But how shall you ID Galapagos Frigatebird when out of range?
In a recent paper from Proceedings of the RSPB, Hailer et al present convincing genetic data (although hard to follow as it is quite technical) that there is no geneflow between Galapagos populations of Magnificent Frigatebird and those on the mainland. This is very much in contrast with other populations that freely intergrade such as the Caribbean and the Pacific populations on both side of the Panama isthmus.
Morphologically the Galapagos populations are significantly larger than other populations of Magnificent Frigatebird, but the paper does not give any more data whether there are other plumage characters that would give away the identity. Banded birds from Galapagos have been found on the South American coast. Probably the new species represents an identification horror for birders. Frigatebirds are hard to ID to species as is.
Sometimes the form on Galapagos has been considered a race, but in most modern literature such as Handbook of the Birds of the World subspecies for Magnificent Frigatebird has not been accepted. To further complicate things the Galapagos form had ssp name magnifcens meaning that the remaining populations of Magnificent Frigatebird would no longer keep its Latin name. It would have to be rothschildi from now on.
Important is that the population of Galapagos Frigatebird is only around 1000 pairs, why it threat status must be reevaluated from the present least concern status. Climate change and natural disasters could easily decimate the population.