Seasonal dietary shifting in yellow-rumped warblers is unrelated to macronutrient targets

Dietary shifting, for example from insects to fruits, is a common mechanism used in migratory songbirds to accumulate fat to fuel migratory flights. We examined a potential underlying cause of dietary shifting in yellow-rumped warblers (Setophaga coronata) by comparing energy and protein intake goals of birds during fall migration and winter. We offered captive warblers pairs of three diets differing in macronutrient composition in both the fall and winter. Using the principles of the geometric framework of nutrition we evaluated protein and energy intake to determine if consumption of the diet pairs was adjusted to meet an energy or protein intake target, and if the target differed seasonally. Regardless of season, the warblers preferred the diet with the lowest protein content and highest carbohydrate content. Total energy intake was maintained relatively constant during migration, at around 60 kJ/day, regardless of diet combination, and at about 50 kJ/day during winter. This suggests that warblers consume macronutrients available to them without protein limitations to reach their total energy intake target. When the diet combination offered allows, the warblers mixed their diet intake to consume roughly 0.5 g/day of protein, regardless of season, which suggested a constant protein target. Our findings suggest that songbirds prefer to alter non-protein energy intake proportionally to meet changing energy demand, rather than an overall increase in macronutrient intake. Additionally, they have the ability to shift their diet based on availability, resulting in high flexibility in their macronutrient intakes to maintain energy intake.