i heard this on a podcast and didn't check it, but here's the story:

when the exxon-valdez oil spill happened, lots of birds got oil on them and died, and the guvmint tried to figure out how many birds died so they'd know how much to fine exxon. but they didn't know how get a count of the dead birds, because they could only count the dead ones that washed ashore, not the ones that stayed in the sea. so they took a hundred normal birds, dunked them in oil, threw them out at sea and counted how many of those washed ashore so they could establish the multiplication factor.