Sexing Blue Tits may be rather straight forward when it comes to typical individuals, but quite many are more difficult (especially 1cy/2cy) and some should be left undetermined. Do not use wing length. Always start with ageing.
Male (1cy) November. Note broad and dark marine blue side of neck as well as a deep blue crown. The dark eye-stripe is on average broader and more distinct (darker), but intermediates are frequently seen. [1ET81027]
Female (3cy+) March. Note less broad and somewhat paler blue side of neck as well as a paler blue crown. On average more narrow and less distinct dark eye-stripe. [1ES23352]
Male (1cy) November. Note on average deeper ultramarine blue in LC, MC, GC and in the edges of SS. [1ET81027]
Female (2cy+) October. On average less deep blue (and often with a faint greenish tinge) in LC, MC, GC and in the edges of SS. However, difficult individuals are regularly seen. [CM10453]