In many cases close (geographic) encounters between two different, but closely related, species results in accentuation of their differences — in song, or behavior, or even their distinctive markings. It’s possible that a female’s choice of mates plays an important part in driving what can often be subtle changes between the two species. However, in southeastern Arizona, Northern Cardinals (Cardinalis cardinalis) and Pyrrhyloxia, or desert cardinals (Cardinalis sinuatus), are playing a little fast and loose, and it looks like they might be sharing more than habitat.
Before I get to the evidence, let’s review the players in this story.
(1) Northern Cardinals have been expanding their range from eastern North America northward toward Canada as well as southward into the deserts of the southwestern U.S. and Mexico. Although they tend to stick to the wetter habitats in riparian areas, they often forage and even nest in drier desert habitat, especially where people have been feeding birds, providing lots of seed and bird baths.
Most everyone easily recognizes this emblematic, cheery red bird, with its black mask, bright red feathers, and wide, straight, orangey-red bill. Newly molted male Cardinals have grayer feathers on their back, but the tips of those feathers wear off during the winter, making spring Cardinals bright red all over.
(2) Pyrrhuloxia are less well known, occurring only in the extreme southern parts of Arizona, New Mexico and parts of Texas, as well as most of Mexico. They prefer the desert scrub and mesquite habitat, foraging for seeds and insects on the desert floor, but will also visit the lusher gardens with bird feeders and bird baths where they no doubt meet up with their close relative, the Northern Cardinal.
At first glance they look completely different from their cousins, with mostly gray plumage, mottled with red in the face and breast in the male only, a much bigger crest of head feathers which they keep erect unlike the Cardinals, and a yellow-orange beak with a distinctive bend in it.
At the Sonoran Desert Museum in Tucson, a captive adult Cardinal mated with a captive adult Pyrrhuloxia, and hybrid offspring of this original pair now populate the aviary and freely interbreed with one another. The hybrids share characteristics of both species, with intermediate plumage (grayer than Cardinals, redder than Pyrrhuloxia) and slightly curved beaks.
The ability to produce viable, reproductively capable offspring means that these two supposedly distinct species, really aren’t different at all! Differences in their habitat preferences and their former separate geographical distributions probably kept the two “species” isolated from one another.
But an aviary is an unnatural situation, in which partnering mistakes might be inevitable. More important is whether interbreeding in these two closely species has actually been observed in the wild? Perhaps. Hybrid “Cardhuloxia” have been reported from Baja California, Mexico, and I’m wondering whether there has been some gene sharing going on in southeastern Arizona as well.