Green-breasted Mangos

David Sibley has a great post summarizing the records of Green-breasted Mango in the US - up to 19 records now. This makes me wonder... do you think the increasing rate of records is due to increased vigilance or an actual increase in the vagrancy of the bird? Something to ask about all of our rarity records. I'm positive this discussion has taken place many times before, if anyone knows some good links on the topic, chime in.