Wild Caught Birds 
A wild caught bird caught in juvenile plumage is called a passager, meaning it is under a year old. Since many of these birds would otherwise die (estimates run from 30-70 percent) within their first year, the taking of juvenile hawks by falconers has no noticeable effect on raptor populations. These passager birds are often caught using traps that catch their feet in nooses when they try to take the bait.
Birds that are in adult plumage at the time of trapping are called haggards and are no longer commonly used in falconry. The reason for this is twofold: first, birds that have matured in the wild are considerably harder to train for return (when released for hunting haggards have a tendency to go off hunting on their own and are easily lost); second, the capture of an adult bird removes a breeding age bird from the local pool of viable adults.
Taking a bird from the wild is illegal in the UK, as is releasing a captive bred bird.