More info on ABA Peeps blog: http://birding.typepad.com/
Isn't this the same site of last year's White-crested Elaenia? 2 firsts in 2 years is not bad.
See below for the Siskin's range, doesn't really have to travel that far and such montane species are expected to undertake long-range dispersal events albeit at a very low frequency....
Strange distribution, if accurately mapped - mostly montane, but also disjunct in tropical lowland eastern Nicaragua. And given the rest of the distribution, the absence from the northern Sierra Madre Oriental north to Monterrey is odd, too.
One to look for in the southeast Arizona 'sky islands' as well.