Specific immunity and polymorphism of breeding plumage in pied flycatcher (Ficedula hypoleuca) males (Aves: Passeriformes)

Abstract

The relationship between the type of melanin-based plumage colouration and the strength of experimentally induced immune response was studied using as an example a pied flycatcher population from the Moscow region. The breeding plumage of pied flycatcher males exhibits the full spectrum of transitions from contrasting black-and-white to cryptic brownish, the latter being very similar to the colouration of females. In spite of numerous studies, the nature of this polymorphism still remains vague. Unlike many other avian species with monocyclic breeding, a considerable fraction of pied flycatchers overlaps two energy-consuming productive processes, breeding and moult, over the whole species range. During the main experimental treatment we activated the humoral immunity of free-living males in chick-rearing period by injection of nonpathogenic multifactorial antigen (sheep red blood cells, SRBC) and estimated the strength of the immune responses after repeated captures in 6–8 days. In addition, after each capture we evaluated the numbers of leucocytes (WBC), heterophil to lymphocyte ratios (H/L) and measured night time basal metabolic rates (BMR). Non-moulting males of different colour types showed the same immune responses. Among moulting birds, the strength of the immune response was significantly higher in pale males (morphs 4–7 by Drost’s scale) than in bright males with rich melanin-based colouration (morphs 2–3). This difference resulted from two opposite processes. During moulting, pale males heightened the antibody titer after immunization, while bright males tended to reduce the strength of immune response. Possibly such an asymmetry in immunocompetence at the first stage of moult reflects the different life strategies of pied flycatcher males—conspicuous birds less commonly overlap breeding with moult than cryptic ones.