The most salient trait of mockingbirds is that, instead of having a species-specific song, each individual mockingbird builds a song by learning the songs of other birds that it hears. Hence the genus name Mimus, which means “mimic.” Thus, those who “bird by ear” recognize a mockingbird, not by a specific song, but by the fact that several different songs are strung together in sequence.
This is very unusual. Species-specific songs seem to be a basal trait in the Passeriformes, which is why “birding by ear” is a thing. This is found even in non-oscine Passeriformes – Empidonax flycatchers have very simple songs, yet these are specific enough that field guides describe them, and even transliterate them into human phonetics (e.g., the sneeze-like “fitz-bew” of the Willow Flycatcher). The question then is, how to become a mockingbird? That is, how to go from a set, species-specific song to copying other species’ songs?
On my Cape May trip last spring, I think that I learned the answer. We heard a long series of notes coming from a thicket, which the guide identified as a Gray Catbird. The Gray Catbird is in the family Mimidae, same as the Mockingbird; and, the guide said, instead of having a song, “they freestyle.” Improv. Catbird the Rapper. In birding by ear, one recognizes a catbird, not by a specific song, but by a series of notes with no discernible pattern.
So there we have it. I would deduce that the catbird’s improv approach is basal in the Mimidae, and that this flexibility – loss of the species-specific song – is a necessary prerequisite to the mockingbird’s copying of other songs. Probably, the ancestral mockingbirds went through a catbird-like improv stage before developing their present-day copycat strategy.
To learn new songs, they first had to forget their old ones.
I think we need a broader look at mimicry first. It’s not just something found in Mimids. I’ve heard gnatcatchers doing a bit of mimicry, for example. One of the best examples is the family Menuridae, the Lyrebirds. They’re basal to the rest of the oscines. (I’m not sure if they’re actually considered oscines, but apparently they do branch off after the suboscines.) There’s also the issue that most oscines show at least some vocal learning, which is why we get micro-geographic variation in some resident birds. (Check out White-crowned Sparrows in California…) Vocal learning has also been demonstrated in one suboscine (the Three-wattled Bellbird), and since most of the suboscines live in South America, they’re likely to be less well-studied than temperate oscines. And of course, vocal learning isn’t limited to passerines – parrots, for instance, are famous for mimicry. I believe hummingbirds have been shown to have the capacity, as well, but that could just be my faulty memory.
There are clear patterns to both mockingbird and grey catbird songs. Mockingbirds, for example, tend to repeat each section of their song four or five times before moving on to the next. Catbirds intersperse the cat-like call that gives them their name and can be recognized just from that call. But yes, all members of Mimidae that I’ve met mimic to varying extents The New York Times has an online article about bird song today that includes a recording of a catbird mimicking a frog. https://www.nytimes.com/interactive/2023/07/21/science/bird-calls-songs-quiz.html
I visited Arizona recently and was surprised by how many species there are mimics. Not just Northern Mockingbirds everywhere, but also Lesser Goldfinch, Yellow-breasted Chat, and maybe a couple others. It made it very challenging to learn the bird calls there when everything else was new to me as well. At home in Ontario mockingbirds are uncommon, and Blue Jays and European Starlings don’t mimic very often and can usually be identified by context (and I’d say they aren’t as good mimics).
I don’t know if they actually mimic, just that they learn their vocalizations instead of having them instinctually. And most of their calls are too high pitched for us poor humans to hear them, anyways.