Eating stone is a common behaviour for Chestnut-headed Bee-eater. Here this bird is eating stone from a riverbed in Manas National Park. They eat stones for calcium which increase the egg shell thickness. They eat stones to grind the harder parts of insects in their gizzard.
Canon EOS 7D + Canon 500mm f4 + Monopod
ISO 200, 1/400, f4, -0.3step EC, cropped