If you have a Windows Mobile device and if the option: "Fill Pocket 'Display Name' field 'Last name, First name' or 'Company name"' is checked in BirdieSync options, the "Display name" synchronized on the mobile device is built from the "First name" and "Last name" field for instance and the original value of the "Display name" present in Thunderbird is ignored. So even if you modify this field, the contact will be considered unchanged and not be synchronized since the values of the "First name" and "Last name" field haven't changed.