Dogfish is right about birds taking in small gravel. It is used in their gizzard to grind their food. Along with small stones waterfowl are more likely to pick up shot from the bottoms of ponds and puddles.
The ifo I have on lead weights lost by fishermen says this is not a problem. Here is a link to an article on the subject.
http://www.sioutdoors.com/2001/leadtackle051901.html