With growing vigor, political philosophers have started questioning the Westphalian system of states as the main actors in the international arena and, within it, the doctrine of Permanent Sovereignty over Natural Resources. In this article I add to these questionings by showing that, when it comes to migratory natural resources, i.e., migratory species, a plausible theory of territorial rights should advocate a regime of shared sovereignty among states. This means that one single entity should represent their interests and maybe also (...) those of third parties, managing and making decisions over the resource as a whole. Although such a regime might be the tacit goal of existing international conventions regarding wildlife, it remains untheorized in political philosophy and largely under-theorized in international law. By presenting the critical situation of the monarch butterfly in North America, I point to the inadequacy of the compartmentalized current regime, which generates injustice in migration; namely, the phenomenon whereby range states of a given species may neglect or over-exploit it while in their territory, to the detriment of others. I suggest that more flexible and imaginative governance arrangements are needed to deal in a better way with these and similar natural resources. (shrink)