My first thought was that there is some artificial event that is timed that sets them off but I can't think of what that would be where I live. Is it usual for birds to sing at a particular time other than, for example, when the sun comes up?
Note: this is happening at like 4 am, nowhere near sunset or dawn. I am not certain it happens every night at this time but it seems very frequent that if I happen to be awake (which is common for me due to work) at 4 am, I hear it.