Many aspects of animal digestive form and function vary with ecological factors including diet composition and food availability. I examined phenotypic flexibility of digestive traits in white-browed sparrow-weavers (Plocepasser mahali), a widespread southern African passerine in which the ratio of insects to plant matter consumed varies seasonally. I predicted that digestive traits of P. mahali are modulated in response to diet composition. For experiment one, I caught forty-five sparrow-weavers from the Kalahari Desert population in South Africa, transported the birds to experimental facilities and acclimated each bird to one of three experimental diets [100 % seed, 100 % insect or control (70% seed, 30% insects), n = 15 per diet] while monitoring food intake and body mass (Mb). All three groups initially received the control diet, and thereafter gradually acclimated to the respective experimental
diets. Once experiment one was concluded, I randomly selected 24 individuals from experiment one, and maintained the birds on the control diet for two weeks. I euthanized the birds after an additional 8 days of acclimation to either a 100 % seed or 100 % insect diet (n=12), and thereafter determined liver mass, pancreas mass, gizzard mass, intestine mass, intestine length, and also quantified intestinal digestive enzyme activity of aminopeptidase-N (APN), maltase and sucrase. Diet emerged as a significant predictor of Mb. Birds acclimated to the seed diet lost ~0.07 % of Mb per day, whereas birds acclimated to the insect diet and the control diet gained ~0.15 % and ~0.08% of Mb per day respectively. Of all the digestive organs assessed, the sizes of only the liver and gizzard varied significantly with diet. Analysis of digestive enzyme activity indicated modulation correlated with diet composition. For example, in birds acclimated to the higher protein insect diet compared to the lower protein seed diet, APN activity summed over the entire intestinal length was 2.6-fold higher. For maltase and sucrase, activity in the proximal and medial section of the small intestine was statistically indistinguishable between the two diet groups, however, for birds acclimated to the insect diet, activity in the distal section was 78 % higher for maltase and 85 % for sucrase. Maltase activity summed over the entire intestinal length was 1.4-fold higher in birds fed the insect diet whereas sucrase activity remained similar between the two diet groups. These results indicate that digestive system features of P. mahali are modulated relative to diet composition, thus providing the capacity to maintain energy and nutrient balance despite variable diet compositions observed in different populations.