Females and young males have the whole upper plumage a rich chocolate brown, with reflections of purplish grey, each feather on the back and upper wing coverts tipped with a somewhat lighter brown, and crossed by a buff bar, which is usually not to be seen except when the plumage is disarranged or ruffled. Each feather also has a black shaft, which is conspicuous in strong lights on a close view. The primaries and all the principal feathers of the wing are very dark brown on the upper surface, barred with several patches of light brown or buff. The under surface of the wing is very light silvery grey, with numerous bars and spots of brownish grey, each feather having a dark grey shaft, which is white underneath. The tail feathers on the upper surface are of a slightly lighter brown than the back, and light grey underneath, barred with more or less oblique bands, which are buff-coloured above, and light grey-brown underneath, and are all tipped with white. The under plumage of the body is creamy white, more or less tinged with light buff, especially on the sides of the head and throat. It is liberally streaked with longitudinal splashes of dark brown, which on the upper throat are very small, but on the lower flanks are broad and large. There is a facial patch and a moustache of dark brown, but these are not so strongly marked as in the peregrine and hobby. The beak is light blue, darkening to indigo, and at the tip to black. The cere and eyelids, light bluish grey. The legs and feet vary from light greenish or blueish grey to light yellow. The toes are long, thin, and flexible. Adult females do not change, except that they lose much or all of the purplish sheen of nestlings, and that the edging of the feathers is less marked. Adult males undergo a very striking transformation. The whole upper plumage changes from brown to a rich bluish slate colour, deepening in the long wing feathers to greyish black. Instead of the light bars on the tail, there is a single broad grey-black band nearly at the extreme end. The breast at the same time assumes a warmer tint, deepening from cream colour at the chin to a rich buff lower down, and deep russet at the flanks. The cere, eyelids, legs, and feet assume a deep golden or light orange colour. The wing and tail feathers have a stronger and stiffer appearance than before the moult, and those of the tail are generally somewhat shorter as well as stouter than they were. Very old females occasionally, but not often, put on the livery of the adult male; and this is the case sometimes also with old female kestrels. In merlins of both sexes the third feather of the wing is usually exactly equal in length to the second, and it is only exceptionally that it is even fractionally shorter.