Male birds sing to let other males know where their territory is and where it's bondaries are. Somtimes you will hear two birds singing back and forth to each other (called countersinging). They will also sing in the spring to attact a mate…to advertise that they have an available territory. Female birds do not "sing" like males but do communicate by calls and chips.

I believe that all birds sing (or communicate) for different reasons. They don't have to sing in order to mate. They don't have to sing to say this is my territory, stay out – even though many birds do.

It's not just a territorial thing or just a mating thing. It can be those.. Or it can be.. Hey mom, I want some food. Or it can be hey bro., look at me, I'm flying! Or it could be. Hey hey hey hey now! – I told you once I told you a million times, put it back where you found it!

Most birds sing high above vegitation and where there is less competition against noisy things such as running water. Some birds will actually fligh up high just to sing (to say something such as all is clear, there's some grub here, but it ain't gonna be an easy one) and then come back down to do whatever they were doing.

Most birds also sing in the evening or early morning hours – usually less wind and less other disturbances. Communicating like this early or later in the day can be 10 to 20 times or more effective and it uses less energy.

But yes., mostly.. Birds sing to mark territory and attract the mates. The males are the ones that are trying to attract the females.

The brown thrasher bird has I believe the most songs of all birds.. Somewhere over 2,000.

Some birds are even bilingual! Because they migrate they learn how to communicate in different song. Pretty cool huh?

The one I remember is the african shrike. They make a song as a duet.. Each has it's own part in the song… Mostly to keep track of one another in the heavy dense forest.

Singing is part instinct, part learned behavior.

"Where do birds get their song from. Research has shown that it is a mixture of innate, pre-programmed knowledge of what their species song is and learning from older singing males. Birds of various species such as Chaffinches and the White-crowned Sparrow (Zonotrichia leucophrys) will develop a song even if raised in complete isolation but the song will be deformed in comparison with normal wild song for that species. Scientists believe that birds are born with some innate capacity to develop a song, this is called the 'Auditory Template Theory'. This template is modified and perfected as a result of practice and listening to older birds singing.

Some species of birds show definite learning skill; they sing a song that is a bit of mess at the beginning of their first season, but after a couple of weeks of practice become much better at singing the species' anthem. This early song is first called a 'plastic song'. Plastic song evolves into 'pre-adult' song which in tern becomes full adult song. There is much variation between species though. Some species have such a strong innate template that they cannot learn anything else, others learn whatever is available and can be taught the song of other species. In the wild though these birds learn from their own species. Finally, some birds learn, even in the wild, from other species. Starlings (Sturnus vulgaris) for instance will copy the whistles of various shore birds as well as the sound of other songbirds, even mechanical sounds, and incorporate them into their song. The greatest song mimics though are the Marsh Warblers. Scientists have recorded and distinguished the calls of over 200 other bird species being used by Marsh Warblers, though not all by the same bird.

A further interesting fact concerning bird song and learning is that because birds learn from each other they tend to develop regional dialects just like people. This is much more evident of non-migratory species. Bird watchers around San Francisco Bay area can place a White-crowned Sparrow to within a couple of miles of its home range on hearing its song. These dialects are maintained because females tend to prefer males which sing the local dialect and because though birds from the centre of a particular dialectal range disperse outwards in a circular radiation, those near the edge tend to disperse in towards the centre of the range."
Http://www.earthlife.net/birds/song.html