Narrow down which module is causing a PHP warning
FIELD_BEHAVIOR_DEFAULT is defined in the core field.module file, which will always be loaded into the page build if you have the core field module enabled.

If you're getting an error that it's not defined then either the field module isn't being loaded for whatever context you're receiving the error in, or the field.module file has been corrupted/changed.

Try getting the field.module file from a fresh copy of Drupal and replacing it on your site, that might fix the problem. If not, grep your sites/all/modules folder for the string FIELD_BEHAVIOR_DEFAULT. That should help you narrow down the modules that use it, and which ones you should try to disable.