Why is there a hole in one of these shells?

We found them alongside a small lake while we were bird watching in the Ibera marshes, Argentina. The area was teaming with wildlife, herons, egrets and a great freshwater reptile, called a Caiman.

 

But this is the bird that was responsible.  The Snail Kite finds freshwater snails are yummy and so they comprise a large proportion of its diet.

The bird is highly specialized. It sees the snail under the water, hovers and dives down, grips it with its very long talons and flies to a perch. Then using its sharp and scimitar like bill, it punctures a hole in the thin shell and by dexterous twists and turns pulls out the snail and eats it.

The Snail Kite feeds exclusively on snails and being a freshwater specialist is always likely to find itself prone to habitat loss, but currently the Snail Kite has a large range and is not threatened.

… and I thought only the French eat snails.