Biological vs. Morphological Species Concept

On paper The Biological Species Concept looks great: if two populations of birds can interbreed, they must be the same species. But it is difficult to apply.
We tend to look at morphology rather than GENE FLOW.

HOW DIFFERENT DO TWO POPULATIONS HAVE TO BE TO QUALIFY AS DISTINCT SPECIES?

MORPHOLOGICAL SPECIES CONCEPT

In birds, enough is known to follow, generally, the biological species concept. We do have problems when two populations show limited hybridization. How much is allowed before they are proclaimed to be two well marked RACES of the same species?

 

Rose Breasted Grosbeak and Black-headed Grosbeak

These two birds interbreed, but are they the same species?