Slagsvold T. (1993) Sex recognition and breast stripe size in Great Tits. ARDEA 81 (1): 35-41

Great Tits Parus major have a conspicuous, black, central breast stripe which is larger in males than in females. I manipulated stripe size in females to make them resemble males. The response of resident pairs was tested by presenting a caged, dyed female on territories during the breeding season, and using caged, unmanipulated males and females as controls. Resident females responded aggressively towards control females and similarly towards dyed females, but not towards males. Thus, females appeared to be able to recognize the sex of a dyed female, even when its lower belly between the legs was dyed black. In contrast, males were significantly more aggressive towards dyed than control females, though less aggressive than towards control males. In comparison, both sexes of Pied Flycatchers Ficedula hypoleuca have difficulty identifying the true sex of conspecific dull-coloured males (female-like) and black dyed females (male-like). I suggest that the female mimicry hypothesis for the evolution of delayed plumage maturation in passerine birds is most relevant for migratory, solitary-living species which have pair bonds of short duration, allowing little sexual experience.