Geographic variation in secondary sexual plumage colour characteristics of the male Pied Flycatcher

The plumage colour of the head and back of the male Pied Flycatcher Ficedula hypoleuca varies from a female-like brown to pure black, and the mean colour of a population varies geographically, with lighter coloured populations found in Central Europe and darker populations in Fennoscandia. In dark populations the blacker males are significantly larger than the brown males, while in light populations there is no significant size dimorphism between brown and black males. Brown males probably survive better than black males, irrespective of the population they belong to, because they have a higher return rate. The tendency of light populations to have higher rates of polygyny among the males than dark populations is insignificant. Two different hypotheses are proposed to explain the adaptive significance of this geographical variation in plumage colouration: (1) The predation rate varies geographically and affects colouration; this is unlikely, however. (2) The overlap in Central Europe with a sympatric species, the Collared Flycatcher Ficedula albicollis has led to selection for lighter colour of the Pied Flycatcher populations there. This seems to provide a reliable explanation, but the present results did not indicate which of the three sub-hypotheses (a) Female choice of the right species, (b) Fighting Ability, or (c) Interspecific Female Mimicry provides the best explanation for this variation. These alternatives need to be further investigated.