- 'reduce' here doesn't feel like ordinary meaning. What does it mean? I quote Etymonline because legalese can deceivingly use common words today but actually aim for some bygone meaning.
- Why not just use 'possess'?
Herring, Criminal Law: Text, Cases, and Materials (8 edn, 2018). p. 493
These are dealt with by section 4(4) of the Theft Act 1968, which provides:
Wild creatures, tamed or untamed, shall be regarded as property; but a person cannot steal a wild creature not tamed nor ordinarily kept in captivity, or the carcase of any such creature, unless either it has been reduced into possession [emboldening mine] by or on behalf of another person and possession of it has not since been lost or abandoned, or another person is in course of reducing it into possession.
The key distinction drawn in this section is between tamed creatures (e.g. pets), wild creatures kept in captivity (e.g. wild animals kept in a zoo) or reduced into possession (e.g. wild animals which have been trapped), and wild creatures not kept in captivity. Tame animals are treated as property and can be stolen. Similarly, wild animals kept in captivity or reduced into possession can be stolen.8 However, wild animals not kept in captivity are not property and cannot be stolen.
8 Cresswell v DPP  EWHC 3379 (Admin) held that wild badgers were not property.