An interesting discussion recently on WP birds http://groups.yahoo.com/group/WestPalBirds/ about the association of vagrant birds and parasites. A vagrant Ivory Gull which was tracked between France and Ireland showed evidence of a parasite infestation. Perhaps this is a vagrant because the parasites weakened it and somehow managed to affect it in a way to wander outside it's range. This would imply an interesting example of how parasites can alter the behaviour of their host. In this case the vagrant bird could help the parasites spread outside it's range. This would be an example of co-evolution of species range between parasites and birds.
It is also possible that the infestation occurred as a result of the vagrant bird arriving in an area where parasites can quickly infest the birds as the host is not adapted to defend against the infestation.
Suggestions to investigate this area include: identifying the species of parasites involved (to see where their natural range is), look at control birds e.g. investigate the incidence of parasites on Ivory gulls within their range and also look at incidences of other vagrants which have been found to host parasite infestations.