There's a range of licences that allow wild animals to be kept in captivity, most of which are issued for conservation monitoring purposes rather than for welfare reasons. Many are linked and include clauses relating to further pieces of legislation and international conventions. The Department of Environment, Food and Rural Affairs (Defra) administers most licences in England and Wales; more details of these complex matters can be found on their website.
Remember that it’s always best to get legal advice if you’re at all unsure of how these laws affect you and whether you need a licence - or licences.
Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES)
If you want to display certain species of animal, including birds, for commercial reasons - including for sale, keeping for sale, offering for sale or transporting for sale - a certificate is required from Defra, who administer CITES. This is often referred to as an 'Article 10 Certificate'. For more information, visit the Defra CITES page.
Displaying animals to the public
If you display any wild animal that is not normally domesticated in the UK to the public for seven or more days in a twelve-month period, you will have to apply for a zoo licence. This is obtained from your local authority, who administers the Zoo Licensing Act 1981 and deals with applications and inspections. The local authority considers each application for exemptions to inspections or for limited inspections where, for example, the establishment is small or contains a very limited number of species. For more detailed information, see the Defra website.
What does 'not normally domesticated' mean?
This is often difficult to understand and there are many definitions. Annex E of Defra's Guide to the provisions of the Zoo Licensing Act might help if you're confused.
Animals that are classed as dangerous under the Dangerous Wild Animals Act 1976 (DWAA) need to be licensed by your local authority. Although licensing under this Act is mainly to ensure that the enclosure in which the animal is held is safe, a visit by a vet may also provide some animal welfare guidance.
The Act itself can be accessed, but keep in mind that the online version may not be completely up-to-date (this applies to all legislation listed on the site).
Birds of prey
Some captive birds of prey (and other birds) need to be registered with the government for conservation purposes. Seven bird of prey species are listed on Schedule 4 of the Wildlife and Countryside Act 1981 - most are large, day-flying species. A further two, the peregrine and merlin, may need to be registered under certain circumstances. Registered birds are uniquely marked with either a leg ring and/or microchip. Have a look at Defra's Animal Health pages covering bird registration for more information.
What about owls?
If they were legally obtained, you don't need a licence to keep them because they're not on Schedule 4 of the Wildlife and Countryside Act. However if you want to use these birds for commercial purposes, you may need an Article 10 certificate from Defra.
Grey squirrel and mink
Under the Destructive Imported Animals Act 1932, both of these animals need a licence (through Natural England) in order to keep them in captivity.