Blue Tit; Herrerillo común

Small passerine bird in the tit family Paridae. The bird is easily recognisable by its blue and yellow plumage, but its scientific classification is disputed. The Blue Tit is usually 12 centimetres long with a wingspan of 18 centimetres. Blue Tit has an azure 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 is 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 irides dark brown. The sexes are similar, but under ultraviolet light, males have a brighter blue crown. Young Blue Tits are noticeably more yellow.