Mimus polyglottos (Northern Mockingbird): photo by Ryan Hagerty, 2008 (U.S, Fish & Wildlife Service)
As all or most singing birds learn their songs from the adults of the same species, it is not strange that there should be a good deal of what we call mimicry in their performances: we may say, in fact, that pretty well all the true singers are mimics, but that some mimic more than others. Thus, the starling is more ready to borrow other birds' notes than the thrush, while the marsh-warbler borrows so much that his singing is mainly composed of borrowings. The nightingale is, perhaps, an exception. His voice excels in power and purity of sound, and what we may call his artistry is exceptionally perfect; this may account for the fact that he does not borrow from other birds' songs. I should say, from my own observation, that all songsters are interested in the singing of other species, or at all events, in certain notes, especially the most striking in power, beauty, and strangeness. Thus, when the cuckoo starts calling, you will see other small birds fly straight to the tree and perch near him, apparently to listen. And among the listeners you will find the sparrow and tits of various species—birds which are never victimized by the cuckoo, and do not take him for a hawk since they take no notice of him until the calling begins. The reason that the double fluting call of the cuckoo is not mimicked by other birds is that they can't; because that peculiar sound is not in their register. The bubbling cry is reproduced by both the marsh warbler and the starling. Again, it is my experience that when a nightingale starts singing, the small birds near immediately become attentive, often suspending their own songs and some flying to perch near him, and listen, just as they listen to the cuckoo. Birds imitate the note or phrase that strikes them most, and is easiest to imitate, as when the thrush copies the piping and trilling of the redshank and the easy song of the ring-ouzel, which, when incorporated into his own music, harmonizes with it perfectly. But he cannot flute, and so never mimics the blackbird's song, although he can and does, as we have seen, imitate its chuckling cry.
There is another thing to be considered. I believe that the bird, like creatures in other classes, has his receptive period, his time to learn, and that, like some mammals, he learns everything he needs to know in his first year or two; and that, having acquired his proper song, he adds little or nothing to it thereafter, although the song may increase in power and brilliance when the bird comes to full maturity. This, I think, holds true of all birds, like the nightingale, which have a singing period of two or three months and are songless for the rest of the year. That long, silent period cannot, so far as sounds go, be a receptive one; the song early in life has become crystallized in the form it will keep through life, and is like an intuitive act. This is not the case with birds like the starling, that sing all the year round—birds that are naturally loquacious and sing instead of screaming and chirping like others. They are always borrowing new sounds and always forgetting.
The most curious example of mimicry I have yet met with is that of a true mocking-bird, Mimus patachonicus, a common resident species in northern Patagonia, on the Atlantic side, very abundant in places. He is a true mocking-bird because he belongs to the genus Mimus, a branch of the thrush family, and not because he mocks or mimics the songs of other species, like others of his kindred. He does not, in fact, mimic the set songs of others, although he often introduces notes and phrases borrowed from other species into his own performance. He sings in a sketchy way all the year round, but in spring has a fuller unbroken song, emitted with more power and passion. For the rest of the time he sings to amuse himself, as it seems, in a peculiarly leisurely, and one may say, indolent manner, perched on a bush, from time to time emitting a note or two, then a phrase which, if it pleases him, he will repeat two or three, or half a dozen times. Then, after a pause, other notes and phrases, and so on, pretty well all day long. This manner of singing is irritating, like the staccato song of our throstle, to a listener who wants a continuous stream of song; but it becomes exceedingly interesting when one discovers that the bird is thinking very much about his own music, if one can use such an expression about a bird; that he is all the time experimenting, trying to get a new phrase, a new combination of the notes he knows and new notes. Also, that when sitting on his bush and uttering these careless chance sounds, he is, at the same time, intently listening to the others, all engaged in the same way, singing and listening. You will see them all about the place, each bird sitting motionless, like a grey and white image of a bird, on the summit of his own bush. For, although he is not gregarious as a rule, a number of pairs live near each other, and form a sort of loose community. The bond that unites them is their music, for not only do they sit within hearing distance, but they are perpetually mimicking each other. One may say that they are accomplished mimics but prefer mimicking their own to other species. But they only imitate the notes that take their fancy, so to speak. Thus, occasionally, one strikes out a phrase, a new expression, which appears to please him, and after a few moments he repeats it again, then again, and so on and on, and if you remain an hour within hearing he will perhaps be still repeating it at short intervals. Now, if by chance there is something in the new phrase which pleases the listeners too, you will note that they instantly suspend their own singing, and for some little time they do nothing but listen. By and by the new note or phrase will be exactly reproduced from a bird on another bush; and he, too, will begin repeating it at short intervals. Then a second one will get it, then a third, and eventually all the birds in that thicket will have it. The constant repeating of the new note may then go on for hours, and it may last longer. You may return to the spot on the second day and sit for an hour or longer, listening, and still hear that same note constantly repeated until you are sick and tired of it, or it may even get on your nerves. I remember that on one occasion I avoided a certain thicket, one of my favourite daily haunts for three whole days, not to hear that one everlasting sound; then I returned and to my great relief the birds were all at their old game of composing, and not one uttered—perhaps he didn't dare—the too hackneyed phrase.
Mimus thenca (Chilean Mockingbird): photo by Alastair Rae, 2007
W.H. Hudson: from Birds in a Village, in Birds in Town & Village, 1920