Perhaps the most difficult job faced by national rarities committees is whether or not to admit 'contentious' species which are considered candidates for vagrancy, yet are kept commonly in captivity. Waterbirds are obviously the major thorny issue, as many exotic species are kept commonly in captivity yet are also excellent candidates for long-distance vagrancy e.g. flamingos, pelicans, Ruddy Shelduck, Baikal Teal etc. Formerly committees were forced to make decisions based on the best available evidence - population trends, numbers kept in captivity, known vagrancy history elsewhere in the range, along with attributes of the individual bird - age, location, behaviour and plumage condition.
Recent scientific advances have allowed us to assess origin of individual birds by stable isotope analysis (and identification of individuals using molecular-genetic techniques) and now robust statistical analyses allow us to 'pattern seek' within large datasets to look for 'signatures of vagrancy'. These would take the form of predictable occurrence patterns correlated with demographic fluctuations and their abiotic (climatic etc) drivers. Jiguet et al. (2008) [attached] have analysed the extralimital occurence patterns of 3 species of pelican and looked for evidence of natural vagrancy. They have uncovered a predictable (and crucially statistically significant) pattern of occurrence that suggests that at least some of the pelicans occurring in Western Europe are of a wild origin. It would therefore be possible to revisit many records on a case-by-case basis and judge whether or not they occurred in years which were more likely to have produced vagrant individuals which would then add to the individual case for acceptance based on the individual's location, plumage etc...