Diet composition and provisioning rates of nestlings determine reproductive success in a subtropical seabird

Abstract

**Abstract:** Understanding how both quality and quantity of prey affect the population dynamics of marine predators is a crucial step toward predicting the effects of environmental perturbations on population-level processes. The Junk Food Hypothesis, which posits that energetic content of prey species may influence reproductive capacity of marine top predators regardless of prey availability, has been proposed as a mechanism by which changes in prey populations could affect predator populations in high latitude systems; however, support for this hypothesis has been inconsistent across studies, and further data are needed to elucidate variation in the relative importance of prey quantity and quality among predator species and across ecological systems. We tested the relative importance of prey quantity and quality to nestling survival, condition, and stress levels in the eastern brown pelican (Pelecanus occidentalis carolinensis) across 9 breeding colonies in the northern Gulf of Mexico. Compared to previous results from cold-water systems, we found lower and less variable energy densities (4.4 kJ g−1, vs. 5.2 to 6.5 kJ g−1 in other studies) and lipid content (9% dry mass, vs. 16 to 23% in other studies) of common prey items. As a result, energy density of meals was not a strong predictor of fledging success, body condition, or stress levels of nestlings. However, both feeding frequency and meal mass were significantly correlated to energy provisioning rates and nestling survival. We also found that levels of the stress hormone corticosterone in nestling feathers collected at 3-4 weeks of age correlated strongly with provisioning rates and predicted both pre-and post-fledging survival more effectively than did one-time measures of body condition. We conclude that quantity rather than quality of prey, particularly small schooling fish, is the main driver of brown pelican reproductive success in this system, and that environmental perturbations affecting biomass, distribution, and abundance of forage fish could substantially affect brown pelican reproductive success. Moreover, our results suggest that feather corticosterone can provide information on both nutritional stress and survival rates in free-living seabird nestlings. **Authors:** Juliet Lamb¹, Yvan Satge², Katie O'Reilly³, Patrick Jodice⁴ ¹University of Rhode Island, ²Clemson University, ³University of Portland, ⁴US Geological Survey