There can often be occasions when it is difficult to know where to draw the line between a ‘song’ and a ‘call’, and it is best not to be too rigid in making those distinctions. Songs are assumed to be complex and can be melodic, and are associated with mating, territory and displaying. Calls are assumed to be simpler and relate to alarm, flight or, in the case of chicks, begging for food. However, this is a drastic over-simplification. Many birds ‘sing’ although we do not associate them with pleasing harmonies (think of ducks, owls, divers, for example), but the purpose behind these sounds is the same as that of a nightingales tuneful song.
Most closely related species will have parallel call or song types for a range of behaviours, because their calls will have evolved from the repertoire of a shared ancestor. These parallels are called homologies. Certain sounds may have evolved little from ancestral calls, while other sounds differ strongly between closely related species. For example, whole families (e.g. tits) have evolved with common sounds in their repertoire of calls and songs, with subtle modifications.
The same bird sound can also convey different messages depending on the recipient (e.g. a predator vs another prey species), so when attempting to interpret the meaning of a sound, you need to be as broad-minded as you can. If a particular sound is exactly right for a basic need, several species may use it even if they are not closely related.
One of the first examples of this was described by Marler (1959) when he showed that many passerines (e.g., Dunnock, Common Blackbird, Common Chaffinch) use a high-pitched seep call in the presence of certain predators, especially hawks. The high frequency and drawn out nature of the call, as well as the soft start and end, give the call ventriloquial qualities. All three features of the sound make it very difficult for a predator to tell the location of the caller. At the same time, all the small birds in the vicinity are warned about the threat, and can take evasive action or just keep perfectly still.
When a predator passes through a flock of birds, those in the path of the predator are silent, and those it has flown past call more as it passes and less as the danger recedes. Thus an individual in the flock can work out where the danger is and in which direction it is moving.
Understanding what a bird is doing or the message it is conveying when it makes a sound is a key part of learning bird sounds. Being aware of habitat, behaviour, seasonality, time of day and what other species are around will help unlock your perception and understanding of bird sounds. It is also worth noting that birds of different ages, different sexes and at different stages of their life cycles will make different sounds, too, so when you’ve mastered the basics, you can expand your knowledge further.