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The blackcap is a mainly grey warbler with distinct male and female plumages. The nominate subspecies is about 13 cm (5.1 in) long with a 7–8 cm (2.8–3.1 in) wing length. The weight is typically 16–25 g (0.56–0.88 oz), but can be up to 31 g (1.1 oz) for birds preparing to migrate. The adult male has olive-grey upperparts, other than a paler grey nape and a neat black cap on the head. The underparts are light grey, becoming silvery white on the chin, throat and upper breast. The tail is dark grey, with an olive tint to the outer edge of each feather. The bill and long legs are grey, and the iris is reddish brown. The female resembles the male, but has a reddish-brown cap and a slightly browner tone to the grey of the upperparts. Juveniles are similar to the female, but their upperparts have a slight rufous tinge, and the breast and flanks have a more olive tone; young males have a darker brown cap than their female counterparts. This species is unmistakable; other dark-headed Sylvia species, such Sardinian and Orphean warblers have extensive black on the head instead of a small cap. They are also larger and have white edges on the tail.
