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; a biasing element configured and arranged to bias the chain guide assembly for rotation in a first rotational direction 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 opposite to the first rotational direction, 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 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; a biasing element configured and arranged to bias the chain guide assembly for rotation in a first rotational direction 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 opposite to the first rotational direction, 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 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.