Color of External Features
Tree Swallows provide an excellent example of countershading (strong dark coloration dorsally and highly contrasted white shading ventrally). Countershading allows for birds to reduce the contrast between their shape and the surrounding environment, therefore aiding in concealment. Generally, greater contrast in coloration of a bird reflects a greater illumination from above (Thayer, 1909).
Tree Swallows are unique in that first year females display delayed plumage maturation, appearing a dull brown dorsally although they are sexually mature. By contrast, first year males do not display delayed plumage maturation and appear blue-green dorsally in their first year. They, too are sexually mature although evidence suggests that they are not as successful as returning males at securing a mate (Thompson, 1991). Although there is no delay in male plumage maturation, it is believed that as males mature, their plumage becomes brighter to optimally reflect light at a shorter wave-length (blue). Additionally, evidence suggests that males who appear more green may express lower nest fidelity and/or a lower survival rate than do their blue counterparts. Therefore both brightness and hue may be reflective of male quality in Tree Swallows (Bitton and Dawson, 2008).
This is further supported by the knowledge that iridescence in color is a function of both the number and quality of feather barbules while hue is a function of the thickness of the feather cortex (Doucet et al., 2006, Prum, 2006). Condition in both the feather cortex and barbules may be influenced by genetic quality or physiological stress during the development period both of which may determine quality at an individual level.
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