In Sickness and in Health: Understanding the interactive effects of immune challenge and foraging behavior on parental investment in a monogamous seabird
Carotenoid ornamentation is quite common in many species of birds, and often serves as an honest indicator of their fitness. The honesty of carotenoid ornamentation in the brown booby (Sula leucogaster brewsteri), a monogamous seabird, is likely driven by rarity of carotenoids in the diet but may also be regulated through additional pathways. For example, the trade-off hypothesis states that carotenoid pigments used in ornaments may also be required for the immune system as an antioxidant, thereby allowing only the healthiest birds to afford the carotenoid ornamentation. To test the trade-off hypothesis, I took advantage of a previously conducted experiment on brown boobies from the Marietas Islands, Mexico, where a subset of individuals were treated with a lipopolysaccharide to produce an immune response, while a control group was treated with a buffer solution. These seabirds were observed during their nesting period with parental behaviors and carotenoid coloration data recorded throughout the given time-frame. I studied the foraging behavior of these birds through bulk stable isotope analysis of d13C and d15N values in their breast contour feathers.
I explored foraging behavior data alongside,–immune responses, parental behaviors, and carotenoid coloration data in order to determine whether brown boobies foraging habits moderated the impact of immune challenge on parental effort. Results indicated that the imposed immune challenge had no effect on carotenoid ornament coloration, suggesting that the carotenoids used for ornamentation are not necessary for the bird’s immune system function, or else not limited in quantity. The results also revealed that immune-challenged birds with increased foraging ability (foraging habits associated with the preferred ornament type) provided better parental care compared to birds with decreased foraging ability. Control birds did not show this trend, and their parental care was not related to my measure of foraging behavior. My results suggest that foraging ability becomes more critical to parental effort when the parent is also producing an immune response.