This is an example of adaptive variation, utilizing information already available within the birds' gene pool. To call this evolution, in the sense of generating a new species, is idiocy. The Galapagos finches have been demonstrating such responses to alterations in their environment for as long as man has been observing them and recording such variations in beak size and shape. But they remain finches.
Yet this very example contributed to the Darwinian mechanisms of descent with modification and common ancestry. Are you saying that Darwin did not know the definition of Evolution?
The acceptance of adaptation by creationists is a relatively recent change from the rejection of any and all changes. Once creationists were forced to admit the reality of adaptation they were quick to lay claim to it and attempt to divorce it from Evolution. Biology, in the adoption of the two terms Microevolution and Macroevolution to mean evolution within a population for the former and evolution above the species level for the latter, provided creationists with terms which could be easily misrepresented to produce an artificial divide between the mechanisms for both. Creationists were all too happy to do so.
I understand your need to divorce adaptation from Evolution but creationists do not define terms used by science and science has always defined Evolution to include adaptation.
Evolution, right from the days of Darwin, has included adaptation and many evolutionary proponents believe that no other mechanisms are needed to produce speciation.
If you disbelieve the ability of accumulated small changes to result in the diversity we see in biology you need to propose a limiting mechanism. Without that mechanism, changes have no option but to accumulate. The differences between any two related (through DNA comparisons) species, even those in separate genera can be described in terms of change in degree of morphological features.
To steal an idea from Ichneumon, the differences between a bear and a dog can all be described as a matter of degree - leg length, weight, hip position, tail length, etc. We can also describe the differences between Ursidae and Mustelidae (weasels, otters, ferrets, badgers,etc.) in terms of degree only. Since the differences are all of degree they can be attributed to accumulated adaptation.
Unless you have a mechanism to prevent the accumulation. Or Canidae, Ursidae and Mustelidae are all the same kind.
Note:Mutations provide additional variation in the expression and number of alleles.