Climate change and breeding success: decline of the capercaillie in Scotland

1. The number of capercaillie in Scotland has fallen since the 1970s. Previ ous work showed that low breeding success, exacerbated by deaths of fully g rown birds flying into forest fences, was the primary cause of the decline. The hypothesis that climate change caused the lower breeding success was i nvestigated in this study. 2, Temperature usually rose during April. There was no trend in mean April temperature during the study (1975-99) but there was a progressive cooling in mid-April relative to the rest of the month, such that the normal April warming was increasingly delayed. 3. Hens reared more chicks when the temperature rose more in early April. I t is suggested that this stimulated timely plant growth, so improving the l aying hens' plane of nutrition and the viability of their chicks. 4. Hens also reared more chicks when late May was warmer and early June was warmer and had fewer rain days. Young chicks may have foraged more success fully in warm dry conditions. However, neither temperature nor rain days in late May or early June showed any trend during the study. 5. Increasingly protracted spring warming seems to have been a major cause of the decline of the capercaillie in Scotland.