Adaptive Value

The major benefit of mate choice based on ornamental traits such as UV coloration is to allow the females to select the fittest mates and provide the best genes for their offspring.  Birds, however, have several different easily visible external characteristics besides UV plumage that might also potentially be honest signals of mate quality; the question is why UV-reflecting feathers make such a good representation of mate value, and why the UV range might be more conducive to sexual signaling than other wavelengths.

Allowing UV light to reach the retina can actually have negative effects; UV tends to damage the retina over time, and it doesn’t provide as clear an image, often making the perceived image slightly unfocused or blurry (Hart and Hunt 2007).  The fact that UV light is used, then, indicates that there is some advantage that outweighs the potential detrimental effects.


This image of the underside of a saw-whet owl’s wing shows bright plumage under black light.


The Honest Signal Hypothesis

One hypothesis proposed to explain the use of UV signals stems from the fact that ultraviolet colors are produced not by pigments in the feather, but by light scattering off the structural conformation of the feather barbs (Prum et. al. 2003).  UV hues are therefore considered to be “structural colors” rather than pigment-based ones.  Because of this, UV colors are a good representation of male quality and condition because unlike pigments, structurally based colors degrade as feather condition declines; basically, a bird with UV plumage becomes less “bright” as the feathers become older or the bird grows unable to properly care for them (Keyser and Hill 1999).  Structural coloration would also be less bright if the male did not get enough proper nutrition during molt when new feathers were growing in; this would also be a sign to females that the male was of low quality (Siefferman and Hill 2003).  In 2009, a study of budgerigars (Melopsittacus undulatus) found that males with greater UV reflectance had stronger immune responses than males with duller UV-colored plumage, indicating that such males were healthier and therefore better mates (Griggio et. al. 2009).


Anti-Predator Benefits of UV

Several other hypotheses exist to explain the use of UV plumage as an honest quality indicator.  For example, ultraviolet light, a shorter wavelength than visible light, tends to be scattered in the air more quickly than longer wavelengths such as blue or green.  UV light is therefore only really useful over short distances, so that it could be a good way for males to display without attracting unwanted attention (Hunt et. al. 2001).  In many cases, mammals are unable to detect UV wavelengths, which means that a potential predator would find it hard to distinguish a bird brightly colored in the UV from its background environment (Hausmann et. al. 2002), though a recent paper suggests that this explanation is largely irrelevant for many songbirds whose primary predators are raptors perfectly capable of seeing UV light (Stevens and Cuthill 2007).


Picture A (left) shows the head of a male budgerigar under normal visible light;
picture B (right) shows the head of the same budgerigar under UV light.


Environmental Effects and Sensory Exploitation

In terms of benefits to the birds, UV light contrasts strongly with the green, grey and brown environment of most woodland birds, making it an ideal method of display in such habitats (Andersson et. al. 1998).  “Sensory exploitation” theory proposes that UV-colored plumage evolved because of birds’ ability to see in the UV range; essentially, that birds could perceive UV light, so their feather patterns evolved to take advantage of that part of the spectrum (Hausmann et. al. 2002).


A woodland environment such as this one, with lots of greens and browns,
provides high contrast for UV signals.


Potentially all of these factors could play a part in the function of UV-related mate choice.  Most importantly, the structural UV colors are honest representations of male quality, and are therefore relatively reliable signals on which females can base their mate choice.