Parasites and plumage: an experimental field test of the parasite-mediated handicap hypothesis

Male Steller's jays (Cyanocitta stelleri) display structurally-based non-iridescent blue plumage which reflects light in the ultraviolet (UV) range, ornamental crests and colored patches. The parasite-mediated handicap hypothesis argues that ornaments are costly to produce and can be influenced by susceptibility to parasites. Consequently, the extent of ornament expression can act as honest indicators of quality, which are assessed by mates or competitors. I quantified structurally-based plumage color and ornament size in relation to individual condition (feather growth rate) and occurrence of disease caused by a parasite infestation (knemidokoptic mange) in wild male jays. I then experimentally reduced parasite load prior to molt and quantified the relative change in plumage and condition. Using reflectance spectrometry to quantify feather color, I found that birds with heavier body mass, independent of body size, had higher feather growth rates which suggest that birds which grew their plumage faster were in better condition during the post-breeding molt. I also found birds in better condition had lower hues (i.e., reflected shorter wavelengths, possibly displaying a more intense, brilliant blue color), suggesting a possible condition-dependent production of feather color. I found no support for the parasite-mediated handicap hypothesis, as UV reflectance was positively related to the occurrence of disease. I detected no effect of experimental treatment on plumage ornaments or condition. In conclusion, I found no evidence in support of the parasite-mediated handicap hypotheses, but found some evidence for condition-dependant development of structurally based blue plumage in adult male Steller's jays.