Plumage Signals & Sexual Selection

A focus of my dissertation research (with Dr. Geoffrey Hill at Auburn University) concerned the function and evolution of plumage coloration in the eastern bluebird. Eastern bluebirds are a model system within which to investigate sexual selection and mate choice because they readily use nest boxes, breed repeatedly in the same location, and tolerate considerable disturbance at the nest. They are socially monogamous, with bi-parental care. Males assist in nest defense, and feeding nestlings and fledglings. Eastern bluebirds have a brilliant blue head, back, and wings and a rusty breast patch. The UV-blue plumage is structurally-based while the rusty breast patch is combination of phaeo- and eu- melanins.

My past research suggests that structural plumage of male eastern bluebirds is an important sexually-selected signal. First, males with more ornamented UV-blue plumage are better able to secure nest boxes, and limiting resource for cavity nesting species (Siefferman & Hill 2005a). Second, field correlations show that more-ornamented males find mates earlier in the season, provision offspring more often, and produce larger offspring than less ornamented males (Siefferman & Hill 2003). Third, males that display brighter UV-blue plumage tend to be both older and in better body condition than duller males (Siefferman et al. 2005). Fourth, brood size manipulations, used to increase parental investment, cause males to become duller in the next breeding season (Siefferman & Hill 2005b) and those male nestlings that were fed less grew less colorful juvenile plumage (Siefferman & Hill 2007). However, there is no evidence that females prefer to mate with bluer males (Liu et al. 2007, Liu et al. 2009).

Female bluebirds show a subdued expression of blue and chestnut coloration. I used a combination of an aviary nutritional-stress experiment and field data to test the hypothesis that female ornamentation functions as a signal of quality. In the laboratory, females who were given ad libitum access to food displayed more ornamented structural coloration than females on a food-restricted diet, but there was no effect of the experiment on melanin ornamentation. In the field, blue color, age, and body condition of females predicted first egg date and nestling condition (Siefferman and Hill 2005c).

Currently, we are continuing this work both in the Alabama population and in a new population in North Carolina. Lately, we are focusing more on juvenal plumage coloration. My former graduate student, Nicole Barrios-Miller, found that fathers showed favoritism toward brighter sons and favored sons over daughers when mated with brighter females (Barrios-Miller and Siefferman 2013). Amanda Doyle, an former undergraduate honors student, showed that supplemental feeding increases the plumage coloration of sons.


Advanced