Moyle et. al. (2005) demonstrated these two forms are different phylogenetically and morphologically and the montane form is a different species endemic to Borneo.
Mann (2008) in his Checklist for Borneo classified the lowland form as Southern White-crowned Forktail E. l. frontalis and the the highland form as Northern White-crowned Forktail E. sinensis borneensis.
Sheldon et. al. suggested in 2009 that the highland form to be called Bornean Forktail Enicurus borneensis here.
These two forktails sound and look identical, birders traditionally distinguish them by habitats. Their specimens collected from Borneo are distinguished by measurements, however, their wing and tail lengths overlap and are confusing. Identification based on observation of tail length is not reliable as it varies among different individuals and may be age related.
Susan Myers (2009) is the first to suggest that Bornean Forktail has lesser white in its forehead than the lowland White-crowned Forktail in her Field guides to the Birds of Borneo.
I have wanted for quite some time to prove her point, but good photos of White-crowned Forktail are hard to take, they are skittish and usually occur in remote forest streams. Bornean Forktail, on the other hand, is relatively easier to photograph in Kinabalu Park, so we have good photos of Bornean Forktail in the Internet while photos of White-crowned Forktail from Borneo are hard to come by.
I have over time accumulated photos of White-crowned Forktail from Borneo, these photos are from Tawau and Tabin, far away from high mountain ranges of Borneo, so it is quite safe to label them as such.
The following composite images show the forehead and front of individual birds, after looking at them and other images in the links, we know we can reliably use the extent of white in the bird's forehead to positively ID the bird, especially in areas where they overlap.
Birds with crown feather lifted (left panel : Bornean Forktail from Kinabalu Park, right panel : White-crowned Forktail from Tawau (upper bird from Andrassy Forest Reserve, lower bird from Tawau Hills Park.)
Birds with crown feather at rest. (left panel : Bornean Forktail from Kinabalu Park, right panel : White-crowned Forktail (upper bird from Andrassy Forest Reserve, Tawau, lower bird from Tabin Wildlife Reserve)
It is evident from above that the extent of white in the forehead and crown of these two species differ, especially visible when the crown feathers are lifted. When at rest the forehead white feather of White-crown Forktail extends well towards the crown touching the upper nape, while Bornean Forktail only has the white forehead feather extends midway to the crown.
More images of Bornean Forktails are here:
More images of White-crowned Forktail from Danum Valley, Sabah, Borneo are here:
Photodocument of the wild birds of Borneo.
Standard references for my blog.