Regulation of breeding expenditure in the blue-footed booby, Sula nebouxii: an experimental approach

Abstract

In the short-term, reproductive expenditure has repercussions for survival and future reproductive output. In long-lived seabirds, parents are expected to adjust their reproductive investment based on their physiological and immune status and then respond to the needs of their offspring without incurring additional costs. However, the impacts of parental expenditures on physiological and immune status have not been well explored. We compared the foraging effort (number and duration of foraging trips), time at nest and physiological status (plasma metabolites, heterophil/lymphocyte (H/L) ratio, creatine kinase (CK) activity) and body condition index (BCI) of blue-footed boobies subjected to experimentally increased or decreased breeding demands. When parental reproductive demands were increased, adults made more foraging trips and decreased their time at the nest. The increases in adult physical activity were reflected in higher levels of creatine kinase/total protein (CK/TP, U/g) and higher H/L ratios, indicating that adjustments in foraging effort were reflected in their physiological status. When parental reproductive demands were reduced, parents adjusted their level of effort according to the lower needs of their offspring; as a result, their overall physiological status was greater than that of the controls, showing lower CK/TP levels and H/L ratios. Our findings indicate that parents modified their foraging effort in response to variation in the food demands of the brood to maintain reproductive value, but this adjustment had consequences for physiological status in both costs and benefits. Blue-footed boobies inhabit upwelling systems where they experience high environmental variability throughout their life span. Thus, the ability to adjust breeding effort may buffer breeding success under different scenarios, with short-term physiological expenditures possibly related to long-term survival.