Nutrient balance and energy-acquisition effectiveness: Do birds adjust their fruit diet to achieve intake targets?

Abstract

According to diet-regulation hypotheses, animals select food to regulate the intake of macronutrients or maximise energy feeding efficiency. Specifically, the nutrient balance model proposes that foraging is primarily a process of balancing multiple nutrients to achieve a nutritional intake target, while the energy maximisation model proposes that foraging aims to maximise energy. Here, we evaluate the adjustment of fruit diets (the fruit-derived component of the diets) to nutritional and energy intake targets, characterising the nutrient balance and energy maximisation strategies across fruit-eating bird species with different fruit-handling behaviours (‘gulpers’, which swallow whole fruits, and ‘mashers’, which process the fruit in the beak) in subtropical Andean forests. Food-handling behaviour determines the food intake rate and, consequently, influences animal efficiency to obtain nutrients and energy. We used extensive field data from the diet of fruit-eating birds to test how species adjust their food intake. We used nutritional geometry to explore macronutrient balance and the effectiveness framework to explore energy-acquisition effectiveness. Observed diets showed a good fit with predictions of a diet balanced in macronutrient proportions. With few exceptions, diets clustered near an optimal macronutrient mixture and did not differ from each other in terms of maximising energy intake. Moreover, when comparing our results with a random diet based on local fruit availability, birds tended to fit better to the nutritional target, and less to the energy target, than expected from a random diet. Fruit-handling behaviour did not affect the ability of bird species to reach a nutritional target but it affected species energy acquisition, which was lower in mashers than in gulpers. This study explores for the first time different diet-regulation strategies in wild fruit-eating birds, and supports the argument that the diet reflects a specific regulation of macronutrients. Understanding why birds select fruits is a complex question requiring multiple considerations. The nutrient balance model explains the relevance of nutrient composition in the fruit selection by fruit-eating birds, although it is still necessary to determine its relative importance with respect to other dietary drivers. Read the free Plain Language Summary for this article on the Journal blog.