Behavioral isolation is actually very important. There are many species of birds that are physically capable of cross-breeding but seldom do, because of behavioral isolation. The most remarkable are red crossbills. Crossbills are finches that have specialized crossed beaks that they use to extract seeds from pine cones. However, a given beak size is typically optimal for only one or two species of pines. So if you have a population of large-beaked crossbills, that feed on one species, and small beaked crossbills that feed on a different species, then if they mate, the beak size will be intermediate, and the poor offspring will be inefficient at extracting seeds from either species.
So what happened is that divergent groups of crossbills adopted different songs, and female crossbills only mate with males that sing songs of the correct type. There are a total of 9 breeding groups in the US, and they seldom crossbreed, even though a birder can't distinguish any of the types by sight. These are in effect nine species.
By the way, it appears this speciation only occurred in the last 10,000 years.