I know that birds sing and tweet to comminicate vital information to
other birds, but do birds ever just sing for the Hell of it; in other
words, are they entertaining themselves and other birds?
Being that birds have "bird-brains," it is unlikely that they know how
much we like it, but what's going on?

Here's an interesting article about why birds sing:

Why Files
http://whyfiles.org/shorties/114bird_song/

Although most birdsong is motivated by mating behavior and the staking
of territory, birds may sometimes sing just for the sheer pleasure of
singing:

"Generally, male birds are the singers. Some species of female birds
probably use a male's song as a measure of his experience. Thus, the
male's song can influence the female's choice of a mate. Songs also
help build bonds between bird couples by letting the female know that
her mate is nearby and all is well. At the same time, bird songs
express territory ownership. Male bird songs send a clear message to
other males of the same species that his territory is occupied and
will be defended. In a few species, females also sing, usually to
deter other females from the territory. The Northern Cardinal is one
example of a singing female bird. Sometimes, a male and female bird
will sing together. This is known as duetting. Scientists believe
that, primarily, hormones cause birds to break into song. However,
some authorities say birds sometimes sing for pleasure, play, or just
to release energy."

Teaching Tools
http://www.teachingtools.com/birdbrain.pdf

"Birdsong serves the two main functions of advertising a territory and
attracting females," says Eliot Brenowitz, a professor of Psychology
and Zoology who studies birdsong at the University of Washington.
Additionally, some scientists think birds also sing for the pure
pleasure of hearing their own voice."

American Museum of Natural History
http://research.amnh.org/users/nyneve/birdsong.html

"Birds sing for the same reason you talk - to communicate. Birds are
territorial - they have a place that they protect so they can use
their song or call to warn birds in their group that an outsider has
come into their territory and also to warn the intruder 'this is my
place - get outta here!' There are over 9000 different types of birds
that each have their own unique way of communicating. Different
birdsongs are like different languages so most birds from a different
species can't understand each other. Most birds sing at particular
times of the day, especially at dawn and dusk. And also like us
they'll sing for enjoyment or to chatter with other birds in their
community."

Suzy's World
http://www.suzy.co.nz/suzysworld/Factpage.asp?FactSheet=124

"...Many birds sing in summer when the season of courtship is over, or
in fall, and even in winter. Why? Evidently because they enjoy singing
for its own sake, just as we do. [F. Schuyler] Mathews goes so far as
to say (and few know birds so intimately) that they sing first for the
love of music and second "for the love of the lady". The skylark
sometimes continues to sing even when fighting. Caged birds sing
because they have nothing else to do. Wild birds announce daybreak by
song because the dangers of the dark are past. The sing when the fog
lifts, or after a shower, because they are glad that the sun has
returned. Like ourselves they sing at a feast. "The songbird", writes
Maurice Thompson, in his charming little book, Sylvan Secrets, "is a
gourmand of the most pronounced type, and we find him going into a
rapture of sweet sounds over a feast of insects or fruit. I have seen
a mocking bird eat the best part of a lucious pear or apricot, and
then leap to the top most stray of the tree and sing as if it would
trill itself into fragments for very joy of the feast."

Web-Helper
http://www.web-helper.net/PDMusic/Articles/51921/article1.asp

Here's something intriguing: while asleep, birds dream of singing!

"Songbirds rehearse their melodies by singing in their sleep, a new
study has found. Scientists believe the birds dream of singing to help
them hone a range of different tunes. The new finding emerged from a
study of the electrical brain activity of zebra finches.

A team from the University of Chicago found that sleeping birds fired
their neurons in complex patterns similar to those produced when the
birds were awake and singing."

BBC News
http://news.bbc.co.uk/1/hi/sci/tech/992538.stm

Search terms used:

"why do birds sing"
"birdsong" + "enjoyment"
"birds sing" + "enjoyment"
"birdsong" + "pleasure"
"birds sing" + "pleasure"
"birdsong" + "fun"
"birds sing" + "fun"

Thanks for asking a fascinating question! I hope I've provided the
kind of information you were seeking. If anything I've said is
unclear, or if a link does not function, please request clarification;
I'll be glad to offer further assistance before you rate my answer.

Best wishes,
pinkfreud

Isn't thinking fun?
I'm glad that you enjoyed this question too.


Yesmam