An estimated 2-5 million birds of many different species are trapped worldwide each year for the cage-bird trade, large numbers of which are transported for sale elsewhere. Some apparent vagrants might therefore be escapees that are reported at varying dates and distances after their escape. It is often possible to tell if a bird has been kept in captivity for a long time, but this is much less easy for a bird that escaped soon after capture, or had lived in the wild long enough to moult before being noticed. Again, however, such processes are unlikely to account for the appearance of the same species at the same offshore islands in numbers at about the same dates every year. Most cage birds are passerines or parrots, and other species, such as waders, are kept in captivity only by a very small number of specialists. In addition, in some parts of the world, including North America, almost no native birds are caged or exported, except small numbers for scientific purposes. Yet vagrancy is just as frequent a phenomenon in the New World as in the Old.
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