Male scarlet tanager. Julio Cortez/Associated Press

Rose-breasted grosbeaks – with their striking black and white plumage with an irregular red patch on the breast – are one of our most striking birds. Their robin-like song is much more melodic and pleasing than the cheerup-cheerio-cheerio song of a robin.

Scarlet tanagers are equally striking. The scarlet body and black wings of a male are unforgettable. Females are more subtly beautiful with a greenish-yellow body and dark wings. The male’s song has the same sing-song quality of a robin or rose-breasted grosbeak but with a scratchy timbre.

Thanks to a heads-up from Cliff Otto, I learned about a remarkable hybrid of these two species seen in Pennsylvania. This hybrid is the first ever recorded for the two species. Here’s a photo.

We’ll circle back to this remarkable hybrid. When I first learned of this hybrid, an admonition by an ecologist when I was in grad school sprang to my mind. This professor said that nature has no interest in being pigeon-holed.

Humans have a propensity to organize. Biologists like to develop schemes to describe different levels of some phenomenon. For instance, animals may be divided up into herbivores, carnivores, and scavengers. But some species don’t play by our rules and a meat-eater may eat berries or feed on carrion. Some people find such messiness discomfiting. I love the variation.

Hybridization falls into this type of uncertainty. We can define a hybrid as the result of mating of a male and female of different species. Probably the most popular definition of a species is that it is a group of interbreeding individuals that do not reproduce with any other groups.

So, a hybrid should not be able to produce viable and fertile offspring. What are the ways that this reproductive incompatibility can arise? The sperm from one prospective parent may not be able to fertilize eggs; eggs might be fertilized but lethal combinations of genes might arise. A hybrid might grow to adulthood but be infertile or have less vigor than intraspecific matings. So far, so good.

But let’s think about mammals. The cross between a male donkey and a female horse is a mule. Mules are not fertile. The reciprocal cross of a female donkey with a male horse is called a hinny. Hinnies are fertile. One species or two?

Now we will move to the Great Plains where a hybrid zone for several species exists. The yellow-rumped warblers in the West have a yellow throat and were once classified as a separate species, Audubon’s warbler. In the east, our yellow-rumped warblers were once called myrtle warblers.

In the hybrid zone, the two forms interbreed producing fertile offspring with no apparent loss in vigor. So, the current classification has the two forms classified as a single species.

Along the same hybrid zone, Baltimore orioles overlap with western Bullock’s orioles, producing fertile offspring. But here, the decision is to split the two forms into two species.

Mallards and American black ducks frequently interbreed and produce viable offspring. Like the orioles, these two forms are classified as separate species. Conservation biologists are worried that frequent crosses of black ducks with mallards may cause genetic swamping and the demise of the less common black ducks.

So, we have situations where hybrids are sometimes fertile, but we still treat the parent species as separate species in some cases but not others. It’s no wonder that many biologists are moving away from interbreeding as a definition of species to morphological or genetic definitions of species.

Returning to our hybrid of a rose-breasted grosbeak and scarlet tanager, I have little doubt that this individual is infertile. The two parents are more closely related than one might think. Recent genetic work shows that all our North American tanagers belong in the cardinal family, not the tanager family. The grosbeak is in the cardinal family, too.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu