A bicycle rear derailleur comprising: a base member mountable to a bicycle frame ; a movable member movably coupled to the base member ; a chain guide assembly rotatably connected to the movable member for rotation about a rotational axis (R); a biasing element configured and arranged to bias the chain guide assembly for rotation in a first rotational direction (T) with respect to the movable member ; and a fluid damper having a fluid cavity containing a volume of fluid, the fluid damper being operatively disposed between the chain guide assembly and the movable member to apply a damping force to the chain guide assembly when the chain guide assembly rotates in a second rotational direction (D) opposite to the first rotational direction (T), wherein the fluid cavity comprises a damping chamber arranged to hold a portion of the volume of fluid to provide the damping force and a return chamber in fluid communication with the damping chamber , characterized in that the fluid damper includes a compensation device in fluid communication with the fluid cavity , the compensation device being configured to accommodate a change in the volume of fluid in the fluid cavity, wherein the compensation device is in fluid communication with the return chamber , wherein the compensation device is configured to adjust the volume of the return chamber upon the change in the volume of fluid, wherein the compensation device includes a variable volume expansion chamber (S) in fluid communication with the return chamber , wherein the compensation device includes a movable body (240; 416; 436) received in an auxiliary bore (412; 432) which is in fluid communication with the return chamber , a surface of the movable body (240; 416; 436) being either directly, or indirectly via a flow channel exposed to the fluid in the return chamber . a base member mountable to a bicycle frame ; a movable member movably coupled to the base member ; a chain guide assembly rotatably connected to the movable member for rotation about a rotational axis (R); a biasing element configured and arranged to bias the chain guide assembly for rotation in a first rotational direction (T) with respect to the movable member ; and a fluid damper having a fluid cavity containing a volume of fluid, the fluid damper being operatively disposed between the chain guide assembly and the movable member to apply a damping force to the chain guide assembly when the chain guide assembly rotates in a second rotational direction (D) opposite to the first rotational direction (T), wherein the fluid cavity comprises a damping chamber arranged to hold a portion of the volume of fluid to provide the damping force and a return chamber in fluid communication with the damping chamber , characterized in that the fluid damper includes a compensation device in fluid communication with the fluid cavity , the compensation device being configured to accommodate a change in the volume of fluid in the fluid cavity, wherein the compensation device is in fluid communication with the return chamber , wherein the compensation device is configured to adjust the volume of the return chamber upon the change in the volume of fluid, wherein the compensation device includes a variable volume expansion chamber (S) in fluid communication with the return chamber , wherein the compensation device includes a movable body (240; 416; 436) received in an auxiliary bore (412; 432) which is in fluid communication with the return chamber , a surface of the movable body (240; 416; 436) being either directly, or indirectly via a flow channel exposed to the fluid in the return chamber . <EOS>