Some birds elaborate their songs with sounds from other bird species.
Mockingbirds and starlings
The mockingbird is so-called because it copies the sounds of other birds and weaves them into its own output. The starling is another “copier”. Indeed, in North America where the starling is established coast to coast as an immigrant from Europe, it even mocks the mockingbird!
The starling is the best known feathered impersonator in Britain and Europe. It will pick up sounds from wild birds like buzzards and golden orioles, curlews and tawny owls, and also from domestic hens and geese. There is a reliable report of a starling in a London suburb that cried like a baby, and other starlings imitated V-1 flying bombs in London towards the end of World War 2. Less reliable - though certainly within the bounds of credibility - is the account of a starling that mimicked a referee's whistle and upset a football match.
A study of the marsh warbler
The sedge warbler is, to some extent, imitative, but its very close relative the marsh warbler is known to be 80% imitative. Further research is expected to show that it is 100% so with the passage of time, the remaining 20% will probably prove to have been derived from birds - or perhaps in a few cases from insects. Like the sedge warbler the marsh warbler crosses the equator twice a year flying to and from Eurasia and Africa. Its song has been studied in both summer and winter quarters (it apparently maintains an off-season feeding territory).
Newly-fledged marsh warblers start to pick up notes within a month, a process which continues during the autumn migration across Europe and stops only in January when the birds are six or seven months old. After this, the birds adopt no new material for the rest of their lives, a fact attested to by sound recordings of colour-ringed individuals made in successive years on the European breeding grounds which show no changes in repertoire.
The startling fact is that each individual studied incorporated notes from between 63 and 84 other species into its song; that is an average of 76. Of those, an average of 31 were European species and an average of 45 were African species. Certain of the African species have such a limited distribution - like the boran cisticola in southern Ethiopia and northern Kenya - that it is possible to tell from this acoustic evidence where the bird has been!
But why do these birds appropriate the sounds from other species? The probable answer is very straightforward: it is simply their way of elaborating their songs. Why these vocal appropriators should choose to diversify their songs with pre-existing material is difficult to understand.