Res nullius

Res nullius (lit: "nobody's thing") is a Latin term derived from private Roman law whereby "res" (an object in the legal sense, anything that can be owned, even a slave, but not a subject in law such as a citizen nor land) is not yet the object of rights of any specific subject. Such items are considered ownerless property and are free to be acquired by means of "occupatio". Its usage as a legal concept continues in modern civilian legal systems.
Examples of "res nullius" in the socio-economic sphere are wild animals ("ferae naturae") or abandoned property ("res derelictae"). Finding can also be a means of occupation (i.e. vesting ownership), since a thing completely lost or abandoned is "res nullius", and therefore belonged to the first taker. Specific legislation may be made, e.g. for beachcombing.
Scope.
Wild Animals.
In common law legal systems, forest laws and game laws have specified which animals are "res nullius" and when they become someone's property. Wild animals are regarded as "res nullius", and as not being the subject of private property until reduced into possession by being killed or captured (see, e.g. "Pierson v. Post"). A bird in the hand is owned; a bird in the bush is not. Even bees do not become property until hived. An exception is the mute swan: the Monarch retains the right to assert ownership of unmarked mute swans, which she currently does on stretches of the Thames and its tributaries.
Likewise in common law systems, abandoned things are generally the property of the owner of the land in which they are found. Exceptions include treasure trove, for which specific law applies, generally making it Crown property; and some types of shipwreck: see Flotsam, jetsam, lagan, and derelict.
Modern Public International Law.
A concept derived from "res nullius" by allegory is "terra nullius". Using it, a state may assert control of an unclaimed territory by occupying it.
This "terra nullius" principle was used to justify colonization of much of the world, as exemplified in the competition for influence within Africa by the European powers (see Scramble for Africa). The concept was applied even where there were indigenous peoples residing in what Europeans considered newly discovered land, as in Australia.