Parasites and plumage in Steller's jays: an experimental field test of the parasite-mediated handicap hypothesis

Abstract

The handicap hypothesis broadly argues that ornamental traits and displays are costly to produce and are a function of individual quality and condition. Consequently, the extent of ornament expression can act as an honest indicator of quality, condition or susceptibility to parasites, and influence the behavior of mates or competitors. We used reflectance spectrometry to quantify non-iridescent, ultraviolet (UV) and blue structurally-based plumage color and character size in relation to individual condition (feather growth rate) and occurrence of disease caused by a parasite infestation (knemidokoptic mange) in wild male Steller's jays (Cyanocitta stelleri). We then attempted to experimentally reduce parasite load prior to molt and quantified the relative change in plumage and condition. Those birds with faster feather growth rates (i.e., in better condition) had lower hues (i.e., reflected shorter wavelengths, displaying a more intense, brilliant UV-blue color), suggesting the possibility of condition or feather growth rate-dependent production of color. We found that UV reflectance (UV chroma) was positively related to the occurrence of disease. We detected no effect of experimental treatment on plumage or condition. In conclusion, our results are suggestive of possible condition-dependent development of structurally-based plumage, and the positive relationship between macro-parasite-caused disease intensity and UV blue plumage color, demonstrated for the first time in this study, did not support the parasite-mediated handicap hypothesis.