in where they feed and what they eat.  This allows various species to forage together as they are not competing for the same prey.  For example, while the long, slightly upturned bill of the Bar-tailed Godwit allows it to probe deep into mud, the curlew Sandpiper feeds on its invertebrate prey just below the surface and the Common Ringed Plover uses its short bill to pluck prey of the surface itself.  The range of habitats in which waders can forage is either constrained or extended by the length of their legs.  Short-legged waders can forage only on open ground or shallow water, while long-legged waders can forage in open habitats and in relatively deep water.

Many of the world`s waders are dependent on wetlands, either throughout or at some stage in their lives.  Worldwide, both coastal and inland wetlands are under increasing threat from industrial development, urbanization, agriculture, pollution and water abstraction.  Because of their dependence on wetland habitats, many species of waders could serve as sensitive biological indicators of the ecological well-being of these habitats.  For us to interpret what they show us, it is necessary that we understand how waders interact with and respond to their wetland environments.

To learn more about this wonderful group of birds I can strongly recommend the Waders book written by Faansie Peacock.