Of course, it doesn't actually matter which one is chosen in this scenario. They could easily have gone one step further to see if they could use this effect to make the sparrows prefer the worse of the two deals - that would be much more interesting.
You walk into a store that sells two identical candies. Would you buy the candy that ordinarily costs $2 at 50% off (for one dollar), or the candy that ordinarily costs 10$ for 80% off (for one dollar)?
In an isomorphic situation, sparrows preferred the latter deal.
http://users.ox.ac.uk/~kgroup/publications/pdf/kacelnik_marsh_2002_animbehav_costs.pdf