Carotenoid-based traits commonly act as condition-dependent signals of quality to both males and females. Such colours are typically quantified using summary metrics (e.g. redness) derived by partitioning measured reflectance spectra into blocks. However perceived colouration is a product of the whole spectrum. Recently, new methods have quantified a range of environmental factors and their impact on reflection data at narrow wavebands across the whole spectrum. Using this approach, we modelled the reflectance of red integumentary eye combs displayed by male black grouse (Lyrurus tetrix) as a function of ornament size and variables related to male quality. We investigated the strength and direction of effect sizes of variables at each waveband. The strongest effect on the spectra came from eye comb size, with a negative effect in the red part of the spectrum and a positive effect in UV reflectance. Plasma carotenoid concentration and body mass were also related to reflectance variance in differing directions across the entire spectra. Comparisons of yearlings and adults showed effects were similar, but stronger on adult reflectance spectra. These findings suggest reflectance in different parts of the spectrum are indicative of differing components of quality. This method also allows a more accurate understanding of how biologically relevant variables may interact to produce perceived colouration and multi-component signals, and where the strongest biological effects are found.

Eye comb size and reflectance

Eye comb size was calculated by first taking a digital video recording of both eye combs against a scale for each individual. Left and right eye comb areas (cm²) were measured using ImageJ (Rasband 2012). Comb reflectance was calculated in the field using spectrophotometry (Ocean Optics USB2000) over the range of 320-700nm (Figure 2; Cuthill 2006, Montgomerie 2006, see Siitari et al. 2007 for methods in full detail). Reflectance was measured by placing the probe perpendicularly against the eye comb at a distance of 12mm from the surface. The strobe illumination area encompassed an area measuring approximately 1.5mm in diameter. This was replicated twice per comb side, varying the probe position and side (left or right) measured on each repetition. After every four measurements, a 99% white standard (LabSphere) was used to recalibrate the spectrophotometer. Sometimes, the spectrophotometer recorded negative values especially at <380nm – negative values were excluded as these were obvious probe errors. 

Other variables

For other condition measures, please see the paper for a detail description of each variable.  

There are two sheets. Sheet 2 contains a description of each column header and any associated notes. If something is not clear, I am happy to help. Please note that these data are also submitted to FigShare (the same file).