I had started at the base of the 10,834-foot mountain at first light. Now at the fourth stop on my vertical transect through Southern California life zones, I could see leafless deciduous oaks and Coulter Pines over an emerald carpet of annual grass. Despite recognizing such California endemics as California Towhee, Thrasher, and Quail in the darkness below, I was confronted here with a song I didn't know. Never mind that I was doing sabbatical research on the bird's sister species in nearby Arizona, never mind that its strange song conformed to the pattern of the bird family I thought I knew best, I didn't know who was singing. So, I played back the recording I had just made, the bird came in, and I learned that it was an Oak Titmouse.
My problem had been that I knew too much. Oak Titmouse and Juniper Titmouse had until recently been subspecies of a single species, Plain Titmouse. That's understandable. They are both little gray birds with crests, and they can't be told apart visually. So, when I wanted an excuse to spend a year in the Chiricahua Mountains, comparing the vocalizations of the Juniper Titmouse with the distinctively colored Bridled Titmouse seemed like a good idea. The Oak and Juniper were so closely related I could assume they sounded alike. Wrong! Do not make assumptions about nature, you might miss something good.
Another facet of my problem was that I was not open to novelty. The novelty in this case is the little trill that precedes every one of those long, upslurred notes. Without it I might have caught on sooner. Do you see any kinship between these songs, minus the little grace-trills, and the simple songs of the Tufted Titmouse on April 5? Yes. So, actually, all five of the species of North American titmice (genus Baeolophus) sing songs that are constructed just like those of yesterday's mockingbird: a note or simple phrase is repeated several times, the whole lasting a second or two. The differences between the phrases used by Oak, Juniper, and Tufted are surprisingly great, but the syntactical pattern is the same. And ancient. Both Great Tits and Oak Titmice follow the same rule, and they last shared an ancestor 7.5 million years ago.
The oak woodlands of California and the Sierra Madre are physiognomically similar, which means they look alike, despite having no species of oak in common. Birds are associated with habitat structure, not species of trees, so the similar structure of Californian and Madrean woodlands leads to many kinds of birds occurring in both woodlands. In most cases, these avian kinds have evolved into different subspecies, or even species, because there is a large patch of desert between the two woodland zones. That discourages gene migration, which occurs when individuals move from one place to the other. The titmouse situation is a little more complicted, though. Yes, there is a titmouse species in Californian oak woodlands and a different one in Madrean woodlands, but they aren't sister species. As mentioned, the sister of the Oak Titmouse is the Juniper Titmouse, while the Madrean species is the Bridled Titmouse. Where does that leave the Juniper? In the junipers of the Great Basin, of course. This troika might be the Rosetta Stone of oak woodland biogeograpy. Stay tuned.