a difficult trade arrangement

I am fortunate that my workplace is a large, campus-like settings, with lots of grassy, open spaces and trees. Birds, of course, like it too, and there are large numbers of mocking birds, greckels and sparrows, plus the occasional crow flying by. I happen to like crows for their entertainment value and feel that this place needs more of them. So, while feeding a group of several dozen sparrows, I began wondering how many sparrows I'd need to trade for each crow. A exchange-rate of sorts, like currency. Like any simple question, there can be complex philosophical ramifications. Exchange, after all, implies compensation, which requires an owner. But since autonomous living beings have no owner (other than themselves), and I didn't want to get metaphysical, I had to conclude this mental exercise with the thought that there was no one to arrange a trade with...the sparrows and crows certainly seemed content with the present arrangement, so that settled the matter.