A header for an agricultural harvester has a frame with a floor surface and an auger to laterally transport crop material towards a central aperture, wherein a cutter bar forms the leading edge of the header and is moveable via a slide between a retracted and an extended position, the header further includes a belt conveyor for transporting crop material from the cutter bar to the auger, the belt conveyor is provided with a belt-length-adapting mechanism so that the length of the belt is changeable between a short track and a long track respectively corresponding to the retracted position and the extended position.