Plumage scoring & spectral reflectance
Methods used to obtain reflectance spectra from forecrown feathers using a spectrophotometer have been described in detail (Nwankwo et al., 2019). In brief, we used a JAZ spectrometer (Ocean Optics) with a fibre-optic reflectance probe (Ocean Optics R-200) and PX xenon light source to obtain two opposite orientation measurements per set of 3-5 forecrown feathers from each of 43 individuals for which we collected forecrown feathers. Reflectance spectra were obtained following calibration of the spectrophotometer with a white standard (Ocean Optics WS-1), and dark standard (by covering the fibre-optic connector with its lid). Replicate spectra were averaged in the R package pavo (Maia et al., 2013). Colour was estimated following (Endler, 1990), by computing measures of chroma and hue.
Plumage scoring was performed by two project participants that had no experience from the contact zone and hitherto had no involvement in the study (MM, BOO). They were asked to score forecrown colour of 78 individuals captured and photographed in the field on an integer scale from yellow (0) through orange (1) to red (2). To test whether the same SNPs might otherwise explain other carotenoid-based plumage differences between the species, supercilium colour was also scored from white (0) through cream (1) to yellow (2), and percentage coverage of yellow over black on the wing bar was also estimated (Fig. 1). We then compared the scores of the two observers using spearman rank correlation for each plumage patch, and with ρ > 0.85 in all cases, we averaged the two scores for further analysis. For forecrown colour we designated intermediate scores between red and orange as reddish and between orange and yellow as amber.