Can anyone explain this pun to me?

[ameliasj]

"Two attractive female birds were showing off in front of some males. Both had spent two hours at the hairdresser, but it was the curly bird that got the perm."

I find this pun in www.punoftheday.com, but I am not quite understand the pun. Can you explain this to me? Why it is a pun? Which word or words is pun?
Thanks!

[Tomasz Klimkiewicz]

..lol..it's a good one!

The pun is based on the similarity (sound-wise) of the phrase:

"...the curly bird that got the perm."

to the popular English proverb:

"The early bird gets the worm."