Age effects on Nazca booby foraging performance are largely constant across variation in the marine environment: Results from a 5‐year study in Galápagos

Abstract

Foraging outcomes dictate the nutritional resources available to an organism and may vary with intrinsic factors, like age. Thus, understanding how age affects foraging performance, alone or in interaction with extrinsic factors (like environmental quality), improves our understanding of aging processes in the wild. We examined how foraging traits, measured across five breeding seasons, change with age, environmental variation, and their interaction in Nazca boobies (Sula granti), a pelagic seabird in Galápagos. We evaluated the hypotheses that (1) foraging performance is better in middle-aged birds than in young ones, and that (2) foraging performance is better in middle-aged birds than in old ones. Furthermore, favorable environmental conditions will either (3) attenuate age differences in foraging performance (by relieving constraints on young, inexperienced and old, senescent age classes), or (4) accentuate age differences (if middle-aged birds can exploit abundant resources better than other age classes can). Incubating birds tagged with GPS loggers (N = 815) provided data on foraging performance (e.g., total distance traveled, mass gained) to evaluate interactions between age and environmental variation (e.g., sea surface temperature). Poor environmental conditions associated with the cool phase of the El Niño-Southern Oscillation increased foraging effort, including foraging distance and duration, for example. Across age classes, foraging boobies responded similarly to environmental variation except for female mass gain rate: age-related declines in mass gain rate were reduced under favorable environmental conditions. Birds of different ages also searched in somewhat distinct areas in the poor conditions of 2016, but not in other years. In several foraging traits, including foraging duration and distance, female boobies showed predicted early-life improvement and late-life decline, following the established pattern for reproductive traits in this species. Thus, deficits in resource acquisition (this study) may contribute to the poor survival and reproductive outcomes previously observed in old Nazca boobies, particularly in females.underlying foraging traits (mass gain, foraging duration; following Howard et al., 2021), and, for the first time, for foraging location, distance traveled, and time spent searching. We evaluate age-byenvironment interactions with respect to a population-level shift in breeding date (Tompkins et al., 2017) and two environmental predictors linked to the El Niño-Southern Oscillation (ENSO; sea surface temperature and cloud cover; Table 1). Using patterns of observed age effects in breeding traits as models (Tompkins et al., 2017;Tompkins & Anderson, 2019, 2021, and assuming a tight link between foraging and breeding performances, we predict that: (1) foraging performance will be better in middle-aged birds than in young ones in our cross-sectional sample (higher rate of mass gain, larger mass gain, and shorter foraging duration, distance traveled, and time searching in middle-aged...