This does not necessarily mean 50 of one species (although this may be the case) but 50 overall. For example if you have 30 chickens, 15 geese, 4 ducks and 1 rhea you need to register.
Details of how and where to register can be found on the Department for Environment, Food and Rural Affairs (DEFRA) website. Please use the link to the Great Britain poultry register on the right.

DEFRA are also accepting voluntary registration of poultry flocks of less than 50 birds.

Other guidance leaflets, published by DEFRA relating to the keeping of poultry and game birds can also be found through the links on the right.

The Health Protection Agency provides information for anyone who may be concerned about the implications of avian influenza to human health.