An oestrus detector 100 includes a reservoir portion 135 which contains a colored liquid. The detector 100 further includes a generally planar portion 110 that is attached to and sealed from the reservoir portion 135, except for an open or closed passage 111 in a seal line separating the reservoir portion 135 and the generally planar portion 110. In use the liquid is squeezed from the reservoir portion 135, through the passage 111, and into the generally planar portion 110, when the reservoir portion 135 receives pressure.