Plumage coloration analyses
To objectively describe plumage colour variation within T.
ruficapillus , we obtained reflectance spectra measurements from museum
skins deposited at the MACN (Table S3). We measured five males and four
females of T. r. cochabambae and 30 males and 18 females ofT. r. ruficapillus , which are the only two subspecies present in
Argentina and available in the MACN collection. We used only adult
specimens that were in excellent preservation condition for reflectance
measurements.
Male plumage in this species is mostly brown (T. r. cochabambaeand T. r. ruficapillus ) or dark gray (T. r. jaczewskii ,T. r. marcapatae and T. r. subfasciatus ), with rufous
wings, a rufous or chestnut cap and barred underparts and tail. The
species is sexually dichromatic, with brown females in all subspecies
(i.e., no gray plumage) and either lack or very faint barring in their
underparts and no white marking in their tales (Figure S2). Taking into
account this colour patterning, we measured reflectance in six plumage
patches on each specimen: throat, breast, belly, crown, nape and back.
All reflectance measurements were performed with an Ocean Optics USB
2000 spectrometer (Ocean Optics Inc., Dunedin, Florida) with a PX-2
pulsed xenon light source (effective range of emission from 220 to 750
nm), calibrated against a WS-1 diffuse reflectance white standard (Ocean
Optics, Inc., Dunedin, Florida, USA). Plumage was illuminated and
reflectance data were collected with a bifurcated probe housed in a
prismatic holder that was held against the chosen region on the study
skin. The probe was held with an angle of 90º to the surface of the
plumage patch. The diameter of the circular measured area was
approximately 6 mm, and the distance between the probe and the plumage
was 23 mm.
We performed analyses using the pavo package in R (Maia et al. 2013).
Because this species is sexually dichromatic, males and females were
analyzed separately. We evaluated colour differentiation in the
chromatic component between subspecies for each plumage patch by
estimating a perceptual distance (∆S) using the Vorobyev and Osorio
(1998) colour discrimination model. Colour perceptual distances are
expressed in terms of just noticeable differences (jnd) and a value of
1.0 jnd represents the theoretical threshold for discrimination of two
colours, meaning that it is the distance in the perceptual colour space
at which two colours would be visually discernible (Barreira et al.
2021). Thus, ∆S > 1.0 jnd indicates significant colour
discrimination by the birds in all light conditions (Vorobyev et al.
1998, Siddiqi et al. 2004). In addition, we determined the statistical
significance of the estimated ∆S values for each plumage patch
comparison between subspecies (Maia and White 2018). For this, we
estimated the geometric average of ∆S values for all comparisons
performed between subspecies and calculated the 95% confidence interval
(CI) through a bootstrap analysis with the bootcoldist function of pavo
in R. We also tested the significance of these comparisons through a
PERMANOVA analysis with the adonis function of the vegan package in R
(Oksanen et al. 2008).