R.J.
The old "quick copyright" was just to us the US Postal system. Make a copy (recorded or written version) of the song and mail it to yourself and leave sealed.
Newer... post it on YouTube.
Of course... to actually copyright... one has to go through the agency.
((Edited to add: Intellectual theft is highly irritating, but it happens all the time. I've heard some of my husband's melodies on the radio played by someone he jammed with the year before... I have friends who've had stories stolen -one thing you'll notice about authors is that we almost never give out storylines before they've been published unless it's with an established character. There's no such thing as a new plot : boy meets girl, good guys chase bad guys, end of the world as we know it, etc.. but storylines are fingerprint like. A fairly notorious one is a college student who told her roommate about a short story she was writing, who then told her dad who told, who told,... and a 18mo later the short story was a tv show. The story had (fortunately) already been published at that point... but it was something of a mess legally speaking. Because while the storyline was the college girl's, thousands of man hours and hundreds of people... not to mention a team of writers, all working out of their own minds had created this show... so it wasn't considered theft. They didn't even have to put a "based on" tag at the beginning of the show because they settled.
Anyhow.. yeah... highly irritating to hear someone else playing a riff my husband created 10 years ago, but that's just one of the dangers of preforming arts. And they can do it, copy right or not, and legally, as long as the entire song isn't my H's. Because it falls under "influences". Art, like science, stands on the shoulders of others. At least, popular art does. Anything truly original gets booed offstage in the beginning at best. There is no piece of popular music that doesn't have it's roots in other popular music. Things people think of as original (like Stomp, for example, or Blues, or Elvis, etc.) all have strong influences from something else that is widely accepted that is just looked at in an innovative way.