This is an interesting question, and while ChatGPT or All About Birds will give you a fairly straightforward answer, the reality seems more complicated.
On the one hand, even old (1950) scientific literature states “It is inconceivable that such pronounced vocal mimicry [in birds] is functionless” (source).
However, 32 years later, a review of the topic by Baylis (Acoustic Communication in Birds) found only minimal experimental verification for the explanations for vocal mimicry in songbirds.
And another 26 years later (we are now in 2008), the authors of another review find not much progress: “In 26 years, then, almost no significant evidence supporting any of the functional hypotheses for vocal mimicry has been produced. Any of the data that do appear somewhat consistent with the predictions of one hypothesis or another are not convincing even for the hypothesis they have been invoked to support. “
That said, there are a few possible reasons for the imitation of other birds (apart from messing with birders and professional ornithologists):
Impressing your mate. Having a broader repertoire of songs can be an indicator of experience and of good health. The fact that males tend to be better mimics than females seems to support this, though, of course, they are also the more active singers in general. Lyrebirds are a prime example of this type of mimicry (source).
Deceiving your mate. Male Superb Lyrebirds may imitate the mobbing calls of whole flocks when females attempt to exit male display arenas or during the lyrebirds’ unusually long copulation. Presumably, the hint of danger from a predator – indicated by the mobbing flock – may keep the female lyrebird from leaving (source).
Attracting flock members. In Sri Lanka, the Greater Racket-tailed Drongo mimics other species that can be part of mixed flocks. Researchers think they may do this to attract these other species, as they find more food as parts of mixed species.
Facilitating Brood Parasitism. Female Common Cuckoos have bubbling calls similar to the calls of Eurasian Sparrowhawks – the use of these calls frightens the host species and thus makes it easier for the cuckoo to place its egg (source). Similarly, the cuckoo chicks may imitate the begging calls of their host species to avoid nest ejection (source).
Facilitating Kleptoparasitism. Fork-tailed Drongos use false mimicked alarm calls to scare other species and steal their food (source).
Deterring Intruders. Male Spotted Bowerbirds mimic alarm sounds to confuse or deter intruders when disturbed at their bowers (source).
Reducing Competition. This was an explanation first suggested in 1977, but it has fallen a bit out of favor. The basic idea was that by using mimicry, a territory holder would make other birds believe that there were already many other birds in his territory, making it less attractive for other birds.
For a more detailed answer to the question, I can recommend a Bachelor’s Thesis that can be found online.
Photo: Greater Racket-tailed Drongo
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