The Oxford Names Companion, OUP "Scottish Forenames" - Donald Whyte, FGH, FSG
Private correspondent [TL
Notes:
A generic English pet name for names starting or ending in the element "-bert-" [see examples above]. We understand that this pet name is also used in the same way Dutch-speaking communities.
There is no connection with the female name Bertha.
Thanks to Donald Whyte who provided much of the initial information for this site.