Sjaerdema came from Ylst or IJlst (Elostoe in Latin) and was elected to Potestaat by the men of Friesland in 1237.
Sicko had a military success in 1252. Abel, the "King of Norway" fell in East Friesland. The Frisians killed king Abel and many of his troops on 12 June 1252. Petrus Thaborita had the year as 1250 which is almost correct, and that Abel was king of Denmark.