Lesley Andrews

I am the proud owner of two stone plants which I keep on the stoep table - these are decorated by a surface layer of small tumbled semi-precious stones. Recently I was astonished to see a threesome of Cape Turtle Doves on the table carefully picking out some of the stones, passing them to each other, rolling them around in their beaks and putting them down carefully all over the table. I knew that many birds eat grit, but why this preference for my ornamentals?

The Cape Turtle Dove (Streptopelia turtur). Photo: Jo Wicht

The source-tumbled stones around a stone plant. Pot diameter 75 mm. Photo: Lesley Andrews

The internet provided interesting answers. Many birds (as well as certain reptiles and marine mammals) use stones and grit in their gizzards to aid digestion. The stones are called gastroliths and vary in size according to the host species, and the length of time in use (wear). The bird therefore needs to replace these. Do my pigeons realise that my gemstones are harder than the many sandy stones in the garden, or are they just attracted by the pretty colours? Stones that were most favoured were quartz/silica varieties such as agate, amethyst, obsidian, tigers’ eye and rose quartz.

Some of the stones picked out by the pigeons – note that some may have been rejected as too large. Photo: Lesley Andrews

Pigeons world-wide, especially the seed eaters, require gizzard stones. Sadly, the beautiful Nicobar pigeons of southeast Asia are threatened, as many are killed not just for their feathers, but also for their gastroliths, which are made into jewellery. This has led to captive breeding programs around the world.

Nicobar Pigeon in Central Park Zoo, New York. Photo: Lesley Andrews

The selection of stones for digestion aids does not seem to be classified as avian stone tool use, such as the behaviour of Egyptian Vultures breaking ostrich eggs with stones, or the use of stones to attract mates to nest sites (Adele penguins and Bower Birds). The study of gastroliths, however, is important in archaeology and palaeontology. Stone and pottery gastroliths of domestic fowls in early communities have been identified, and even fossil gastroliths (including large stones from dinosaur gizzards) can be found where fossil bone is scarce. There is a specialised branch of mineralogy which involves the study of markings and erosion patterns on such gastroliths to distinguish them from other stones on site.

There is a great deal more information for those who are interested; here are examples of sites to start off with: