Dietary protein influences the rate of ¹⁵N incorporation in blood cells and plasma of Yellow-vented bulbuls (Pycnonotus xanthopygos)

The rate at which an animal's tissues incorporate the isotopic composition of food determines the time window during which ecologists can discern diet changes. We investigated the effect of protein content in the diet on the incorporation rate of ¹⁵N into the plasma proteins and blood cells of Yellow-vented bulbuls (Pycnonotus xanthopygos). Using model comparison analyses, we found that one-compartment models described incorporation data better than two-compartment models. Dietary protein content had a significant effect on the residence time of ¹⁵N in plasma proteins and blood cells. The diet with the highest protein content led to a ¹⁵N retention time of 21 and 5 days for cells and plasma, respectively. In contrast, average ¹⁵N retention time in the cells and plasma of birds fed on the diet with the lowest protein was 31 and 7 days, respectively. The isotopic discrimination factor Δ¹⁵N=δ¹⁵N _tissues-δ¹⁵N_diet was also dependent on dietary protein content, and was lowest in birds fed the diet with the highest protein content. Blood, plasma and excreta were enriched in ¹⁵N relative to diet. In contrast, ureteral urine was either significantly depleted of ¹⁵N in birds fed the diet with the lowest protein content or did not differ in δ¹⁵N from the diets with the intermediate and high protein content. Thus, isotopic incorporation rates and tissue-to-diet discrimination factors cannot be considered fixed, as they depend on diet composition.