The common whitethroat is a summer migrant to Britain that winters in the Sahel zone of Africa. A catastrophic drought in 1968-69 reduced the UK population by 75%, demonstrating the vulnerability of Sahel-dependent migrants.
Saharan crossings are energetically demanding and dangerous, with many birds dying of exhaustion or predation. Droughts in the Sahel reduce body condition before the northward migration begins, increasing mortality during crossing. Scrub clearance in the UK removes the nesting habitat that migrants need upon arrival after a 4000km journey. Cats and sparrowhawks cause significant predation pressure on migrant warblers in spring when birds are exhausted and weight-depleted.