The earliest careful count of what a Northern mockingbird borrows is Frank Chapman's, from a sycamore in Englewood, New Jersey, in 1916. Chapman sat under the tree with a notebook for ninety minutes and recorded 138 distinct phrases.
He could identify the source of forty-one. Cardinal, brown thrasher, blue jay, robin, tufted titmouse. The rest were either species he did not know or original phrases of the mockingbird's own.
Chapman's count was the start of a research record that is now over a century old. The Northern mockingbird, Mimus polyglottos, is the most-studied avian mimic in North America.
What the research has established, in the broad sense, is this. A single male can hold somewhere between 150 and 400 distinct phrase types in his repertoire. He acquires new phrases throughout his life, not only as a juvenile.
He borrows from other birds but also from frogs, dogs, sirens, car alarms, and the contact calls of his own species. He is selective about what he borrows, in ways that are not fully understood.
The most thorough recent work is David Gammon's, at Elon University in North Carolina. Gammon has been recording mockingbirds in central North Carolina since 2009 and has published a string of papers on the mechanics of mimicry.
His 2013 paper, in The Auk, looked at whether mockingbirds can mimic with the same acoustic fidelity as the original. The answer is that they can, for many species, to a degree that confuses sound-recognition software designed for the original.
Gammon's 2019 paper looked at which species the mockingbird borrows from. The strongest predictor was overlap in pitch range. The mockingbird is unable to produce sounds outside its own physical range, regardless of how much it hears them.
This is why mockingbirds rarely mimic the upper registers of warblers like the blackpoll, which exceed six kilohertz. They mimic, instead, species in the two-to-four kilohertz band that overlaps with their own song.
Marius Doyle visited Gammon at Elon in March of this year and spent two mornings in a residential neighbourhood near campus. They followed one banded male, a six-year-old known to Gammon as B-114.
B-114 held a territory that ran along Trollinger Avenue. He sang from a power line, from the top of a magnolia, and from a rain gutter on the east side of a brick house belonging to a retired professor named Margaret Eisley.
In four hours of recording, B-114 produced 219 distinct phrases. Gammon, who had heard the bird for two seasons, recognized perhaps half from prior recordings. New phrases included a passable Carolina chickadee chick-a-dee-dee-dee and what Gammon was confident was the dropped-call tone of a particular cellular network.
The cellular tone is interesting because it is brief, electronic, and unlike anything in the bird's evolutionary environment. The mockingbird's mimicry is constrained by what its syrinx can produce, but the syrinx is more flexible than most.
What Gammon's work has not been able to settle is the function. The leading hypotheses are mate choice, territorial defense, and a third possibility that the repertoire size is simply a by-product of a learning architecture that never shuts off.
The mate-choice hypothesis has some support. Older males with larger repertoires tend to attract mates earlier in the season. But the effect is modest and inconsistent across populations.
The territorial-defense hypothesis is harder to test. Mockingbirds do appear to use mimicked calls of predators or competitors in agonistic encounters, but the data are anecdotal.
The by-product hypothesis is the most modest and probably the most accurate. The mockingbird has a long lifespan for a songbird, sometimes eight or ten years. It has the neural machinery to learn new sounds throughout that life. It hears, in its territory, a great variety of sounds. It learns many of them because the cost of learning is low.
Doyle and Gammon discussed this over coffee at a place on Williamson Avenue. Gammon was cautious. The function question, he said, may not have a single answer.
What is striking about B-114 is what is striking about the species generally. He sang for nearly three hours that morning without obvious pause. His perch was visible from Mrs. Eisley's kitchen window and she has, by her own count, been watching mockingbirds in the same yard for twenty-six years.
She does not recognize particular birds, but she recognizes the pattern. A male sings from the magnolia in early spring. A pair nests in the holly. The young fledge in late May. The cycle repeats.
The mockingbird is the Audubon-era model of an American bird, which is to say it is everywhere people are. It learns from us and gives back, in scrambled and repeated form, what it has heard. The research record is still adding to itself.
