It seems that bird calls evolve like human language. Research on the chestnut-crowned babbler shows the cooperative bird is able to change the order of sounds to make new "sentences" with different meanings. The babbler does not sing like other birds. It makes a series of unique sounds.
Analysis shows that the bird is communicating in different ways by stringing sounds together. It has two main categories of calls A and B. If flying, only AB calls are made. In the nest with young birds BAB calls predominate.
When different calls were played back, AB calls initiated flight in the birds that heard it and BAB sounds caused them to go the nest. These "sentences" were definitely perceived as unique instructions. Even when sound prompt elements were changed for the sentences the birds could still tell the difference.
This is the first time a vocabulary type structure in communication has been observe in any animal other than human. The first sound in The A and B structure determines what the overall meaning is, This is similar to human words, for example like the C in CAT (when AT can be used multiple times in any human sentence). It would be pertinent to assume that language in humans began with a very simple system as in these birds.