The number of capercaillie in Scotland has fallen since the 1970s. Previous work showed that low breeding success, exacerbated by deaths of fully grown birds flying into forest fences, was the primary cause of the decline. The hypothesis that climate change caused the lower breeding success was investigated in this study. 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. Hens reared more chicks when the temperature rose more in early April. It is suggested that this stimulated timely plant growth, so improving the laying hens’ plane of nutrition and the viability of their chicks. 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 successfully in warm dry conditions. However, neither temperature nor rain days in late May or early June showed any trend during the study. Increasingly protracted spring warming seems to have been a major cause of the decline of the capercaillie in Scotland.

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