When do most Southwest snowbirds leave?

I've watched this for 11 winters here in Yuma. I won't comment on the Canadians leaving because their out of country insurance usually determines when they must leave. I have heard so many reasons why US folks who spend the bucks to get out of lousy weather, drive many miles to spend time in the warmer climates leave early to just drive 500-700 miles to be sitting out a snow storm etc. on their way home. A very common answer is "to do income taxes". Shoot that can be taken care of any where in the world today. March, it is starting to get nice here, April is terrific. We always have emails from friends who will say "we left too early". I really think "hitch itch" sets in and they see the outstanding weather here and for some unknown reason thinks it will be similar at home. I always get a hoot out of when the temp hits about 82 you here that familiar "hmmmmmm" as the roof ACs are kicked on. Unbeivable. From the later part of March thru April the traffic is gone and Wally_Mart has thinned out. We ourselves usually leave the end of April or beginning of May depending where we are headed for the summer.