The Effect of Male Plumage Coloration on Parental Effort in Eastern Bluebirds (Sialia sialis)
Type of Degreethesis
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In iteroparous species, there is tradeoff between current and future reproductive investment. Parents that invest more in current offspring will provide resources that give those offspring better chances of survival and reproductive success. If the current brood is of poor quality, parents may invest less and save energy for other processes, as well as future reproduction, maximizing lifetime reproductive success. The differential allocation hypothesis predicts that individuals will adjust their parental investment based on the quality of their mate. Females will invest more in offspring of attractive males than unattractive males. In response, attractive males will invest less in current reproduction and allocate more time and resources to other processes. In the field, I tested the differential allocation hypothesis in the socially monogamous Eastern Bluebird (Sialia sialis). I experimentally manipulated the brightness of the structural blue coloration of Eastern Bluebirds, which is a known condition-dependent trait in the species. The brightness of male birds was either enhanced using a violet marker or reduced using a black marker. I recorded nest visitation rates prior to and after marker application to test the effects of treatment on both female and male visitation rates. I then tested the effect of treatment on offspring growth rates. I predicted that 1) females mated to brighter males would visit the nest more than females mated to duller males, 2) brighter males would visit the nest less than duller males, and 3) offspring of brighter males would grow at a faster rate than offspring of duller males. I found a trend suggesting that females of brighter males provision offspring at a higher rate than females mated to duller males, but there was no significant difference between males of each group. Tarsus growth rate of chicks between the groups did not differ, but chicks of the bright treatment gained mass at a faster rate than chicks of the dull treatment. The significant effect of treatment on chick body mass supported the trend demonstrated in female provisioning. I suggest that differential allocation by female bluebirds has the potential to create a selective pressure on male plumage brightness through enhanced offspring growth rates.