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

1. 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. 2. Here, we evaluate the adjustment
of fruit diets (the fruit-derived component of the diets) to nutritional
and energy intake targets, characterizing 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. 3. 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. 4.
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. 5. 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.