Limited variation in visual sensitivity among bowerbird species suggests that there is no link between spectral tuning and variation in display colouration
Variation in visual spectral tuning has evolved in concert with signal colour in some taxa, but there is limited evidence of this pattern in birds. To further investigate this possibility, we compared spectral sensitivity among bowerbird species that occupy different visual habitats and are highly diverged in plumage and decoration colour displays, which are important in mate choice and possibly reproductive isolation. Microspectrophotometry of violet-, short-, medium- and long-wavelength-sensitive cones revealed no significant interspecific variation in visual pigment peak spectral absorbance values that ranged between 404–410, 454, 503–511 and 558–568 nm, respectively. Mean cut-off wavelength values for C-, Y-, R- and P-type coloured oil droplets were 418–441, 508–523, 558–573 and 412–503 nm, respectively, with values at longer wavelengths in ventral compared with dorsal retina cones. Low ocular media mid-wavelength transmission values (340–352 nm) suggest that bowerbirds may represent a transitional stage in the evolution from the ancestral violet-sensitive- to the derived ultraviolet-sensitive-type short-wavelength-sensitive-1-based visual system found in younger passerine lineages. Sequence data obtained for rod opsin and four cone opsin genes were identical at key tuning sites, except for an interspecific leucine-52-alanine polymorphism in the short-wavelength-sensitive 2 opsin. There was no obvious relationship between relative proportions of cone classes and either visual habitat or display colour. Overall, we detected little interspecific variation in bowerbird spectral sensitivity and no association between sensitivity and display diversity, which is consistent with the general trend among avian taxa.
