Macronutrients, such as proteins and fats, play a vital role in host immunity and can influence host-pathogen dynamics, potentially through dietary effects on gut microbiota. To increase our understanding of how feeding behavior and macronutrient selection are influenced by a direct and perceived immune threat and whether shifts in macronutrient intake affect the composition of the gut microbiome, we conducted two experiments. First, we determined if zebra finches (Taeniopygia guttata) exhibit shifts in physiology and gut microbiota when fed diets differing in macronutrient ratios. Second, we simulated an infection in birds using the bacterial endotoxin lipopolysaccharide (LPS) and quantified feeding behavior in immune challenged and control individuals, as well as birds housed near either a control pair (no immune threat), or birds housed near a pair given an immune challenge with LPS (social cue of heightened infection risk). We also examined whether social cues of infection alter physiological responses relevant to responding to an immune threat, an effect that could be mediated through shifts in feeding behavior. In the first experiment, protein diets decreased the abundance of the bacterial Phylum Campylobacterota. Further, diet treatment disrupted relationships between gut microbiota alpha diversity and physiological metrics. In the second experiment, LPS induced a reduction in caloric intake driven by a decrease in protein, but not fat consumption. No evidence was found for socially induced shifts in feeding behavior, physiology, or gut microbiota. However, fat consumption decreased gut microbial diversity regardless of treatment. Our findings carry implications for host health, as sickness-induced anorexia and diet-induced shifts in the microbiome could shape host-pathogen interactions.

Nutrient-specific foraging is the ecological theory that generalist consumers select food resources based on their nutritional content. While laboratory experiments support this idea, it has yet to be demonstrated in invertebrates in the field. We combined dietary DNA metabarcoding with prey availability data and macronutrient content in the field to analyze nutrient-based prey choice. We show that spider nutrient intake and prey choice deviates from what we would expect if individuals randomly chose their prey. Through a novel nutrient-based taxonomy and null modelling, we reveal a stable average macronutrient intake. There was disproportionate foraging for different macronutrients by individual spiders. Although, this might be expected as individual prey are biased toward particular nutrients and individual spiders were at different stages of nutrient balancing when collected. This finding suggests that spiders are redressing nutritional deficits to obtain a target nutrient intake, as would be expected of nutrient-specific foraging. This evidence for nutrient-specific foraging under field conditions is a significant advance, extending our understanding beyond lab-based behavioral assays to now resolving complex real-world systems.

Nutrients are a critical driver of ecological interactions (e.g., plant-herbivore, predator-prey and host-parasite) but are not yet integrated into ecological networks. Ecological concepts like nutrient-specific foraging and nutrient-dependent functional responses provide invaluable mechanistic context to complex ecological interactions. These concepts in turn offer an opportunity to predict dynamic network processes such as interaction rewiring and cascading extinction events. Here, we propose the concept of nutritional networks. By integrating nutritional data into ecological networks, we envisage significant advances to our understanding of ecological dynamics at every scale from individuals to ecosystems. We summarise the potential influence of nutrients on the structure and complexity of ecological networks, with specific reference to niche partitioning, predator-prey dynamics, spatiotemporal patterns and robustness. Using an empirical example of an inter-specific trophic network, we show that networks can be constructed with nutritional data to disentangle the drivers of ecological interactions in natural systems. Throughout, we identify fundamental ecological hypotheses that can be explored in a nutritional network context, and highlight methodological frameworks to facilitate their operationalisation.

Insectivores gain macronutrients and elements from consuming arthropod prey, but must also deal with indigestible components (i.e., exoskeleton) of prey. For example, avian chicks (e.g. northern bobwhites; Colinus virginianus) have limited gut space, and ingesting prey with relatively higher proportions of indigestible components may impact assimilation efficiency, growth, and survival. The ability of insectivores to choose higher quality prey would depend on prey taxa varying consistently in nutritional content. We tested if there were consistent differences among taxonomic orders of arthropod prey in their macronutrient (protein and lipid), elemental (C and N), and exoskeleton content. We used northern bobwhite chicks as our focal insectivore and focused on their potential prey as a case study. We also tested the influence of indigestible exoskeleton on the measurement of macronutrient content and the ability of elemental content to predict macronutrients. We found large and consistent variation in macronutrient and elemental content within and between arthropod orders. Some orders had consistently high protein content and low exoskeleton content (i.e., Araneae) and are likely higher quality prey for insectivores. Abundant orders common in the diets of insectivores, like Hymenoptera and Coleoptera, had high exoskeleton content and low protein content. We also found support for the ability of elements to predict macronutrients, and found that metabolizable (i.e. exoskeleton removed) elemental content better predicted macronutrient content. A better understanding of arthropod nutrient content is critical for elucidating the role of spatial and temporal variation in prey communities in shaping the growth and survival of insectivores.