Lake Natron in northern Tanzania is no ordinary lake. As the African Sun beats down on it, water is evaporated far more quickly than any rain can replace it. The lake is replenished by springs as well as rain, but these add a strong solution of sodium carbonate (the basis of household washing soda). Natron is a soda lake, a hell-hole of hot, stinking, corrosive water and mud.
Despite the hostility of this landscape, half a million pairs of flamingos have been counted breeding on the mud flats of Lake Natron, which are transformed into an improbable lunar landscape by their nests – mounds of mud that bake hard in the Sun to look just like miniature volcanoes. From time to time a vast pink cloud of birds rises from the edge of the lake – an astonishing blaze of color in such a barren terrain.
In all, about three million flamingos – half the world's population – inhabit Natron and similar lakes along the length of East Africa's Rift Valley. Flamingos are highly specialized feeders, straining their food out of bitter, salty shallow waters. In other parts of the world, in the Caribbean or in the Camargue region of southern France for example, they share their environment with other water birds, but no other bird or mammal has evolved to cope with life on a soda lake as concentrated as Lake Natron.
Two flamingo species breed at Lake Natron, the greater and the lesser flamingo. The lesser feed on algae that flourish in the bitter water, and the greater flamingos eat tiny brine shrimps that harvest the algae. So efficient are the flamingos' bills, designed uniquely for use upside down, that they can trawl the shallows for food without drinking the water. The lesser flamingos 'bills are equipped with fine hairs that act as a sieve, and the birds' tongues pump water in and out. The highly alkaline water is poisonous, even to flamingos, and in order to drink, they have to fly off to neighboring freshwater springs.
The greatest flocks of all are to be seen at Lake Nakuru, 240 km (150 miles) north of Natron. Nakuru is a much more pleasant environment, with a far lower concentration of soda, and it is home to many other species of large bird, including pelicans and marabou storks. It is there that vast numbers of flamingos go through their elaborate courtship rituals. But in order to mate and nest, they head for the isolation of Natron, where the danger of a visit from their enemies – such as a scavenging jackal that might feed on the eggs, or a marabou stork that might kill a fledgling with its stabbing bill – is almost nil.