From this article:
"But being on the way to becoming a new species isn’t the same thing as actually speciating. Actual speciation without isolation is quite rare, and even the Santa Cruz Island jays have not actually speciated, and may never even do so. But the implications for long-held evolutionary principles are intriguing. Darwin’s famous Galapagos finches certainly prove that isolation leads to speciation, but now it may be that isolation isn’t always necessary to get species to diverge."
1) The theory of evolution never states what the parameters of "separation" are, it simply demonstrated that strict spacial separations do lead isolated populations to local adaptations and eventually speciation over time. This is a separate thing however from saying that to have speciation one must have *strict spacial separation*. This is key because....
2)There is one other way where by populations can become separated. What is it? Culturally. This would include in other animals specialization within a given strata like the various beak shapes of these birds...in this case adaptations sufficient to exploit the available food sources optimally but not sufficient to induce true speciation ...at least as measured by the *ability* for the birds to mate and create viable progeny...but with culture some times you don't want to do that with a mate that is not appealing to you.
We actually as a species are a very clear example of this at any given moment of time...human beings are a single species but in the past our ancestors via separation and isolation have evolved into divergent lines of homonids...from the ancient homo erectus populations that left Africa for the first time around 2 million years ago to the homo neanderthals that evolved from those erectus to later radiations of early homo sapiens and then homo sapiens...which over a very short period of relative time were able to populate pretty much every planetary niche and quickly begin cultural and physical adaptation to those regimes while still maintaining genetic reproductive capability.
Yet when we come together our cultural elements...our languages our physical distinctions of appearance place strong vectors of conformance to remain in group reproducing despite our very close physical /spacial proximity in our large cities. It's not at all a surprise that we do this, or why more or less we tend to mate along phenotype alignment but that's exactly what we are seeing with these birds.
Over a longer time of observation this pattern of course goes away as culture melts together and phenotypes munge together as individuals mate across the gap to create hybrid populations (which tend to be more fit in a mixed environment)...so it would seem that though physical separation is sufficient to create and maintain phenotypic isolation it is not necessary...as such isolation seems to be a natural transition period that may persist briefly before hybridization occurs OR before true physical isolation occurs..or both.
When mammals and birds and Cetaceans evolved culture it began to exert an interesting feedback on what formerly was a purely spacial separation induced pressure toward speciation....once animal brains got smart enough to *choose not to pay attention to spacial separation* all sorts of crossing events became possible and non speciation in this case of birds stands as a single snapshot in a dynamic that is at various states across flora and fauna.
So really what we have here is a refined look at how evolution proceeds, one that includes the nuance of cultural feedback induced separation with or without spacial isolation as a prerequisite state.