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.