A recent experiment (
Torben Dabelsteen, Nicolas Mathevon) sought to shed light on your question, which has puzzeled ornithologists for some time. They, initially, believed that the quality of the sound transmission was enhanced in the early morning hours. The experiments, however, failed to corroborate their theory. In fact, due to a number of technical factors, it was found that sound quality (distance, clarity, etc.) actually reached its zenith in mid to late afternoon. So much for that.
Sometimes, I think it's possible that birds
do sing for the sheer joy of it. Here in the western U.S., we have a species identified as the Rose-breasted Grosbeak (seen here:
http://www.northrup.org/photos/rose-breasted-g rosbeak/male-rose-breasted-grosbeak%20(6).htm )that lands on the top most branch of the tallest tree around, just before sunset and sings his heart out for about 30 minutes. It can only be described as utter joyfulness. I'm sure dedicated ornithologists will attribute purely natural instincts to the song... but I enjoy it immensely regardless...