In The History and Topography of Ireland, Gerald of Wales* claims to have actually seen them:
I have myself seen many times with my own eyes more than a thousand of these small bird-like creatures hanging from a single log upon the sea shore.
Other accounts rely on other authorities, but Gerald says he saw them with his own two eyes "many times." Assuming he isn't lying (and since he has a lot more interesting creatures, why lie about this one?), are there any modern theories about what exactly Gerald was looking at to cause him to think these geese grow out of barnacles on trees?
*By the way, Wikipedia, in all its glory, refers to Gerald of Wales as "a contemporary English author." I wonder if they thought his last name was "of Wales?" Heck, since he was half-Welsh and half-Norman, he'd no doubt have taken that as a double insult.