Orkney Egg

A very large (maybe 10cm) empty egg that I came across when walking across from Houton Bay to Ring of Brodgar on Orkney Mainland. I've not worked out what bird this is from, as there was nothing nearby to hide a large nest, and not near any open water, but perhaps goose or swan.
Taken with K5, DA * 300mm F4 as I was set up for distant wildlife.