Luc Lens and his collegues investigated whether high levels of asymmetry in bird populations could be used to predict vulnerability to extinction. They compared the length of the left and right foot bones of 260 Taita Thrush over a period of 3 years. Fewer than 1,400 of the critically endangered Taita Thrush survive in just three remnants of the Taita Hills forest in southeastern Kenya - the highly degraded Chawai, the moderately degraded Ngangao and the virtually undegraded Mbololo forest fragment.