Thurstan is a very talented falconer who finds and trains the best birds in the world. His job as Grand Falconer to King William was prestigious and important.
The real Thurstan was born in Oiully de Basset, a small village in Normandy. Through family connections he was drawn into the orbit of Guillaume, Duke of Normandy and future King. He fought against Norman nobles who tried to steal Guillaume’s title and so earned Guillaume’s never-ending gratitude and trust. In the story, Thurstan is especially sensitive to Guillame’s status as a batard, or bastard, hence his reluctance to discuss with Guillame Isabelle’s desire to annul their marriage, which would make their son Ralph a bastard.
In real life William punished anyone who brought up his bastard status, so he really is friends with Thurstan. In real life, Thurstan is buried in Abingdon, England, about 80 miles from his estate in Drayton Bassett.