1. 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.
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. It is sug-gested
that this stimulated timely plant growth, so improving the laying 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 successfully 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.
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