I would like to add to the fine comments made by Michael Dossett:

Genetic studies generally look at one portion of one gene. Mitochondrial DNA is most commonly used these days, although some studies also look at a portion of the nuclear DNA, or even better, combine data sets from these two regions.
Keep in mind that DNA sequences used in these studies are only ~2000 base pairs, which is just a miniscule fraction of the total genome.

Mitochondrial DNA (MtDNA) tends to be more variable, i.e. accumulates more mutations, so it is particularly informative. However, differences in Mt DNA sequences tell us NOTHING about selection on these mutations, or how they impact things like migration, vocalizations, breeding displays, plumage variation, etc. These are all factors involved in gene flow (hence speciation) and I would bet that the genetic influence on these characteristics is considerable.

Regarding crow taxonomy:
Any study of the validity of the species status of the northwestern crow needs to examine crows from throughout their range. If we only look at the situation in this region we will get a very tainted picture. When I was in SE Alaska some years ago, I was struck by how tiny and how different the crows (which would be northwestern crows) sounded from the crows I was familiar with in the Puget Sound region.

Mike Donahue
Seattle