Ravens center Matt Birk missed practice for a fourth consecutive day Thursday. The reason for his absence has not been disclosed, though head coach John Harbaugh said on Tuesday that the 36-year-old is at the stage of his career where he doesn't have to practice every day.

Rookie Gino Gradkowski has been filling in for him at center.

There was still no sign of defensive tackle Haloti Ngata (hamstring) and offensive tackle Bryant McKinnie (back). Neither has practiced in training camp so far, though Harbaugh said Wednesday that Ngata is getting closer.

Rookie linebacker Courtney Upshaw (arm) and rookie running back Bernard Pierce (hamstring) also did not practice.

There were two new players sitting out Thursday: defensive tackle Arthur Jones and guard Bobbie Williams.

The list of non-participants included wide receivers Patrick Williams (undisclosed), David Reed (knee) and Tandon Doss (hamstring); linebackers Josh Bynes (undisclosed) and Terrell Suggs (hamstring); and offensive tackle Jah Reid (calf). Tight end Dennis Pitta (hand) remains sidelined, but he did make a cameo at Thursday's practice.