Birds have long been maligned for being dumb--hence the term "birdbrain"--but they're actually pretty smart. They can navigate thousand-mile migrations, they use tools, they solve problems, they recognize faces. And their songs for defending territory and attracting mates are complex. Interesting note: in temperate zones, the males sing and the females call; in tropical zones, males and females will often sing in "duet."
For today's purposes, I'll just mention how much I enjoy hearing my backyard birds sing, the occasional hooting of an owl if I'm very lucky, and even the screech of a blue jay.
Bird Academy has great resources about birdsong. And I'll leave you with this short clip: