Life is not always sweet! A nutritional geometry analysis of protein and sugar intake by hummingbirds

• Sympatric hummingbird species differed markedly in protein and sugar intake when exposed to identical diets. • Ingesting large amounts of protein negatively affected the health of hummingbirds. • Higher protein tolerance was associated with higher protein ingestion when sugar availability was limited. • Differences in nutrient intake patterns may help explain ecological differences between sympatric hummingbird species. Nectar-feeding birds have low nitrogen requirements that tend to be paired with low protein dietary ingestion. As a result, they must acquire and efficiently use proteins for maintenance, growth, and reproduction, thereby avoiding being limited by this nutrient. While these birds regularly consume arthropods, particularly during periods of low nectar availability, little is known about their intake and regulation of nutrients other than sugar and water. In this study, we used a nutritional geometry approach to examine protein's role in nutrient and energy balance in hummingbirds. We conducted free-choice feeding trials with solutions varying in protein (0.2%-2%) and sucrose (5%-30%) concentrations to compare the nutritional strategies of Broad-billed ( Cynanthus latirostris ) and Berylline ( Saucerottia beryllina ) Hummingbirds. Results show that daily and cumulative protein and sucrose intake per unit of metabolic mass were significantly higher in C. latirostris than in S. beryllina. During the first days, both species presented similar accumulated feeding trajectories with nearly identical protein:sucrose ratios. However, as trials proceeded, their slopes diverged, emphasizing a higher protein intake in C. latirostris . These species-specific intake patterns are consistent with differences in protein tolerance and nutrient-handling constraints under the offered diets. We suggest that such differences could contribute to contrasting ecological strategies in the wild (e.g., residency vs. local movements) when floral resources decline, a hypothesis that requires comparative and field-based testing. Overall, this study is the first to explore protein and sugar ingestion in hummingbirds, offering new insights into how nutrient-handling abilities reflect the ecology of nectar-feeding birds.