Selling a Sculpture

In the current New Yorker magazine, Rebecca Mead reports on a work being auctioned at Christie’s:  “‘Flying Rats,’ by the French-Algerian artist Kader Attia. It consists of a hundred-and-fifty-square-foot cage, a couple of dozen figures of children molded from birdseed, and a hundred and fifty hungry pigeons.”

Not the sort of thing to catch my eye on most days, except for this description of how the sale will be consummated:

The work, which Cappellazzo described as “being about the innocence of childhood as something that is elusive and phantomlike,” goes on sale this week, for a price estimated at between sixty thousand and eighty thousand dollars. The buyer receives neither cage nor birds nor birdseed children but a certificate permitting the future staging of the work, along with a series of photographs of the Lyons installation. The cost to Christie’s of building the coop, buying eight hundred and twenty-five pounds of birdseed, and hiring the birds has been about equal to the estimated purchase price.

If the buyer doesn’t get the sculpture itself, then in what sense has the sculpture been sold?  The New Yorker story says that the buyer’s certificate “permits” the future staging of the work, but I hope that’s a journalistic slip.  It is certainly conceivable that the artist is selling nonexclusive licenses for $60,000 to $80,000 apiece, but improbable; each buyer likely would be surprised to find Flying Rats in other collections around the world. 

Specifically, then, what does this certificate say?  Is Christie’s auctioning the copyright?  Is it selling certain rights (reproduction and public display, for example)?  Does the purchaser get an exclusive license?