If you want to use BirdieSync with calDAV calendars, it is necessary to patch Lightning or Sunbird to correct some issues.
Download calDavCalendar.js version 2 (2007/10/29) and after backuping the original file close Thunderbird or Sunbird and install it in:
Thunderbird:
C:\Documents and Settings\<users name>\Application Data\Thunderbird\Profiles\<profile id>\extensions\{e2fda1a4-762b-4020-b5ad-a41df1933103}\components
Sunbird:
C:\Program Files\Mozilla Sunbird\components
This patch was released for Lightning and Sunbird 0.7. Don't use it with another version.