The observation that most likely led to the reclassification of the myrtle warbler and the Audubon's warbler as one species is:
The myrtle warbler and the Audubon's warbler interbreed and produce fertile offspring.
This is a key factor in determining whether two populations are separate species, as the ability to interbreed and produce fertile offspring indicates that they are genetically similar enough to be considered the same species, despite differences in appearance.