Birds communicate using a variety of vocalizations, body language and combinations of both for an array of reasons. This presentation is only concerned with vocalization by birds. The primary motivations of bird vocalization can be attributed to Danger, Territory and Breeding purposes. The most prominent of which is for sexual selection and breeding. This rule is not set in stone and may alter depending on the species of bird, their behavior and predator-prey relations. As with all things in nature the primary drive is to stay alive and pass on genes.
The sound is produced in the syrinx, unlike the mammalian larynx, it has no vocal folds but instead is created by vibrations of the larynx walls when air flows over it. The vocalization is created at the bottom of the syrinx where it splits into two independent channels leading to the bronchi or lungs. This split is regulated by independent nerves for each, meaning the bird can control each individually. This allows birds to produce two different notes at the same time. Birds colloquially termed 'songbirds' have an especially intricate syrinx design capable of producing complex sounds and subtle changes. This allows some birds to mimic human speech and machines, or along with their advanced breathing system, call continuously while breathing both in and out.
Bird songs and calls are distinguished from one another based on their purpose and structure. In general, experts are capable of distinguishing the two, however, the parameters are not fixed and in some cases are difficult to define. Songs are deemed longer, more complex sets of syllabic diversity comparable to the changing patterns of typical music. Some birds who incorporate other methods, such as the drumming of woodpeckers also fall into this category even though the sounds are not produced in the syrinx. On the other hand, shorter vocalizations of one or a few notes are deemed calls, which in general serve different purposes than that of birdsong such as alarming due to imminent danger.
The primary reason "songbirds" sing is to attract a mate. It is thought that the song illustrates the birds' health and wellbeing. A bird that is sick, injured or past its prime has less of a healthy sounding song, which aids prospective partners in choosing a mate with adequate fitness. Birds that are capable of mimicry also show off their devious skills as mimics, which displays health and some even believe intelligence to possible mates. When already partnered, some birds use their song in duets in order to strengthen their bond. Thus the more synchronous the duet, the more established their bond already is, while also illustrating their health to their partner.
The secondary reason birds tend to sing, dependent on species, is for territoriality. Some birds, such as robin chats are fiercely territorial and warn visitors that their area, including its resources, and mates are theirs. The procurement of good territory is also a sign of strength, which help prospective mates identify which birds have suitable genes to pass on. In the mornings and evenings when sound travels further in the cold air, birds notify neighboring competitors that they have survived the day or night, thus the territory they hold remains theirs until defeated or preyed upon. If there is no song, the competitor knows that new territory is available to them.
Much more can be elaborated on concerning this most beautiful subject. I urge anyone who is interested in this topic to research it further and simply to enjoy the symphony that one wakes to in the morning.