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The Eurasian blue tit is usually 10.5–12 cm (4.1–4.7 in) long with a wingspan of 17.5–20 cm (6.9–7.9 in) for both sexes, and weighs about 11 g (0.39 oz) A typical blue tit has an intensively blue crown and dark blue line passing through the eye, and encircling the white cheeks to the chin, giving the bird a very distinctive appearance. The forehead and a bar on the wing are white. The nape, wings and tail are blue, and the back is yellowish green. The underparts are mostly Sulphur-yellow with a dark line down the abdomen—the yellowness is indicative of the number of yellowy-green caterpillars eaten, due to high levels of carotene pigments in the diet The bill is black, the legs bluish grey, and the irises dark brown. The sexes are similar and often indistinguishable to human eyes, but under ultraviolet light, males have a brighter blue crown Juvenile birds are more yellowish and have fewer contrasting colors. They become similar to mature ones in September, although some parts of the wings are kept until May/June next year