Hi Peter,
Food caching is common in corvids and research into this behaviour suggests that it is a compulsive motivational instinct.
A number of studies have recorded stone-collecting by corvids. The generally accepted theory is that this behaviour is conducted mostly by juvenile or immature corvids who are instinctively drawn to collecting objects, but haven’t yet refined their behaviour to cache edible food items only.
A good summary of this behaviour is given in the following paper:
Grodzinski, U. & Clayton, N.S. (2010). Problems faced by food-caching corvids and evolution of cognitive solutions. Philosophical Transactions of the Royal Society. Biological Sciences. 365 (1542): 977-987. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2830244/
Regards, Stephen
Stephen Ambrose Ryde, NSW