Frugivorous birds adjust their diet to achieve their nutritional goals

In our latest post, Dr. Pedro Blendinger, a professor at the National University of Tucumán, Argentina, discusses their paper: “Nutrient balance and energy-acquisition effectiveness: do birds adjust fruit diet to achieve intake targets?”.

A Spanish version of this blogpost is available here.

About the paper

This work was inspired by cross-collaboration between colleagues with whom we share a common interest in plant-animal interactions. Almost a decade ago, we organized ourselves into the “Mutualism Club”, a relaxed space for discussion and exchange of ideas. One of these ideas germinated, then grew while avoiding various encounters and disagreements, eventually ending up as the subject of our recent article.

Mariana Valoy and Mariano Ordano, two of the co-authors, taking data in the field.

Frugivorous birds, like many animals, do not roam their territory consuming food at random, but choose what and how many fruits they consume within the possibilities provided by the environment they inhabit. But what are the rules that govern fruit-eating behaviour? Why do birds consume a combination of different fruits at different times? After decades of study, the researchers found that answering these questions is more complex than it seems, given the multiplicity of factors involved. The dietary regulation hypotheses attempt to answer, in part, these questions, and there were two hypotheses that were particularly attractive to us in order to do so. These are “energy maximization”, which proposes that birds consume fruits in such a way as to maximize energy gain, and “nutrient balance”, which proposes that animals mix nutritionally complementary foods to balance the acquisition of multiple nutrients.

A view of the montane forest in the subtropical Andes.

We set out to study whether the diet of frugivorous birds can be explained by any of these hypotheses. We completed our study in the subtropical Andean forests (Southern Yungas), in north-western Argentina. It is a diverse ecoregion that hosts rich communities of frugivorous bird species and fruits with fleshy pulp that make up their diet. Specifically, we investigated whether the diet of bird species approaches a nutritional optimum that balances the intake of macronutrients (namely carbohydrates, lipids, and proteins), and how effective birds are at acquiring energy from their diet. These two initial diagnoses allowed us to assess whether birds adjust their diet to achieve the nutritional and energy intake goals proposed by both hypotheses.

We found that most bird species tended towards an optimal balance of macronutrients, meaning that the most common strategy among frugivorous birds in subtropical Andean forests is the regulation of nutrient intake in diets and not a consumption that maximizes energy gain. To achieve this optimum, birds selected both the species and amount of fruit that they consumed, in a way that allowed them to regulate their intake of macronutrients. Complementary analyses showed that birds would use regulatory mechanisms that favour carbohydrate intake and minimize protein intake during fruit consumption. The latter highlights that birds can select fruits to meet particular physiological demands, without straying from a nutritional optimum.

Our study allows us to understand the drivers of feeding strategies used by frugivores to satisfy their ecophysiological needs. If we scale our results from individuals to the functioning of communities, the predominance of a strategy based on macronutrient balance could explain why we find a mixture of fruit species that differ in their nutritional content in communities.

About the research

To achieve our research objectives, we combined extensive field and laboratory data from our previous studies: field records of fruit consumption by bird species in different study sites, counts of fruits available for frugivores, and the chemical content of the different fruit species. We also had to complete an important piece of information that we were missing—how many fruits of each species consume a bird during a foraging bout. To do this, we supplemented our own data with published data on the foraging behaviour of frugivorous birds from around the world, with which we calculated an equation to estimate the missing data.

The Andean Slaty Thrush (Turdus nigriceps), one of the frugivorous species we studied (credit: Michael Blendinger)

Studying diet regulation in small free-living frugivores has its complications, which may explain the scarcity of previous studies on the subject. In particular, we had to overcome two difficulties. On the one hand, the nutrient balance and energy maximization models were designed to understand the behaviour of individuals; that is, they required knowing the diet of an individual bird. However, when we study birds in mountain forests, it is almost impossible to know what the same individual eats over time, since once we find a bird we can only observe it for a short period of time before losing sight of it. To overcome this problem, we combined the fruit consumption records of different individuals at one site, which shows the central tendency in the diet of individuals in a given population. On the other hand, there is no information on what is an optimal mixture of macronutrients for a wild bird. As a proxy for this, we calculated the mean percentage of macronutrient mixtures in commercial pellet brands used to keep birds in captivity.

This study explores—for the first time—different dietary regulation strategies in wild frugivorous birds and supports the argument that nutrient balance is useful in explaining fruit selection by frugivorous birds. But what happens when birds cannot balance their diet? Frugivores may suffer from dietary limitations that can affect their physical condition, health, and reproductive success. Habitat transformations due to deforestation and the invasion of exotic plant species are two mechanisms that can lead to this situation. The next step is to study how these human-induced disturbances may affect the diet and health of frugivorous birds.

About the author

I am passionate about nature and fortunate enough to be able to bring that passion to my daily work. I combine my interests in ornithology and ecology in the study of mutualistic interactions like frugivory and seed dispersal.

Pedro Blendinger

I completed my education at the free public university in Argentina, and my professional career at CONICET—the main public organization dedicated to the promotion of science in the country. Currently, I am a researcher at said organization and a professor at the National University of Tucumán, Argentina.

I think that mutualistic interactions between plants and animals are essential to maintain the functioning of ecosystems. They also provide ecosystem services that directly and indirectly influence human well-being. However, these functions and services are usually not appreciated outside those who work on the subject, so it is key to disclose its importance.

Enjoyed the blogpost? Read the research here!

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