Capsule The blue colour of the coverts and crowns of Blue Tits Cyanistes caeruleus and Great Tits Parus major is influenced by sex, age and environmental factors.\ud Aims To quantify the blueness of two species of tits breeding in different habitats in Britain.\ud Methods By manipulating daylength in the laboratory, adult Blue Tits were induced to moult at fast and slow speeds. When they had finished, the blue colour of their wing coverts and crowns was measured (in the range visible to human observers), and compared with the colour of birds caught in the field.\ud Results As well as highly significant sex and age differences in colour, Blue Tits were 26% bluer in 1998 than 2000, and male Great Tits were 15% less blue in small woods than in large woods in our study population in East Anglia, England. Both species became more saturated with blue up to the age of three years. British Cy. c. obscurus were darker blue than Cy. c. caeruleus of Continental Europe .\ud Conclusions There are significant environmental influences on the blueness of British tits in addition to the well-known age and sex effects.\u
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