A harvester (400, 500) comprising: a pair of oppositely facing sidewalls, mechanically attached to a subframe; a pickup assembly (410, 510); a bale chamber (450, 540), positioned between the pair of sidewalls, comprising a crop inlet (451, 541) at the front end of the bale chamber (450, 540), and an outlet at the rear end of the bale chamber (450, 540); a crop supply chamber (200, 420, 520), positioned between the pair of sidewalls and in front of the bale chamber (450, 540), comprising: a supply inlet (203, 425, 523) at the bottom of the crop supply chamber (200, 420, 520); a front wall (201, 421, 521); a rear wall (202, 422, 522); a storage volume (210, 430, 528); a rear wall pivot point; and wherein said rear wall pivot point is positioned between the pair of sidewalls and operably connected to the rear wall (202, 422, 522), such that the rear wall (202, 422, 522) is selectively adjustable in a forward and a rearward direction around the rear wall pivot point; characterized in that the crop supply chamber (200, 420, 520) further comprising a movable control surface (204, 426, 524); wherein the storage volume (210, 430, 528) is defined by the space between the front wall (201, 421, 521), the rear wall (202, 422, 522), the pair of oppositely facing sidewalls, the control surface (204, 426, 524) and the supply inlet (203, 425, 523); and wherein the control surface (204, 426, 542) is able to move to a higher position in between the front wall (201, 421, 521) and the real wall (202, 422, 522), thus increasing the size of the storage volume (210, 430, 528); and to a lower position in between the front wall (201, 421, 521) and the rear wall (202, 422, 522), thus decreasing the size of the storage volume (210, 430, 528). a pair of oppositely facing sidewalls, mechanically attached to a subframe; a pickup assembly (410, 510); a bale chamber (450, 540), positioned between the pair of sidewalls, comprising a crop inlet (451, 541) at the front end of the bale chamber (450, 540), and an outlet at the rear end of the bale chamber (450, 540); a crop supply chamber (200, 420, 520), positioned between the pair of sidewalls and in front of the bale chamber (450, 540), comprising: a supply inlet (203, 425, 523) at the bottom of the crop supply chamber (200, 420, 520); a front wall (201, 421, 521); a rear wall (202, 422, 522); a storage volume (210, 430, 528); a rear wall pivot point; and a supply inlet (203, 425, 523) at the bottom of the crop supply chamber (200, 420, 520); a front wall (201, 421, 521); a rear wall (202, 422, 522); a storage volume (210, 430, 528); a rear wall pivot point; and wherein said rear wall pivot point is positioned between the pair of sidewalls and operably connected to the rear wall (202, 422, 522), such that the rear wall (202, 422, 522) is selectively adjustable in a forward and a rearward direction around the rear wall pivot point; characterized in that the crop supply chamber (200, 420, 520) further comprising a movable control surface (204, 426, 524); wherein the storage volume (210, 430, 528) is defined by the space between the front wall (201, 421, 521), the rear wall (202, 422, 522), the pair of oppositely facing sidewalls, the control surface (204, 426, 524) and the supply inlet (203, 425, 523); and wherein the control surface (204, 426, 542) is able to move to a higher position in between the front wall (201, 421, 521) and the real wall (202, 422, 522), thus increasing the size of the storage volume (210, 430, 528); and to a lower position in between the front wall (201, 421, 521) and the rear wall (202, 422, 522), thus decreasing the size of the storage volume (210, 430, 528).