Birds that feed on fibrous or hard food of vegetable origin often swallow sand or small stones ('grit'). Grit accumulates in the gizzard or crop, and together with the muscular action of the crop it helps macerate their food facilitating digestion and increasing food processing rate. The limited size of the gizzard means that herbivorous birds need to spend time just processing food, waiting until the gizzard empties, before another foraging bout. Grit can be ingested accidentally with the food or purposefully, by visiting places where it is easy to find. Wintering flocks of Greylag goose in Doñana National Park (Spain) visit sand dunes to ingest sand, and their crop can hold up to 10 g of sand. Birds can adjust the amount of grit to their diet and in turn, crops containing more grit become more muscular, flexibly responding to the demands of different coarseness or amount of fibre in the diet.
While watching the flock of wintering geese in my local park, I noticed them stopping in a bare area. First I thought they were feeding on the small tufts of grass there, but when I looked closer I realised they were ingesting grit. They spent a few minutes doing this, and then moved onto the grass to feed. The distinctive individual with a white head (above) serves as an identifier for this flock. I have seen him or her both at East Park and Pearson Park.
A pair gritting together.
Here is a short video of the behaviour:
More information
Amat, J. A., & Varo, N. (2008). Grit ingestion and size-related consumption of tubers by Graylag Geese. Waterbirds, 31(1), 133-137.
here.
VerCauteren, K. C., Lavelle, M. J., & Shively, K. J. (2003). Characteristics of grit in Canada goose gizzards. Wildlife Society Bulletin, 265-269.
Here.
