Do mocking birds mock

I live in Phoenix, AZ. As a bird watcher, I have observed some of the Northern Mockingbirds in my area over the years. They seem to disappear from Phoenix during the hottest months (over 100F+). I believe they must migrate north to where it is cooler between May and late September. (I have no specific evidence of where they go though so it's just a guess.) One particular individual bird arrived back to his tree outside my office building entrance this week. I arrived to work Oct 1 to hear his voice singing out his distinctive mockingbird song. Sure enough there he was at the top of his favorite tree. :)

I live in Texas, about an hour east of Dallas... My wife and I have also noticed that the Mockingbirds vanish about August 1st and are not seen again until about September 10th... We noticed this behavior 3 years ago and have verified the occurance each year aferward...I've talked to game & fish ornithologists and they seem to be totally unaware of this annual exodus... I would like to know more about it... If any of you know more, please contact me... Thanks...