In Texas, and some other states, there is a documented shortage of physics teachers, in terms of both number and qualifications. The shortage in Texas is due as much to teachers leaving the field (attrition) as to a lack of teachers entering. There are efforts under way to prepare more and better-qualified physics teachers who will stay in the field longer, but increasing the overall supply and retention will not necessarily address "localized" teacher shortages. To investigate this issue, we obtained a database cataloging every teacher who taught science in Texas public schools from 2003 to 2008, indicating the school where they taught during each of those years, the subjects they taught, and their route to certification. This allowed us to track not only teachers entering and leaving the public school system, but also migration between schools within the system. We found that migration poses a much bigger problem than attrition for some schools. We also found patterns in the movement of physics teachers in Texas that we would not necessarily have predicted and that varied substantially depending on certification.