Nutrition is fundamental to almost all aspects of an animal’s existence including fitness, growth and reproduction. Foraging theory proposes that the nutritional driver of food choice and foraging in carnivores is energy. We use nutritional geometric models (right-angled mixture triangle -RMT-) to examine macronutrient variability in the prey and diets of a group of wild carnivores and marine top predators, the Australasian gannets (Morus serrator). The study was conducted between 2012 and 2015 breeding seasons at Farewell Spit colony on the South Island, New Zealand (40° 33'S 173° 01'E). Diets were measured using regurgitations collected form individual chick-rearing adults, and the proximate composition (protein, lipid, ash and moisture) of prey samples was obtained following certified laboratory analyses. Our study reveals nutritional variability in the proportion of protein and lipids at multiple scales. First, we observed intra- and inter-specific variability of prey within and between seasons. Second, we found significant individual and seasonal variation of diets. Third, we unravel the use of macronutrient sex specific foraging strategies. We discuss the importance of integrating nutritional modelling for dietary assessments with a long-term systematic collection of samples for proximate analysis. Our approach allows us to comprehend mechanisms governing dietary choices in wild carnivores that forage in complex and fluctuating environments.